How to check if PHP array is associative or sequential

Question

PHP treats all arrays as associative  so there aren t any built in functions  Can anyone recommend a fairly efficient way to check if an array contains only numeric keys  Basically  I want to be able to differentiate between this   sequentialArray          apple    orange    tomato    carrot      and this   assocArray          fruit1    gt   apple        fruit2    gt   orange        veg1    gt   tomato        veg2    gt   carrot

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function is array assoc  foo        if  is array  foo             return  count array filter array keys  foo    is string     gt  0             return false

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To merely check whether the array has non-integer keys  not whether the array is sequentially-indexed or zero-indexed    function has string keys array  array      return count array filter array keys  array    is string     gt  0      If there is at least one string key   array will be regarded as an associative array

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One more fast from source  Fit encoding of json encode  and bson encode   So has javascript Array compliance   function isSequential  value       if is array  value       value instanceof  Countable  amp  amp   value instanceof  ArrayAccess            for   i   count  value  - 1   i  gt   0   i--                if   isset  value  i    amp  amp   array key exists  i   value                     return false                                  return true        else           throw new  InvalidArgumentException              sprintf  Data type   s  is not supported by method  s   gettype  value     METHOD

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answers are already given but there s too much disinformation about performance  I wrote this little benchmark script that shows that the foreach method is the fastest   Disclaimer  following methods were copy-pasted from the other answers   lt  php  function method 1 Array  amp  arr        return  arr     array values  arr      function method 2 Array  amp  arr        for  reset  arr    i   0  key  arr       i    next  arr        return is null key  arr       function method 3 Array  amp  arr        return array keys  arr      range 0  count  arr  - 1      function method 4 Array  amp  arr         idx   0      foreach   arr as  key   gt   val            if   key      idx               return FALSE           idx              return TRUE        function benchmark Array  methods  Array  amp  target           foreach  methods as  method            start   microtime true           for   i   0   i  lt  1000   i                 dummy   call user func  method   target             end   microtime true           echo  Time taken with  method     round   end- start  1000 0 3   ms n               targets          Huge array    gt  range 0  30000        Small array    gt  range 0  1000       methods          method 1        method 2        method 3        method 4      foreach  targets as  targetName   gt   target       echo       Benchmark using  targetName      n       benchmark  methods   target       echo   n       results        Benchmark using Huge array      Time taken with method 1   5504 632ms Time taken with method 2   4509 445ms Time taken with method 3   8614 883ms Time taken with method 4   2720 934ms       Benchmark using Small array      Time taken with method 1   77 159ms Time taken with method 2   130 03ms Time taken with method 3   160 866ms Time taken with method 4   69 946ms

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Speed-wise   function isAssoc  array        return   array     array values  array        Memory-wise   function isAssoc  array         array   array keys  array   return   array     array keys  array

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If your looking for just non-numeric keys  no matter the order  then you may want to try  function IsAssociative  array        return preg match    a-z  i   implode array keys  array

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There are many answers already  but here is the method that Laravel relies on within its Arr class           Determines if an array is associative        An array is  associative  if it doesn t have sequential numerical keys beginning with zero         param  array   array     return bool     public static function isAssoc array  array         keys   array keys  array        return array keys  keys       keys      Source  https   github com laravel framework blob 5 4 src Illuminate Support Arr php

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This question is actually useless when it comes to array of php because with the nature of php an array should not have to be fully associative or indexing  it can be  combination of both  the way user have define and assigned the value an array can be a combination of both  see the example below   y  array 5    y  0x    n    y  vbg    12132    y 1     k    var dump  y     this will output 4 element array  echo   lt  br gt     y  0x     lt  br gt    y 0    for  x 0  x lt sizeof  y   x        this will output all index elements  amp  gives error after that     echo   lt  br gt  index elements    y  x       so the correct question that has to ask is   is all the element in array are associative or index  if you really know that it will only be either associative or indexing not a combination of these two  you can just simply use this method to find wether it is an index or associative array   function AssocTest  amp  arr       if is array  arr             reset  arr      reset pointer to first element of array          if gettype key  arr       string      get the type nature  of first element key              return true           else              return false                 else          return false            you can use it as normal function  echo AssocTest  y     Associative array    Not an Associative array  Not an array at all      and important thing to remember evan you have initialize an array as associative but the names that you have gave the associative array is just numbers it will treat as an index array when it being read by php if you have not explicitly gave the string names  take a look at the example below    y  0    n    y  1    12132    y  22      k      both will get the same output echo   lt br  gt  s0    y  22    echo   lt br  gt  s0    y 22    for  x 0  x lt count  y   x        echo   lt br  gt  arr    y  x      this will output up to 2nd element and give an error after      so if you need to be sure all the elements of the array to be exactly indexed or either associative   there is no other way but go true all the elements and check each and every element key by generated index array as post by many people here   function fullAssocTest  amp  arr        if is array  arr            return  array keys  arr      range 0  count  arr  - 1              its less coding  but this thing is really process intensive and really un-necessary work

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A lot of the solutions here are elegant and pretty  but don t scale well  and are memory intensive or CPU intensive  Most are creating 2 new data points in memory with this solution from both sides of the comparison  The larger the array the harder and longer the process and memory used  and you lose the benefit of short circuit evaluation  I Did some testing with a few different ideas  Trying to avoid array key exists as it is costly  and also avoiding creating new large datasets to compare  I feel this is a simple way to tell if an array is sequential   public function is sequential   arr             if   is array   arr      empty   arr     return false        i   0        total   count   arr         foreach   arr as  key   gt   value   if   key      i     return false       return true      You run a single count on the main array and store a single integer  You then loop through the array and check for an exact match while iterating the counter  You should have from 1 to count  If it fails it will short circuit out which gives you performance boost when it is false    Originally I did this with a for loop and checking for isset   arr  i    but this will not detect null keys which requires array key exists  and as we know that is the worst function to use for speed   Constantly updating variables via foreach to check along with the iterator never growing past it s integer size let s PHP use it s built in memory optimization  caching and garbage collection to keep you at very low resource usage    Also  I will argue that using array keys in a foreach is silly when you can simply run  key     value and check the key  Why create the new data point  Once you abstract away the array keys you ve consumed more memory immediately

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answers are already given but there s too much disinformation about performance  I wrote this little benchmark script that shows that the foreach method is the fastest   Disclaimer  following methods were copy-pasted from the other answers   lt  php  function method 1 Array  amp  arr        return  arr     array values  arr      function method 2 Array  amp  arr        for  reset  arr    i   0  key  arr       i    next  arr        return is null key  arr       function method 3 Array  amp  arr        return array keys  arr      range 0  count  arr  - 1      function method 4 Array  amp  arr         idx   0      foreach   arr as  key   gt   val            if   key      idx               return FALSE           idx              return TRUE        function benchmark Array  methods  Array  amp  target           foreach  methods as  method            start   microtime true           for   i   0   i  lt  1000   i                 dummy   call user func  method   target             end   microtime true           echo  Time taken with  method     round   end- start  1000 0 3   ms n               targets          Huge array    gt  range 0  30000        Small array    gt  range 0  1000       methods          method 1        method 2        method 3        method 4      foreach  targets as  targetName   gt   target       echo       Benchmark using  targetName      n       benchmark  methods   target       echo   n       results        Benchmark using Huge array      Time taken with method 1   5504 632ms Time taken with method 2   4509 445ms Time taken with method 3   8614 883ms Time taken with method 4   2720 934ms       Benchmark using Small array      Time taken with method 1   77 159ms Time taken with method 2   130 03ms Time taken with method 3   160 866ms Time taken with method 4   69 946ms

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function is associative  arr      return  array merge  arr       arr    count array filter  arr   is string   ARRAY FILTER USE KEY    gt  0

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Speed-wise   function isAssoc  array        return   array     array values  array        Memory-wise   function isAssoc  array         array   array keys  array   return   array     array keys  array

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I ve come up with next method  function isSequential array  list   bool        i   0       count   count  list       while  array key exists  i   list              i    1          if   i      count                return true                       return false      var dump isSequential array         false var dump isSequential array  a    b    c         true var dump isSequential array  quot 0 quot    gt   a    quot 1 quot    gt   b    quot 2 quot    gt   c         true var dump isSequential array  quot 1 quot    gt   a    quot 0 quot    gt   b    quot 2 quot    gt   c         true var dump isSequential array  quot 1a quot    gt   a    quot 0b quot    gt   b    quot 2c quot    gt   c         false var dump isSequential array  quot a quot    gt   a    quot b quot    gt   b    quot c quot    gt   c         false   Note empty array is not considered a sequential array  but I think it s fine since empty arrays is like 0 - doesn t matter it s plus or minus  it s empty  Here are the advantages of this method compared to some listed above   It does not involve copying of arrays  someone mentioned in this gist https   gist github com Thinkscape 1965669 that array values does not involve copying - what    It certainly does - as will be seen below  It s faster for bigger arrays and more memory friendly at the same time  I ve used benchmark kindly provided by Artur Bodera  where I changed one of the arrays to 1M elements  array fill 0  1000000  uniqid        big numeric array   Here are the results for 100 iterations  PHP 7 1 16  cli   built  Mar 31 2018 02 59 59    NTS    Initial memory  32 42 MB Testing my method  isset check  - 100 iterations   Total time  2 57942 s   Total memory  32 48 MB  Testing method3  array filter of keys  - 100 iterations   Total time  5 10964 s   Total memory  64 42 MB  Testing method1  array values check  - 100 iterations   Total time  3 07591 s   Total memory  64 42 MB  Testing method2  array keys comparison  - 100 iterations   Total time  5 62937 s   Total memory  96 43 MB   Methods are ordered based on their memory consumption   I used echo  quot   Total memory   quot    number format memory get peak usage   1024 1024  2     quot  MB n quot   to display memory usage

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iszba - Is Zero Based Array  Detects if an array is zero based or not   PARAMS       chkvfnc         Callback in the loop allows to check the values of each element          Signature              bool function chkvfnc  v               return                  true    continue looping                 false   stop looping  iszba returns false too   NOTES    assert   array is an array    May be memory efficient    it doesn t get extra arrays via array keys   or ranges   into the function    Is pretty fast without a callback    With callback it s  2 4 times slower     function iszba  array   chkvfnc null         ncb     chkvfnc       i   0       foreach  array as  k   gt   v           if  k      i                if  ncb     chkvfnc  v                   continue           return false             return true          Without callback it is  30  faster than current leading answer  and possibly more memory efficient       Just negate the answer to know if the array should be considered associative

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function array is assoc array  a         i   0      foreach   a as  k   gt   v            if   k      i                  return true                      return false      Fast  concise  and memory efficient  No expensive comparisons  function calls or array copying

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Unless PHP has a builtin for that  you won t be able to do it in less than O n  - enumerating over all the keys and checking for integer type  In fact  you also want to make sure there are no holes  so your algorithm might look like   for i in 0 to len your array       if not defined your-array i              this is not an array array  it s an associative array      But why bother  Just assume the array is of the type you expect  If it isn t  it will just blow up in your face - that s dynamic programming for you  Test your code and all will be well

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This function can handle    array with holes in index  e g  1 2 4 5 8 10   array with  0x  keys  e g  key  08  is associative while key  8  is sequential    the idea is simple  if one of the keys is NOT an integer  it is associative array  otherwise it s sequential   function is asso  a       foreach array keys  a  as  key   if   is int  key   return TRUE       return FALSE

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One more fast from source  Fit encoding of json encode  and bson encode   So has javascript Array compliance   function isSequential  value       if is array  value       value instanceof  Countable  amp  amp   value instanceof  ArrayAccess            for   i   count  value  - 1   i  gt   0   i--                if   isset  value  i    amp  amp   array key exists  i   value                     return false                                  return true        else           throw new  InvalidArgumentException              sprintf  Data type   s  is not supported by method  s   gettype  value     METHOD

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Here is another simple but powerful logic   which is also used by great Laravel framework in it s internal mechanism          Determines if an array is associative      param  array   array     return bool     function isAssoc array  array         keys   array keys  array        return array keys  keys       keys

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Or you can just use this   Arr  isAssoc  array    which will check if array contains any non-numeric key or   Arr isAssoc  array  true    to check if array is strictly sequencial   contains auto generated int keys 0 to n-1   using this library

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Here is another simple but powerful logic   which is also used by great Laravel framework in it s internal mechanism          Determines if an array is associative      param  array   array     return bool     function isAssoc array  array         keys   array keys  array        return array keys  keys       keys

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function is associative  arr      return  array merge  arr       arr    count array filter  arr   is string   ARRAY FILTER USE KEY    gt  0

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function isAssoc  arr         a   array keys  arr       for  i   0   t   count  a    i  lt   t   i                  if  a  i      i                        return false                      return true

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function array is assoc array  a         i   0      foreach   a as  k   gt   v            if   k      i                  return true                      return false      Fast  concise  and memory efficient  No expensive comparisons  function calls or array copying

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This is my function -   public function is assoc array  array        if is array  array      true           return false       else            check   json decode json encode  array             if is object  check      true               return true           else              return false                        Some examples      print r  is assoc array   one   two   three       true  Yes   No      No     print r is assoc array   one   gt  one   two   gt  two   three   gt  three     Yes   No      Yes     print r is assoc array   1   gt  one   2   gt  two   3   gt  three     Yes   No      Yes     print r is assoc array   0   gt  one   1   gt  two   2   gt  three     Yes   No      No   There was a similar solution by  devios1 in one of the answers but this was just another way using the inbuilt json related functions of PHP  I haven t checked how this solution fairs in terms of performance compared to other solutions that have been posted here  But it certainly has helped me solve this problem  Hope this helps

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function isAssoc  arr         a   array keys  arr       for  i   0   t   count  a    i  lt   t   i                  if  a  i      i                        return false                      return true

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As stated by the OP      PHP treats all arrays as associative   it is not quite sensible  IMHO  to write a function that checks if an array is associative  So first thing first  what is a key in a PHP array       The key can either be an integer or a string    That means there are 3 possible cases    Case 1  all keys are numeric   integers  Case 2  all keys are strings  Case 3  some keys are strings  some keys are numeric   integers    We can check each case with the following functions   Case 1  all keys are numeric   integers   Note  This function returns true for empty arrays too       Check whether the input is an array whose keys are all integers           param in   InputArray           array  Input array       return                          bool   b true iff the input is an array whose keys are all integers     function IsArrayAllKeyInt  InputArray        if  is array  InputArray                 return false             if count  InputArray   lt   0                return true             return array unique array map  is int   array keys  InputArray        array true       Case 2  all keys are strings   Note  This function returns true for empty arrays too       Check whether the input is an array whose keys are all strings           param in   InputArray           array  Input array       return                          bool   b true iff the input is an array whose keys are all strings     function IsArrayAllKeyString  InputArray        if  is array  InputArray                 return false             if count  InputArray   lt   0                return true             return array unique array map  is string   array keys  InputArray        array true       Case 3  some keys are strings  some keys are numeric   integers   Note  This function returns true for empty arrays too       Check whether the input is an array with at least one key being an integer and at least one key being a string           param in   InputArray           array  Input array       return                          bool   b true iff the input is an array with at least one key being an integer and at least one key being a string     function IsArraySomeKeyIntAndSomeKeyString  InputArray        if  is array  InputArray                 return false             if count  InputArray   lt   0                return true             return count array unique array map  is string   array keys  InputArray      gt   2      It follows that    If the value is not an array  all 3 functions return false  If the value is an empty array  all 3 functions return true  which is by definition  as in  the empty set is a subset of any set A because all its elements belong to A    If the value is a non-empty array  exactly 1 function returns true      Now  for an array to be a  genuine  array that we are all accustomed to  meaning    Its keys are all numeric   integers  Its keys are sequential  i e  increasing by step 1   Its keys start from zero    We can check with the following function   Case 3a  keys are numeric   integers  sequential  and zero-based   Note  This function returns true for empty arrays too       Check whether the input is an array whose keys are numeric  sequential  and zero-based           param in   InputArray           array  Input array       return                          bool   b true iff the input is an array whose keys are numeric  sequential  and zero-based     function IsArrayKeyNumericSequentialZeroBased  InputArray        if  is array  InputArray                 return false             if count  InputArray   lt   0                return true             return array keys  InputArray      range 0  count  InputArray  - 1       Caveats   Pitfalls  or  even more peculiar facts about array keys in PHP   Integer keys  The keys for these arrays are integers   array 0   gt   b    array 13   gt   b    array -13   gt   b                Negative integers are also integers  array 0x1A   gt   b               Hexadecimal notation    String keys  The keys for these arrays are strings   array  fish and chips    gt   b    array      gt   b                                         An empty string is also a string  array  stackoverflow email example com    gt   b          Strings may contain non-alphanumeric characters  array  stack t  over   r nflow s cool    gt   b           Strings may contain special characters  array   ta   k   v rfl  w     gt   b                          Strings may contain all kinds of symbols  array  funct on    gt   b                                 You think this looks fine  Think again   see https   stackoverflow com q 9246051 1402846  array       L    gt   b                               How about Japanese Korean Chinese Russian Polish  array  fi x0sh    gt   b                                  Strings may contain null characters  array file get contents  https   www google com images nav logo114 png     gt   b         Strings may even be binary    Integer keys that look like strings  If you think the key in array  13    gt   b   is a string  you are wrong  From the doc here      Strings containing valid integers will be cast to the integer type  E g  the key  8  will actually be stored under 8  On the other hand  08  will not be cast  as it isn t a valid decimal integer    For example  the key for these arrays are integers   array  13    gt   b    array  -13    gt   b                              Negative  ok    But the key for these arrays are strings   array  13     gt   b    array   13    gt   b                              Positive  not ok  array  -013    gt   b    array  0x1A    gt   b                             Not converted to integers even though it s a valid hexadecimal number  array  013    gt   b                              Not converted to integers even though it s a valid octal number  array  18446744073709551616    gt   b             Not converted to integers as it can t fit into a 64-bit integer    What s more  according to the doc      The size of an integer is platform-dependent  although a maximum value of about two billion is the usual value  that s 32 bits signed   64-bit platforms usually have a maximum value of about 9E18  except for Windows  which is always 32 bit  PHP does not support unsigned integers    So the key for this array may or may not be an integer - it depends on your platform   array  60000000000    gt   b                      Array key could be integer or string  it can fit into a 64-bit  but not 32-bit  integer    Even worse  PHP tends to be buggy if the integer is near the 231   2 147 483 648 boundary  see bug 51430  bug 52899   For example  on my local environment  PHP 5 3 8 on XAMPP 1 7 7 on Windows 7   var dump array  2147483647    gt   b    gives  array 1         2147483647   gt      string 1   b         but on this live demo on codepad  PHP 5 2 5   the same expression gives  array 1          2147483647    gt      string 1   b      So the key is an integer in one environment but a string in another  even though 2147483647 is a valid signed 32-bit integer

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Actually the most efficient way is thus   function is assoc  array       keys   array keys  array      return  keys     array keys  keys       This works because it compares the keys  which for a sequential array are always 0 1 2 etc  to the keys of the keys  which will always be 0 1 2 etc

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Checking if array has all assoc-keys  With using stdClass  amp  get object vars       assocArray   array  fruit1    gt   apple                         fruit2    gt   orange                         veg1    gt   tomato                         veg2    gt   carrot      assoc object    object   assocArray   isAssoc    count  assocArray      count  get object vars  assoc object       var dump  isAssoc      true   Why  Function get object vars returns only accessible properties  see more about what is occuring during converting array to object here   Then  just logically  if count of basic array s elements equals count of object s accessible properties - all keys are assoc   Few tests     assocArray   array  apple    orange    tomato    carrot     assoc object    object   assocArray    isAssoc    count  assocArray      count  get object vars  assoc object     var dump  isAssoc      false          assocArray   array  0   gt   apple    orange    tomato    4    gt   carrot     assoc object    object   assocArray    isAssoc    count  assocArray      count  get object vars  assoc object     var dump  isAssoc      false           assocArray   array  fruit1    gt   apple                        NULL   gt   orange                         veg1    gt   tomato                         veg2    gt   carrot      assoc object    object   assocArray   isAssoc    count  assocArray      count  get object vars  assoc object       var dump  isAssoc     false   Etc

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I compare the difference between the keys of the array and the keys of the result of array values   of the array  which will always be an array with integer indices  If the keys are the same  it s not an associative array   function isHash  array        if   is array  array   return false       diff   array diff assoc  array  array values  array        return  empty  diff     false   true

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This function can handle    array with holes in index  e g  1 2 4 5 8 10   array with  0x  keys  e g  key  08  is associative while key  8  is sequential    the idea is simple  if one of the keys is NOT an integer  it is associative array  otherwise it s sequential   function is asso  a       foreach array keys  a  as  key   if   is int  key   return TRUE       return FALSE

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I met this problem once again some days ago and i thought to take advantage of the array merge special property       If the input arrays have the same string keys  then the later value for that key will overwrite the previous one  If  however  the arrays contain numeric keys  the later value will not overwrite the original value  but will be appended  Values in the input array with numeric keys will be renumbered with incrementing keys starting from zero in the result array      So why not to use     function Is Indexed Arr  arr        arr copy    arr      if  2 count  arr      count array merge  arr   arr copy             return 1            return 0

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In simple way you can check is array is associative or not by below steps   convert all keys of array into one array by using array keys   filter out non numeric key from array using array filter   and is numeric   compare number of elements in filtered array and actual array  If number of elements are not equals in both array then it is associative array    Function for above step is as below    function isAssociative array  array                return count array filter array keys  array   function  v  return is numeric  v          count  array

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One way to approach this is to piggyback on json encode  which already has its own internal method of differentiating between an associative array and an indexed array in order to output the correct JSON   You can do this by checking to see if the first character returned after encoding is a    associative array  or a    indexed array       Too short    function is assoc  arr        ksort  arr       return json encode  arr  0

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I just use the key   function  Observe    lt  php var dump key array  hello   gt  world    hello   gt  world        string 5   hello  var dump key array  world    world                         int 0  var dump key array  0    gt   a    1    gt   b    2    gt   c        int 0  who makes string sequetial keys anyway       gt    Thus  just by checking for false  you can determine whether an array is associative or not

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If your looking for just non-numeric keys  no matter the order  then you may want to try  function IsAssociative  array        return preg match    a-z  i   implode array keys  array

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I m surprised no one has mentioned array key first   For your test cases   sequentialArray   array  apple    orange    tomato    carrot     isIndexedArray   is int  array key first  sequentialArray        true  whereas  assocArray   array  fruit1    gt   apple                     fruit2    gt   orange                     veg1    gt   tomato                     veg2    gt   carrot      isIndexedArray   is int  array key first  assocArray        false  Read more about this function here

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After some local benchmarking  debugging  compiler probing  profiling  and abusing 3v4l org to benchmark across more versions  yes  I got a warning to stop  and comparing against every variation I could find     I give you an organically derived best-average-worst-case scenario associative array test function that is at worst roughly as good as or better than all other average-case scenarios          Tests if an array is an associative array         param array  array An array to test      return boolean True if the array is associative  otherwise false      function is assoc array  amp  arr           don t try to check non-arrays or empty arrays     if  FALSE     is array  arr     0       l   count  arr              return false                shortcut by guessing at the beginning     reset  arr       if  key  arr      0            return true                shortcut by guessing at the end     end  arr       if  key  arr       l-1            return true                rely on php to optimize test by reference or fast compare     return array values  arr       arr      From https   3v4l org rkieX    lt  php     array values function method 1 Array  amp  arr        return  arr     array values  arr         method 2 was DQ  did not actually work     array keys function method 3 Array  amp  arr        return array keys  arr      range 0  count  arr  - 1         foreach function method 4 Array  amp  arr         idx   0      foreach   arr as  key   gt   val            if   key      idx               return FALSE             idx            return TRUE        guessing function method 5 Array  amp  arr        global  METHOD 5 KEY       i   0       l   count  arr -1       end  arr       if   key  arr       l           return FALSE       reset  arr       do           if    i     key  arr                return FALSE             i          next  arr         while   i  lt   l       return TRUE        naieve function method 6 Array  amp  arr         i   0       l   count  arr       do           if   NULL       arr  i                return FALSE             i        while   i  lt   l       return TRUE        deep reference reliance function method 7 Array  amp  arr        return array keys array values  arr       array keys  arr          organic  guessing   array values  function method 8 Array  amp  arr        reset  arr       if   key  arr      0           return FALSE       end  arr       if   key  arr      count  arr -1           return FALSE       return array values  arr       arr     function benchmark Array  amp  methods  Array  amp  target   expected           foreach  methods as  method            start   microtime true           for   i   0   i  lt  2000     i                   dummy   call user func  method   target               if    method  target       expected                     echo  Method  method is disqualified for returning an incorrect result  n                   unset  methods array search  method  methods true                      i   0                  break                                  if    i    0                  end   microtime true               echo  Time taken with  method     round   end- start  1000 0 3   ms n                         true targets          Giant array    gt  range 0  500        Tiny array    gt  range 0  20         g   range 0 10   unset  g 0      false targets          Large array 1    gt  range 0  100      a   gt  a     range 101  200        Large array 2    gt    a   gt  a     range 0  200        Tiny array    gt  range 0  10      a   gt  a     range 11  20        Gotcha array    gt   g       methods          method 1        method 3        method 4        method 5        method 6        method 7        method 8       foreach  false targets as  targetName   gt   target       echo       Benchmark using  targetName expecing FALSE      n       benchmark  methods   target  false       echo   n     foreach  true targets as  targetName   gt   target       echo       Benchmark using  targetName expecting TRUE      n       benchmark  methods   target  true       echo   n

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One cheap and dirty way would be to check like this   isset  myArray count  myArray  - 1        you might get a false positive if your array is like this    myArray   array  1    gt   apple    b    gt   banana      A more thorough way might be to check the keys   function arrayIsAssociative  myArray        foreach  array keys  myArray  as  ind   gt   key            if   is numeric  key      isset  myArray  ind   1    amp  amp   myArray  ind   1      key   1                 return true                      return false       this will only return true if all the keys are numeric AND sequential  which    is what you get when you define an array like this     array  a    b    c    d    e      or  function arrayIsAssociative  myArray         l   count  myArray       for   i   0   i  lt   l     i            if   isset  myArray  i    return true            return false       this will return a false positive on an array like this   x   array 1   gt   b   0   gt   a   2   gt   c   4   gt   e   3   gt   d

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A lot of the solutions here are elegant and pretty  but don t scale well  and are memory intensive or CPU intensive  Most are creating 2 new data points in memory with this solution from both sides of the comparison  The larger the array the harder and longer the process and memory used  and you lose the benefit of short circuit evaluation  I Did some testing with a few different ideas  Trying to avoid array key exists as it is costly  and also avoiding creating new large datasets to compare  I feel this is a simple way to tell if an array is sequential   public function is sequential   arr             if   is array   arr      empty   arr     return false        i   0        total   count   arr         foreach   arr as  key   gt   value   if   key      i     return false       return true      You run a single count on the main array and store a single integer  You then loop through the array and check for an exact match while iterating the counter  You should have from 1 to count  If it fails it will short circuit out which gives you performance boost when it is false    Originally I did this with a for loop and checking for isset   arr  i    but this will not detect null keys which requires array key exists  and as we know that is the worst function to use for speed   Constantly updating variables via foreach to check along with the iterator never growing past it s integer size let s PHP use it s built in memory optimization  caching and garbage collection to keep you at very low resource usage    Also  I will argue that using array keys in a foreach is silly when you can simply run  key     value and check the key  Why create the new data point  Once you abstract away the array keys you ve consumed more memory immediately

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lt  php  function is list  array        return array keys  array      range 0  count  array  - 1      function is assoc  array        return count array filter array keys  array    is string       count  array        gt    Both of these examples  which scored the most points do not work correctly with arrays like  array   array  foo    gt   bar   1

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You have asked two questions that are not quite equivalent    Firstly  how to determine whether an array has only numeric keys Secondly  how to determine whether an array has sequential numeric keys  starting from 0   Consider which of these behaviours you actually need   It may be that either will do for your purposes    The first question  simply checking that all keys are numeric  is answered well by Captain kurO   For the second question  checking whether the array is zero-indexed and sequential   you can use the following function   function isAssoc array  arr        if  array        arr  return false      return array keys  arr      range 0  count  arr  - 1      var dump isAssoc   a    b    c         false var dump isAssoc   0    gt   a    1    gt   b    2    gt   c         false var dump isAssoc   1    gt   a    0    gt   b    2    gt   c         true var dump isAssoc   a    gt   a    b    gt   b    c    gt   c         true

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One cheap and dirty way would be to check like this   isset  myArray count  myArray  - 1        you might get a false positive if your array is like this    myArray   array  1    gt   apple    b    gt   banana      A more thorough way might be to check the keys   function arrayIsAssociative  myArray        foreach  array keys  myArray  as  ind   gt   key            if   is numeric  key      isset  myArray  ind   1    amp  amp   myArray  ind   1      key   1                 return true                      return false       this will only return true if all the keys are numeric AND sequential  which    is what you get when you define an array like this     array  a    b    c    d    e      or  function arrayIsAssociative  myArray         l   count  myArray       for   i   0   i  lt   l     i            if   isset  myArray  i    return true            return false       this will return a false positive on an array like this   x   array 1   gt   b   0   gt   a   2   gt   c   4   gt   e   3   gt   d

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This would work too  demo    function array has numeric keys only array  array        try           SplFixedArray  fromArray  array  true         catch  InvalidArgumentException  e            return false            return true      Please note that the main point of this answer is to inform you about the existence of SplFixedArray and not to encourage you to use Exceptions for these kinds of tests

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function is associative  arr      return  array merge  arr       arr    count array filter  arr   is string   ARRAY FILTER USE KEY    gt  0

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I met this problem once again some days ago and i thought to take advantage of the array merge special property       If the input arrays have the same string keys  then the later value for that key will overwrite the previous one  If  however  the arrays contain numeric keys  the later value will not overwrite the original value  but will be appended  Values in the input array with numeric keys will be renumbered with incrementing keys starting from zero in the result array      So why not to use     function Is Indexed Arr  arr        arr copy    arr      if  2 count  arr      count array merge  arr   arr copy             return 1            return 0

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To merely check whether the array has non-integer keys  not whether the array is sequentially-indexed or zero-indexed    function has string keys array  array      return count array filter array keys  array    is string     gt  0      If there is at least one string key   array will be regarded as an associative array

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I think the definition of a scalar array will vary by application   That is  some applications will require a more strict sense of what qualifies as a scalar array  and some applications will require a more loose sense   Below I present 3 methods of varying strictness    lt  php        Since PHP stores all arrays as associative internally  there is no proper    definition of a scalar array         As such  developers are likely to have varying definitions of scalar array     based on their application needs         In this file  I present 3 increasingly strict methods of determining if an    array is scalar          author David Farrell  lt DavidPFarrell gmail com gt              isArrayWithOnlyIntKeys defines a scalar array as containing    only integer keys         If you are explicitly setting integer keys on an array  you    may need this function to determine scalar-ness          param array  a     return boolean      function isArrayWithOnlyIntKeys array  a        if   is array  a           return false      foreach   a as  k   gt   v          if   is int  k               return false      return true            isArrayWithOnlyAscendingIntKeys defines a scalar array as    containing only integer keys in ascending  but not necessarily    sequential  order         If you are performing pushes  pops  and unsets on your array     you may need this function to determine scalar-ness          param array  a     return boolean      function isArrayWithOnlyAscendingIntKeys array  a        if   is array  a           return false       prev   null      foreach   a as  k   gt   v                if   is int  k      null      prev  amp  amp   k  lt    prev               return false           prev    k            return true            isArrayWithOnlyZeroBasedSequentialIntKeys defines a scalar array    as containing only integer keys in sequential  ascending order     starting from 0         If you are only performing operations on your array that are    guaranteed to either maintain consistent key values  or that    re-base the keys for consistency  then you can use this function          param array  a     return boolean     function isArrayWithOnlyZeroBasedSequentialIntKeys array  a        if   is array  a           return false       i   0      foreach   a as  k   gt   v          if   i        k              return false      return true

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function checkAssoc  array       return  ctype digit  implode     array keys  array

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In simple way you can check is array is associative or not by below steps   convert all keys of array into one array by using array keys   filter out non numeric key from array using array filter   and is numeric   compare number of elements in filtered array and actual array  If number of elements are not equals in both array then it is associative array    Function for above step is as below    function isAssociative array  array                return count array filter array keys  array   function  v  return is numeric  v          count  array

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Many commenters in this question don t understand how arrays work in PHP  From the array documentation      A key may be either an integer or a string  If a key is the standard representation of an integer  it will be interpreted as such  i e   8  will be interpreted as 8  while  08  will be interpreted as  08    Floats in key are truncated to integer  The indexed and associative array types are the same type in PHP  which can both contain integer and string indices    In other words  there is no such thing as an array key of  8  because it will always be  silently  converted to the integer 8  So trying to differentiate between integers and numeric strings is unnecessary   If you want the most efficient way to check an array for non-integer keys without making a copy of part of the array  like array keys   does  or all of it  like foreach does    function keyedNext   amp  arr   amp  k        k   key  arr       return next  arr      for   k   key reset  my array    is int  k   keyedNext  my array  k        onlyIntKeys   is null  k     This works because key   returns NULL when the current array position is invalid and NULL can never be a valid key  if you try to use NULL as an array key it gets silently converted to

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One way to approach this is to piggyback on json encode  which already has its own internal method of differentiating between an associative array and an indexed array in order to output the correct JSON   You can do this by checking to see if the first character returned after encoding is a    associative array  or a    indexed array       Too short    function is assoc  arr        ksort  arr       return json encode  arr  0

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After some local benchmarking  debugging  compiler probing  profiling  and abusing 3v4l org to benchmark across more versions  yes  I got a warning to stop  and comparing against every variation I could find     I give you an organically derived best-average-worst-case scenario associative array test function that is at worst roughly as good as or better than all other average-case scenarios          Tests if an array is an associative array         param array  array An array to test      return boolean True if the array is associative  otherwise false      function is assoc array  amp  arr           don t try to check non-arrays or empty arrays     if  FALSE     is array  arr     0       l   count  arr              return false                shortcut by guessing at the beginning     reset  arr       if  key  arr      0            return true                shortcut by guessing at the end     end  arr       if  key  arr       l-1            return true                rely on php to optimize test by reference or fast compare     return array values  arr       arr      From https   3v4l org rkieX    lt  php     array values function method 1 Array  amp  arr        return  arr     array values  arr         method 2 was DQ  did not actually work     array keys function method 3 Array  amp  arr        return array keys  arr      range 0  count  arr  - 1         foreach function method 4 Array  amp  arr         idx   0      foreach   arr as  key   gt   val            if   key      idx               return FALSE             idx            return TRUE        guessing function method 5 Array  amp  arr        global  METHOD 5 KEY       i   0       l   count  arr -1       end  arr       if   key  arr       l           return FALSE       reset  arr       do           if    i     key  arr                return FALSE             i          next  arr         while   i  lt   l       return TRUE        naieve function method 6 Array  amp  arr         i   0       l   count  arr       do           if   NULL       arr  i                return FALSE             i        while   i  lt   l       return TRUE        deep reference reliance function method 7 Array  amp  arr        return array keys array values  arr       array keys  arr          organic  guessing   array values  function method 8 Array  amp  arr        reset  arr       if   key  arr      0           return FALSE       end  arr       if   key  arr      count  arr -1           return FALSE       return array values  arr       arr     function benchmark Array  amp  methods  Array  amp  target   expected           foreach  methods as  method            start   microtime true           for   i   0   i  lt  2000     i                   dummy   call user func  method   target               if    method  target       expected                     echo  Method  method is disqualified for returning an incorrect result  n                   unset  methods array search  method  methods true                      i   0                  break                                  if    i    0                  end   microtime true               echo  Time taken with  method     round   end- start  1000 0 3   ms n                         true targets          Giant array    gt  range 0  500        Tiny array    gt  range 0  20         g   range 0 10   unset  g 0      false targets          Large array 1    gt  range 0  100      a   gt  a     range 101  200        Large array 2    gt    a   gt  a     range 0  200        Tiny array    gt  range 0  10      a   gt  a     range 11  20        Gotcha array    gt   g       methods          method 1        method 3        method 4        method 5        method 6        method 7        method 8       foreach  false targets as  targetName   gt   target       echo       Benchmark using  targetName expecing FALSE      n       benchmark  methods   target  false       echo   n     foreach  true targets as  targetName   gt   target       echo       Benchmark using  targetName expecting TRUE      n       benchmark  methods   target  true       echo   n

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Or you can just use this   Arr  isAssoc  array    which will check if array contains any non-numeric key or   Arr isAssoc  array  true    to check if array is strictly sequencial   contains auto generated int keys 0 to n-1   using this library

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If your looking for just non-numeric keys  no matter the order  then you may want to try  function IsAssociative  array        return preg match    a-z  i   implode array keys  array

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Most answers have sub-optimal time space complexity or are changing semantics  So  here is another answer with the fastest and most functionally correct solution  function is sequential array Array  amp  a         n   count  a       for  i 0   i lt  n   i              if  array key exists  i   a                 return false                      return true     This answer has following advantages over other answers   Space complexity of O 1   many answers here use O n  space   Does not apply equality on keys  which is an unwanted and expensive operation  Treats input array as immutable  many answers have implicitly created a copy by applying mutating functions  Uses function array key exists instead of isset  remember  isset additionally checks for  is not null   thereby changing semantics  Worst-case time complexity is O n   many answers here have best-case time complexity of O n

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My solution   function isAssociative array  array        return array keys array merge  array       range 0  count  array  - 1       array merge on a single array will reindex all integer keys  but not other  For example   array merge  1   gt   One   3   gt   Three    two    gt   Two   6   gt   Six         This will returns  0   gt   One   1   gt   Three    two    gt   Two   2   gt   Six     So if a list  a non-associative array  is created   a    b    c   then a value is removed unset  a 1   then array merge is called  the list is reindexed starting from 0

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iszba - Is Zero Based Array  Detects if an array is zero based or not   PARAMS       chkvfnc         Callback in the loop allows to check the values of each element          Signature              bool function chkvfnc  v               return                  true    continue looping                 false   stop looping  iszba returns false too   NOTES    assert   array is an array    May be memory efficient    it doesn t get extra arrays via array keys   or ranges   into the function    Is pretty fast without a callback    With callback it s  2 4 times slower     function iszba  array   chkvfnc null         ncb     chkvfnc       i   0       foreach  array as  k   gt   v           if  k      i                if  ncb     chkvfnc  v                   continue           return false             return true          Without callback it is  30  faster than current leading answer  and possibly more memory efficient       Just negate the answer to know if the array should be considered associative

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Simple and performance friendly solution which only checks the first key   function isAssoc  arr   NULL        if   arr  amp  amp  is array  arr                 foreach   arr as  key   gt   val                        if  is numeric  key     return true                 break                       return false

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I ve come up with next method  function isSequential array  list   bool        i   0       count   count  list       while  array key exists  i   list              i    1          if   i      count                return true                       return false      var dump isSequential array         false var dump isSequential array  a    b    c         true var dump isSequential array  quot 0 quot    gt   a    quot 1 quot    gt   b    quot 2 quot    gt   c         true var dump isSequential array  quot 1 quot    gt   a    quot 0 quot    gt   b    quot 2 quot    gt   c         true var dump isSequential array  quot 1a quot    gt   a    quot 0b quot    gt   b    quot 2c quot    gt   c         false var dump isSequential array  quot a quot    gt   a    quot b quot    gt   b    quot c quot    gt   c         false   Note empty array is not considered a sequential array  but I think it s fine since empty arrays is like 0 - doesn t matter it s plus or minus  it s empty  Here are the advantages of this method compared to some listed above   It does not involve copying of arrays  someone mentioned in this gist https   gist github com Thinkscape 1965669 that array values does not involve copying - what    It certainly does - as will be seen below  It s faster for bigger arrays and more memory friendly at the same time  I ve used benchmark kindly provided by Artur Bodera  where I changed one of the arrays to 1M elements  array fill 0  1000000  uniqid        big numeric array   Here are the results for 100 iterations  PHP 7 1 16  cli   built  Mar 31 2018 02 59 59    NTS    Initial memory  32 42 MB Testing my method  isset check  - 100 iterations   Total time  2 57942 s   Total memory  32 48 MB  Testing method3  array filter of keys  - 100 iterations   Total time  5 10964 s   Total memory  64 42 MB  Testing method1  array values check  - 100 iterations   Total time  3 07591 s   Total memory  64 42 MB  Testing method2  array keys comparison  - 100 iterations   Total time  5 62937 s   Total memory  96 43 MB   Methods are ordered based on their memory consumption   I used echo  quot   Total memory   quot    number format memory get peak usage   1024 1024  2     quot  MB n quot   to display memory usage

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Another variant not shown yet  as it s simply not accepting numerical keys  but I like Greg s one very much           Returns true if  var associative array        function is associative array   array           return is array  array   amp  amp   is numeric implode     array keys  array

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I noticed two popular approaches for this question  one using array values   and other using key    To find out which is faster  I wrote a small program     arrays   Array     Array  1    gt  Array 1  2  3  54  23  212  123  1  1      Array  2    gt  Array  Stack   1 5  20  Array 3 4       Array  3    gt  Array 1   gt  4  2   gt  2      Array  4    gt  Array 3 0   2   3000   Stack   5   gt   4       Array  5    gt  Array  3    gt  4   2    gt  2      Array  6    gt  Array  0    gt   One   1 0   gt   Two   2   gt   Three       Array  7    gt  Array 3   gt   asdf   4   gt   asdf       Array  8    gt  Array  apple    gt  1   orange    gt  2       function is indexed array 1 Array  amp  arr      return  arr     array values  arr      function is indexed array 2 Array  amp  arr      for  reset  arr    i   0  key  arr       i    next  arr           return is null key  arr          Method  1  start   microtime true   for   i   0   i  lt  1000   i        foreach   arrays as  array         dummy   is indexed array 1  array          end   microtime true   echo  Time taken with method  1     round   end- start  1000 0 3   ms n       Method  2  start   microtime true   for   i   0   i  lt  1000   i        foreach   arrays as  array         dummy   is indexed array 2  array          end   microtime true   echo  Time taken with method  1     round   end- start  1000 0 3   ms n     Output for the program on PHP 5 2 on CentOS is as follows      Time taken with method  1   10 745ms   Time taken with method  2   18 239ms   Output on PHP 5 3 yielded similar results  Obviously using array values   is much faster

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Improvement from Mark Amery  function isAssoc  arr           Is it set  is an array  not empty and keys are not sequentialy numeric from 0     return isset  arr   amp  amp  is array  arr   amp  amp  count  arr   0  amp  amp  array keys  arr      range 0  count  arr  - 1       This tests if variable exists  if it is an array  if it is not an empty array and if the keys are not sequential from 0   To see if the array is associative  if  isAssoc  array         To see if it numeric  if   isAssoc  array

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I think the following two functions are the best way to go for checking  if an array is associative or numeric   Since  numeric  could mean only numeric keys or only sequential numeric keys  two functions are listed below that check either condition   function is indexed array  amp  arr      for  reset  arr   is int key  arr    next  arr      return is null key  arr       function is sequential array  amp  arr   base   0      for  reset  arr    base    int   base  key  arr       base    next  arr      return is null key  arr        The first function checks if each key is an integer value  The second function checks if each key is an integer value and in addition checks if all keys are sequential starting at  base  which defaults to 0 and thus can be omitted if you do not need to specify another base value  key  my array  returns null if the read pointer is moved past the end of the array  which is what ends the for loop and makes the statement after the for loop return true if all keys were integer  If not  the loop ends prematurely because a key is of type string  and the statement after the for loop will return false  The latter function in addition adds one to  base after each compare  to be able to check if the next key is of the correct value  The strict compare makes it also check if the key is of type integer  The  base    int   base part in the first section of the for loop can be left out when  base is omitted or if you make sure it is only called using an integer  But since I can t be sure for everybody  I left it in  The statement is executed only once  anyway  I think these are the most efficient solutions    Memory wise  No copying of data or key ranges  Doing an array values or array keys may seem shorter  less code  but keep in mind what goes on in the background once you make that call  Yes there are more  visible  statements than in some other solutions  but that is not what counts  is it  Time wise  Besides the fact that copying extracting data and or keys also takes time  this solution is more efficient than doing a foreach  Again a foreach may seem more efficient to some because it is shorter in notation  but in the background foreach also calls reset  key and next to do it s looping  But in addition it also calls valid to check the end condition  which is avoided here due to the combination with the integer check    Remember that an array key can only be an integer or a string  and a strictly numeric string such as  1   but not  01   will be translated into an integer  Which is what makes checking for an integer key the only needed operation besides counting if you want the array to be sequential  Naturally  if is indexed array returns false the array can be seen as associative  I say  seen   because in fact they all are

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Most answers have sub-optimal time space complexity or are changing semantics  So  here is another answer with the fastest and most functionally correct solution  function is sequential array Array  amp  a         n   count  a       for  i 0   i lt  n   i              if  array key exists  i   a                 return false                      return true     This answer has following advantages over other answers   Space complexity of O 1   many answers here use O n  space   Does not apply equality on keys  which is an unwanted and expensive operation  Treats input array as immutable  many answers have implicitly created a copy by applying mutating functions  Uses function array key exists instead of isset  remember  isset additionally checks for  is not null   thereby changing semantics  Worst-case time complexity is O n   many answers here have best-case time complexity of O n

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Surely this is a better alternative    lt  php  arr   array 1 2 3 4    isIndexed   array values  arr       arr

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Surely this is a better alternative    lt  php  arr   array 1 2 3 4    isIndexed   array values  arr       arr

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There are many answers already  but here is the method that Laravel relies on within its Arr class           Determines if an array is associative        An array is  associative  if it doesn t have sequential numerical keys beginning with zero         param  array   array     return bool     public static function isAssoc array  array         keys   array keys  array        return array keys  keys       keys      Source  https   github com laravel framework blob 5 4 src Illuminate Support Arr php

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One cheap and dirty way would be to check like this   isset  myArray count  myArray  - 1        you might get a false positive if your array is like this    myArray   array  1    gt   apple    b    gt   banana      A more thorough way might be to check the keys   function arrayIsAssociative  myArray        foreach  array keys  myArray  as  ind   gt   key            if   is numeric  key      isset  myArray  ind   1    amp  amp   myArray  ind   1      key   1                 return true                      return false       this will only return true if all the keys are numeric AND sequential  which    is what you get when you define an array like this     array  a    b    c    d    e      or  function arrayIsAssociative  myArray         l   count  myArray       for   i   0   i  lt   l     i            if   isset  myArray  i    return true            return false       this will return a false positive on an array like this   x   array 1   gt   b   0   gt   a   2   gt   c   4   gt   e   3   gt   d

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Yet another way to do this   function array isassociative  array           Create new Array   Make it the same size as the input array      compareArray   array pad array    count  array   0           Compare the two array keys     return  count array diff key  array   compareArray      true   false

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My solution is to get keys of an array like below and check that if the key is not integer   private function is hash  array        foreach  array as  key   gt   value            return   is int  key             return false      It is wrong to get array keys of a hash array like below   array keys array          abc    gt   gfb           bdc    gt   dbc                will output   array         0   gt   abc          1   gt   bdc      So  it is not a good idea to compare it with a range of numbers as mentioned in top rated answer  It will always say that it is a hash array if you try to compare keys with a range

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This question is actually useless when it comes to array of php because with the nature of php an array should not have to be fully associative or indexing  it can be  combination of both  the way user have define and assigned the value an array can be a combination of both  see the example below   y  array 5    y  0x    n    y  vbg    12132    y 1     k    var dump  y     this will output 4 element array  echo   lt  br gt     y  0x     lt  br gt    y 0    for  x 0  x lt sizeof  y   x        this will output all index elements  amp  gives error after that     echo   lt  br gt  index elements    y  x       so the correct question that has to ask is   is all the element in array are associative or index  if you really know that it will only be either associative or indexing not a combination of these two  you can just simply use this method to find wether it is an index or associative array   function AssocTest  amp  arr       if is array  arr             reset  arr      reset pointer to first element of array          if gettype key  arr       string      get the type nature  of first element key              return true           else              return false                 else          return false            you can use it as normal function  echo AssocTest  y     Associative array    Not an Associative array  Not an array at all      and important thing to remember evan you have initialize an array as associative but the names that you have gave the associative array is just numbers it will treat as an index array when it being read by php if you have not explicitly gave the string names  take a look at the example below    y  0    n    y  1    12132    y  22      k      both will get the same output echo   lt br  gt  s0    y  22    echo   lt br  gt  s0    y 22    for  x 0  x lt count  y   x        echo   lt br  gt  arr    y  x      this will output up to 2nd element and give an error after      so if you need to be sure all the elements of the array to be exactly indexed or either associative   there is no other way but go true all the elements and check each and every element key by generated index array as post by many people here   function fullAssocTest  amp  arr        if is array  arr            return  array keys  arr      range 0  count  arr  - 1              its less coding  but this thing is really process intensive and really un-necessary work

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lt  php  function is list  array        return array keys  array      range 0  count  array  - 1      function is assoc  array        return count array filter array keys  array    is string       count  array        gt    Both of these examples  which scored the most points do not work correctly with arrays like  array   array  foo    gt   bar   1

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This is my function -   public function is assoc array  array        if is array  array      true           return false       else            check   json decode json encode  array             if is object  check      true               return true           else              return false                        Some examples      print r  is assoc array   one   two   three       true  Yes   No      No     print r is assoc array   one   gt  one   two   gt  two   three   gt  three     Yes   No      Yes     print r is assoc array   1   gt  one   2   gt  two   3   gt  three     Yes   No      Yes     print r is assoc array   0   gt  one   1   gt  two   2   gt  three     Yes   No      No   There was a similar solution by  devios1 in one of the answers but this was just another way using the inbuilt json related functions of PHP  I haven t checked how this solution fairs in terms of performance compared to other solutions that have been posted here  But it certainly has helped me solve this problem  Hope this helps

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You have asked two questions that are not quite equivalent    Firstly  how to determine whether an array has only numeric keys Secondly  how to determine whether an array has sequential numeric keys  starting from 0   Consider which of these behaviours you actually need   It may be that either will do for your purposes    The first question  simply checking that all keys are numeric  is answered well by Captain kurO   For the second question  checking whether the array is zero-indexed and sequential   you can use the following function   function isAssoc array  arr        if  array        arr  return false      return array keys  arr      range 0  count  arr  - 1      var dump isAssoc   a    b    c         false var dump isAssoc   0    gt   a    1    gt   b    2    gt   c         false var dump isAssoc   1    gt   a    0    gt   b    2    gt   c         true var dump isAssoc   a    gt   a    b    gt   b    c    gt   c         true

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Here s the method I use   function is associative    a         return in array false  array map  is numeric   array keys  a        assert  true     is associative array 1  2  3  4       assert  false     is associative array  foo    gt   bar    bar    gt   baz        assert  false     is associative array 1  2  3   foo    gt   bar         Note that this doesn t account for special cases like    a   array  1  2  3  4     unset  a 1     assert  true     is associative  a       Sorry  can t help you with that  It s also somewhat performant for decently sized arrays  as it doesn t make needless copies  It is these little things that makes Python and Ruby so much nicer to write in     P

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By using xarray PHP extension  You can do this very fast  about 30  times faster in PHP 5 6     if  array is indexed  array          Or   if  array is assoc  array

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Here s the method I use   function is associative    a         return in array false  array map  is numeric   array keys  a        assert  true     is associative array 1  2  3  4       assert  false     is associative array  foo    gt   bar    bar    gt   baz        assert  false     is associative array 1  2  3   foo    gt   bar         Note that this doesn t account for special cases like    a   array  1  2  3  4     unset  a 1     assert  true     is associative  a       Sorry  can t help you with that  It s also somewhat performant for decently sized arrays  as it doesn t make needless copies  It is these little things that makes Python and Ruby so much nicer to write in     P

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By using xarray PHP extension  You can do this very fast  about 30  times faster in PHP 5 6     if  array is indexed  array          Or   if  array is assoc  array

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Many commenters in this question don t understand how arrays work in PHP  From the array documentation      A key may be either an integer or a string  If a key is the standard representation of an integer  it will be interpreted as such  i e   8  will be interpreted as 8  while  08  will be interpreted as  08    Floats in key are truncated to integer  The indexed and associative array types are the same type in PHP  which can both contain integer and string indices    In other words  there is no such thing as an array key of  8  because it will always be  silently  converted to the integer 8  So trying to differentiate between integers and numeric strings is unnecessary   If you want the most efficient way to check an array for non-integer keys without making a copy of part of the array  like array keys   does  or all of it  like foreach does    function keyedNext   amp  arr   amp  k        k   key  arr       return next  arr      for   k   key reset  my array    is int  k   keyedNext  my array  k        onlyIntKeys   is null  k     This works because key   returns NULL when the current array position is invalid and NULL can never be a valid key  if you try to use NULL as an array key it gets silently converted to

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I know it s a bit pointless adding an answer to this huge queue  but here s a readable O n  solution that doesn t require duplicating any values   function isNumericArray  array         count   count  array       for   i   0   i  lt   count   i              if   isset  array  i                  return FALSE                      return TRUE      Rather than check the keys to see if they are all numeric  you iterate over the keys that would be there for a numeric array and make sure they exist

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I know it s a bit pointless adding an answer to this huge queue  but here s a readable O n  solution that doesn t require duplicating any values   function isNumericArray  array         count   count  array       for   i   0   i  lt   count   i              if   isset  array  i                  return FALSE                      return TRUE      Rather than check the keys to see if they are all numeric  you iterate over the keys that would be there for a numeric array and make sure they exist

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I compare the difference between the keys of the array and the keys of the result of array values   of the array  which will always be an array with integer indices  If the keys are the same  it s not an associative array   function isHash  array        if   is array  array   return false       diff   array diff assoc  array  array values  array        return  empty  diff     false   true

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Could this be the solution     public static function isArrayAssociative array  array          reset  array         return  is int key  array          The caveat is obviously that the array cursor is reset but I d say probably the function is used before the array is even traversed or used

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I noticed two popular approaches for this question  one using array values   and other using key    To find out which is faster  I wrote a small program     arrays   Array     Array  1    gt  Array 1  2  3  54  23  212  123  1  1      Array  2    gt  Array  Stack   1 5  20  Array 3 4       Array  3    gt  Array 1   gt  4  2   gt  2      Array  4    gt  Array 3 0   2   3000   Stack   5   gt   4       Array  5    gt  Array  3    gt  4   2    gt  2      Array  6    gt  Array  0    gt   One   1 0   gt   Two   2   gt   Three       Array  7    gt  Array 3   gt   asdf   4   gt   asdf       Array  8    gt  Array  apple    gt  1   orange    gt  2       function is indexed array 1 Array  amp  arr      return  arr     array values  arr      function is indexed array 2 Array  amp  arr      for  reset  arr    i   0  key  arr       i    next  arr           return is null key  arr          Method  1  start   microtime true   for   i   0   i  lt  1000   i        foreach   arrays as  array         dummy   is indexed array 1  array          end   microtime true   echo  Time taken with method  1     round   end- start  1000 0 3   ms n       Method  2  start   microtime true   for   i   0   i  lt  1000   i        foreach   arrays as  array         dummy   is indexed array 2  array          end   microtime true   echo  Time taken with method  1     round   end- start  1000 0 3   ms n     Output for the program on PHP 5 2 on CentOS is as follows      Time taken with method  1   10 745ms   Time taken with method  2   18 239ms   Output on PHP 5 3 yielded similar results  Obviously using array values   is much faster

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Checking if array has all assoc-keys  With using stdClass  amp  get object vars       assocArray   array  fruit1    gt   apple                         fruit2    gt   orange                         veg1    gt   tomato                         veg2    gt   carrot      assoc object    object   assocArray   isAssoc    count  assocArray      count  get object vars  assoc object       var dump  isAssoc      true   Why  Function get object vars returns only accessible properties  see more about what is occuring during converting array to object here   Then  just logically  if count of basic array s elements equals count of object s accessible properties - all keys are assoc   Few tests     assocArray   array  apple    orange    tomato    carrot     assoc object    object   assocArray    isAssoc    count  assocArray      count  get object vars  assoc object     var dump  isAssoc      false          assocArray   array  0   gt   apple    orange    tomato    4    gt   carrot     assoc object    object   assocArray    isAssoc    count  assocArray      count  get object vars  assoc object     var dump  isAssoc      false           assocArray   array  fruit1    gt   apple                        NULL   gt   orange                         veg1    gt   tomato                         veg2    gt   carrot      assoc object    object   assocArray   isAssoc    count  assocArray      count  get object vars  assoc object       var dump  isAssoc     false   Etc

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Yet another way to do this   function array isassociative  array           Create new Array   Make it the same size as the input array      compareArray   array pad array    count  array   0           Compare the two array keys     return  count array diff key  array   compareArray      true   false

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My solution   function isAssociative array  array        return array keys array merge  array       range 0  count  array  - 1       array merge on a single array will reindex all integer keys  but not other  For example   array merge  1   gt   One   3   gt   Three    two    gt   Two   6   gt   Six         This will returns  0   gt   One   1   gt   Three    two    gt   Two   2   gt   Six     So if a list  a non-associative array  is created   a    b    c   then a value is removed unset  a 1   then array merge is called  the list is reindexed starting from 0

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Best function to detect associative array  hash array    lt  php function is assoc  arr    return  array values  arr       arr       gt

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function isAssoc  arr         a   array keys  arr       for  i   0   t   count  a    i  lt   t   i                  if  a  i      i                        return false                      return true

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You have asked two questions that are not quite equivalent    Firstly  how to determine whether an array has only numeric keys Secondly  how to determine whether an array has sequential numeric keys  starting from 0   Consider which of these behaviours you actually need   It may be that either will do for your purposes    The first question  simply checking that all keys are numeric  is answered well by Captain kurO   For the second question  checking whether the array is zero-indexed and sequential   you can use the following function   function isAssoc array  arr        if  array        arr  return false      return array keys  arr      range 0  count  arr  - 1      var dump isAssoc   a    b    c         false var dump isAssoc   0    gt   a    1    gt   b    2    gt   c         false var dump isAssoc   1    gt   a    0    gt   b    2    gt   c         true var dump isAssoc   a    gt   a    b    gt   b    c    gt   c         true

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One cheap and dirty way would be to check like this   isset  myArray count  myArray  - 1        you might get a false positive if your array is like this    myArray   array  1    gt   apple    b    gt   banana      A more thorough way might be to check the keys   function arrayIsAssociative  myArray        foreach  array keys  myArray  as  ind   gt   key            if   is numeric  key      isset  myArray  ind   1    amp  amp   myArray  ind   1      key   1                 return true                      return false       this will only return true if all the keys are numeric AND sequential  which    is what you get when you define an array like this     array  a    b    c    d    e      or  function arrayIsAssociative  myArray         l   count  myArray       for   i   0   i  lt   l     i            if   isset  myArray  i    return true            return false       this will return a false positive on an array like this   x   array 1   gt   b   0   gt   a   2   gt   c   4   gt   e   3   gt   d

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Unless PHP has a builtin for that  you won t be able to do it in less than O n  - enumerating over all the keys and checking for integer type  In fact  you also want to make sure there are no holes  so your algorithm might look like   for i in 0 to len your array       if not defined your-array i              this is not an array array  it s an associative array      But why bother  Just assume the array is of the type you expect  If it isn t  it will just blow up in your face - that s dynamic programming for you  Test your code and all will be well

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Unless PHP has a builtin for that  you won t be able to do it in less than O n  - enumerating over all the keys and checking for integer type  In fact  you also want to make sure there are no holes  so your algorithm might look like   for i in 0 to len your array       if not defined your-array i              this is not an array array  it s an associative array      But why bother  Just assume the array is of the type you expect  If it isn t  it will just blow up in your face - that s dynamic programming for you  Test your code and all will be well

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Actually the most efficient way is thus   function is assoc  array       keys   array keys  array      return  keys     array keys  keys       This works because it compares the keys  which for a sequential array are always 0 1 2 etc  to the keys of the keys  which will always be 0 1 2 etc

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Modification on the most popular answer  This takes a little more processing  but is more accurate      lt  php    a is a subset of  b function isSubset  a   b        foreach  a   gt  v          if array search  v   b      false              return false       return true         less effecient  clearer implementation   uses     for comparison        return array intersect  a   b       a     function isAssoc  arr        return  isSubset array keys  arr   range 0  count  arr  - 1       var dump isAssoc array  a    b    c         false var dump isAssoc array 1   gt   a   0   gt   b   2   gt   c         false var dump isAssoc array  0    gt   a    1    gt   b    2    gt   c         false     use     in isSubset to get  true  for above statement  var dump isAssoc array  a    gt   a    b    gt   b    c    gt   c         true   gt

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I m surprised no one has mentioned array key first   For your test cases   sequentialArray   array  apple    orange    tomato    carrot     isIndexedArray   is int  array key first  sequentialArray        true  whereas  assocArray   array  fruit1    gt   apple                     fruit2    gt   orange                     veg1    gt   tomato                     veg2    gt   carrot      isIndexedArray   is int  array key first  assocArray        false  Read more about this function here

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function isAssoc  arr         a   array keys  arr       for  i   0   t   count  a    i  lt   t   i                  if  a  i      i                        return false                      return true

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I ve used both array keys  obj      range 0  count  obj  - 1  and array values  arr       arr  which are duals of each other  although the second is cheaper than the first  but both fail for very large arrays   This is because array keys and array values are both very costly operations  since they build a whole new array of size roughly that of the original    The following function is more robust than the methods provided above   function array type   obj         last key   -1       type    index       foreach   obj as  key   gt   val            if   is int   key       key  lt  0                return  assoc                     if   key      last key   1                 type    sparse                      last key    key            return  type      Also note that if you don t care to differentiate sparse arrays from associative arrays you can simply return  assoc  from both if blocks   Finally  while this might seem much less  elegant  than a lot of  solutions  on this page  in practice it is vastly more efficient  Almost any associative array will be detected instantly  Only indexed arrays will get checked exhaustively  and the methods outlined above not only check indexed arrays exhaustively  they duplicate them

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My solution is to get keys of an array like below and check that if the key is not integer   private function is hash  array        foreach  array as  key   gt   value            return   is int  key             return false      It is wrong to get array keys of a hash array like below   array keys array          abc    gt   gfb           bdc    gt   dbc                will output   array         0   gt   abc          1   gt   bdc      So  it is not a good idea to compare it with a range of numbers as mentioned in top rated answer  It will always say that it is a hash array if you try to compare keys with a range

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function is associative  arr      return  array merge  arr       arr    count array filter  arr   is string   ARRAY FILTER USE KEY    gt  0

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Actually  I found myself in a similar situation trying to take an array and parse it into XML   XML element names cannot begin with numbers -- and the code snippets I found did not correctly deal with arrays with numeric indexes      Details on my particular situation are below   The answer provided above by  null   http    stackoverflow  com a 173589 293332   was actually pretty darn close   I was dismayed that it got voted down tho   Those who do not understand regex lead very frustrating lives   Anyway  based upon his answer  here is what I ended up with           Checks if an array is associative by utilizing REGEX against the keys     param    arr     lt array gt  Reference to the array to be checked     return  boolean          private function    isAssociativeArray   amp  arr         return   bool   preg match     D    implode  array keys   arr              See the PCRE Escape Sequences and PCRE Syntax pages for further details   My Particular Situation  Here is an example array that I am dealing with   Case A  return  array       GetInventorySummary     gt  array           Filters     gt  array                Filter     gt  array                  array                       FilterType     gt   Shape                        FilterValue     gt   W                                      array                       FilterType     gt   Dimensions                        FilterValue     gt   8 x 10                                      array                       FilterType     gt   Grade                        FilterValue     gt   A992                                                         SummaryField     gt   Length               The catch is that the filter key is variable   For example   Case B  return  array       GetInventorySummary     gt  array           Filters     gt  array                Filter     gt  array                   foo      gt    bar                    bar      gt    foo                                      SummaryField     gt   Length               Why I Need Assoc  Array Checker  If the array I am transforming is like Case A  what I want returned is    lt  xml version  1 0  encoding  UTF-8  standalone  yes   gt   lt GetInventorySummary gt       lt Filters gt           lt Filter gt               lt FilterType gt Shape lt  FilterType gt               lt FilterValue gt W lt  FilterValue gt           lt  Filter gt           lt Filter gt               lt FilterType gt Dimensions lt  FilterType gt               lt FilterValue gt 8 x 10 lt  FilterValue gt           lt  Filter gt           lt Filter gt               lt FilterType gt Grade lt  FilterType gt                lt FilterValue gt A992 lt  FilterValue gt           lt  Filter gt       lt  Filters gt       lt SummaryField gt Length lt  SummaryField gt   lt  GetInventorySummary gt        However  if the array I am transforming is like Case B  what I want returned is    lt  xml version  1 0  encoding  UTF-8  standalone  yes   gt   lt GetInventorySummary gt       lt Filters gt           lt Filter gt               lt foo gt bar lt  foo gt               lt bar gt foo lt  bar gt           lt  Filter gt       lt  Filters gt       lt SummaryField gt Length lt  SummaryField gt   lt  GetInventorySummary gt

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Modification on the most popular answer  This takes a little more processing  but is more accurate      lt  php    a is a subset of  b function isSubset  a   b        foreach  a   gt  v          if array search  v   b      false              return false       return true         less effecient  clearer implementation   uses     for comparison        return array intersect  a   b       a     function isAssoc  arr        return  isSubset array keys  arr   range 0  count  arr  - 1       var dump isAssoc array  a    b    c         false var dump isAssoc array 1   gt   a   0   gt   b   2   gt   c         false var dump isAssoc array  0    gt   a    1    gt   b    2    gt   c         false     use     in isSubset to get  true  for above statement  var dump isAssoc array  a    gt   a    b    gt   b    c    gt   c         true   gt

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Sometimes you can get away with only checking if the first array s Key is 0    isSequential   array keys  arr  0      0    or  faster  but more verbose version   reset  arr    isSequential   key  arr      0

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Actually  I found myself in a similar situation trying to take an array and parse it into XML   XML element names cannot begin with numbers -- and the code snippets I found did not correctly deal with arrays with numeric indexes      Details on my particular situation are below   The answer provided above by  null   http    stackoverflow  com a 173589 293332   was actually pretty darn close   I was dismayed that it got voted down tho   Those who do not understand regex lead very frustrating lives   Anyway  based upon his answer  here is what I ended up with           Checks if an array is associative by utilizing REGEX against the keys     param    arr     lt array gt  Reference to the array to be checked     return  boolean          private function    isAssociativeArray   amp  arr         return   bool   preg match     D    implode  array keys   arr              See the PCRE Escape Sequences and PCRE Syntax pages for further details   My Particular Situation  Here is an example array that I am dealing with   Case A  return  array       GetInventorySummary     gt  array           Filters     gt  array                Filter     gt  array                  array                       FilterType     gt   Shape                        FilterValue     gt   W                                      array                       FilterType     gt   Dimensions                        FilterValue     gt   8 x 10                                      array                       FilterType     gt   Grade                        FilterValue     gt   A992                                                         SummaryField     gt   Length               The catch is that the filter key is variable   For example   Case B  return  array       GetInventorySummary     gt  array           Filters     gt  array                Filter     gt  array                   foo      gt    bar                    bar      gt    foo                                      SummaryField     gt   Length               Why I Need Assoc  Array Checker  If the array I am transforming is like Case A  what I want returned is    lt  xml version  1 0  encoding  UTF-8  standalone  yes   gt   lt GetInventorySummary gt       lt Filters gt           lt Filter gt               lt FilterType gt Shape lt  FilterType gt               lt FilterValue gt W lt  FilterValue gt           lt  Filter gt           lt Filter gt               lt FilterType gt Dimensions lt  FilterType gt               lt FilterValue gt 8 x 10 lt  FilterValue gt           lt  Filter gt           lt Filter gt               lt FilterType gt Grade lt  FilterType gt                lt FilterValue gt A992 lt  FilterValue gt           lt  Filter gt       lt  Filters gt       lt SummaryField gt Length lt  SummaryField gt   lt  GetInventorySummary gt        However  if the array I am transforming is like Case B  what I want returned is    lt  xml version  1 0  encoding  UTF-8  standalone  yes   gt   lt GetInventorySummary gt       lt Filters gt           lt Filter gt               lt foo gt bar lt  foo gt               lt bar gt foo lt  bar gt           lt  Filter gt       lt  Filters gt       lt SummaryField gt Length lt  SummaryField gt   lt  GetInventorySummary gt

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Could this be the solution     public static function isArrayAssociative array  array          reset  array         return  is int key  array          The caveat is obviously that the array cursor is reset but I d say probably the function is used before the array is even traversed or used

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Unless PHP has a builtin for that  you won t be able to do it in less than O n  - enumerating over all the keys and checking for integer type  In fact  you also want to make sure there are no holes  so your algorithm might look like   for i in 0 to len your array       if not defined your-array i              this is not an array array  it s an associative array      But why bother  Just assume the array is of the type you expect  If it isn t  it will just blow up in your face - that s dynamic programming for you  Test your code and all will be well

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Best function to detect associative array  hash array    lt  php function is assoc  arr    return  array values  arr       arr       gt

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Simple and performance friendly solution which only checks the first key   function isAssoc  arr   NULL        if   arr  amp  amp  is array  arr                 foreach   arr as  key   gt   val                        if  is numeric  key     return true                 break                       return false

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Another variant not shown yet  as it s simply not accepting numerical keys  but I like Greg s one very much           Returns true if  var associative array        function is associative array   array           return is array  array   amp  amp   is numeric implode     array keys  array

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function checkAssoc  array       return  ctype digit  implode     array keys  array

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Yet another way to do this   function array isassociative  array           Create new Array   Make it the same size as the input array      compareArray   array pad array    count  array   0           Compare the two array keys     return  count array diff key  array   compareArray      true   false

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This would work too  demo    function array has numeric keys only array  array        try           SplFixedArray  fromArray  array  true         catch  InvalidArgumentException  e            return false            return true      Please note that the main point of this answer is to inform you about the existence of SplFixedArray and not to encourage you to use Exceptions for these kinds of tests

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Improvement from Mark Amery  function isAssoc  arr           Is it set  is an array  not empty and keys are not sequentialy numeric from 0     return isset  arr   amp  amp  is array  arr   amp  amp  count  arr   0  amp  amp  array keys  arr      range 0  count  arr  - 1       This tests if variable exists  if it is an array  if it is not an empty array and if the keys are not sequential from 0   To see if the array is associative  if  isAssoc  array         To see if it numeric  if   isAssoc  array

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Surely this is a better alternative    lt  php  arr   array 1 2 3 4    isIndexed   array values  arr       arr

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Sometimes you can get away with only checking if the first array s Key is 0    isSequential   array keys  arr  0      0    or  faster  but more verbose version   reset  arr    isSequential   key  arr      0

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Yet another way to do this   function array isassociative  array           Create new Array   Make it the same size as the input array      compareArray   array pad array    count  array   0           Compare the two array keys     return  count array diff key  array   compareArray      true   false

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I just use the key   function  Observe    lt  php var dump key array  hello   gt  world    hello   gt  world        string 5   hello  var dump key array  world    world                         int 0  var dump key array  0    gt   a    1    gt   b    2    gt   c        int 0  who makes string sequetial keys anyway       gt    Thus  just by checking for false  you can determine whether an array is associative or not

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I think the following two functions are the best way to go for checking  if an array is associative or numeric   Since  numeric  could mean only numeric keys or only sequential numeric keys  two functions are listed below that check either condition   function is indexed array  amp  arr      for  reset  arr   is int key  arr    next  arr      return is null key  arr       function is sequential array  amp  arr   base   0      for  reset  arr    base    int   base  key  arr       base    next  arr      return is null key  arr        The first function checks if each key is an integer value  The second function checks if each key is an integer value and in addition checks if all keys are sequential starting at  base  which defaults to 0 and thus can be omitted if you do not need to specify another base value  key  my array  returns null if the read pointer is moved past the end of the array  which is what ends the for loop and makes the statement after the for loop return true if all keys were integer  If not  the loop ends prematurely because a key is of type string  and the statement after the for loop will return false  The latter function in addition adds one to  base after each compare  to be able to check if the next key is of the correct value  The strict compare makes it also check if the key is of type integer  The  base    int   base part in the first section of the for loop can be left out when  base is omitted or if you make sure it is only called using an integer  But since I can t be sure for everybody  I left it in  The statement is executed only once  anyway  I think these are the most efficient solutions    Memory wise  No copying of data or key ranges  Doing an array values or array keys may seem shorter  less code  but keep in mind what goes on in the background once you make that call  Yes there are more  visible  statements than in some other solutions  but that is not what counts  is it  Time wise  Besides the fact that copying extracting data and or keys also takes time  this solution is more efficient than doing a foreach  Again a foreach may seem more efficient to some because it is shorter in notation  but in the background foreach also calls reset  key and next to do it s looping  But in addition it also calls valid to check the end condition  which is avoided here due to the combination with the integer check    Remember that an array key can only be an integer or a string  and a strictly numeric string such as  1   but not  01   will be translated into an integer  Which is what makes checking for an integer key the only needed operation besides counting if you want the array to be sequential  Naturally  if is indexed array returns false the array can be seen as associative  I say  seen   because in fact they all are

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Surely this is a better alternative    lt  php  arr   array 1 2 3 4    isIndexed   array values  arr       arr

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You have asked two questions that are not quite equivalent    Firstly  how to determine whether an array has only numeric keys Secondly  how to determine whether an array has sequential numeric keys  starting from 0   Consider which of these behaviours you actually need   It may be that either will do for your purposes    The first question  simply checking that all keys are numeric  is answered well by Captain kurO   For the second question  checking whether the array is zero-indexed and sequential   you can use the following function   function isAssoc array  arr        if  array        arr  return false      return array keys  arr      range 0  count  arr  - 1      var dump isAssoc   a    b    c         false var dump isAssoc   0    gt   a    1    gt   b    2    gt   c         false var dump isAssoc   1    gt   a    0    gt   b    2    gt   c         true var dump isAssoc   a    gt   a    b    gt   b    c    gt   c         true

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If your looking for just non-numeric keys  no matter the order  then you may want to try  function IsAssociative  array        return preg match    a-z  i   implode array keys  array

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I ve used both array keys  obj      range 0  count  obj  - 1  and array values  arr       arr  which are duals of each other  although the second is cheaper than the first  but both fail for very large arrays   This is because array keys and array values are both very costly operations  since they build a whole new array of size roughly that of the original    The following function is more robust than the methods provided above   function array type   obj         last key   -1       type    index       foreach   obj as  key   gt   val            if   is int   key       key  lt  0                return  assoc                     if   key      last key   1                 type    sparse                      last key    key            return  type      Also note that if you don t care to differentiate sparse arrays from associative arrays you can simply return  assoc  from both if blocks   Finally  while this might seem much less  elegant  than a lot of  solutions  on this page  in practice it is vastly more efficient  Almost any associative array will be detected instantly  Only indexed arrays will get checked exhaustively  and the methods outlined above not only check indexed arrays exhaustively  they duplicate them

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As stated by the OP      PHP treats all arrays as associative   it is not quite sensible  IMHO  to write a function that checks if an array is associative  So first thing first  what is a key in a PHP array       The key can either be an integer or a string    That means there are 3 possible cases    Case 1  all keys are numeric   integers  Case 2  all keys are strings  Case 3  some keys are strings  some keys are numeric   integers    We can check each case with the following functions   Case 1  all keys are numeric   integers   Note  This function returns true for empty arrays too       Check whether the input is an array whose keys are all integers           param in   InputArray           array  Input array       return                          bool   b true iff the input is an array whose keys are all integers     function IsArrayAllKeyInt  InputArray        if  is array  InputArray                 return false             if count  InputArray   lt   0                return true             return array unique array map  is int   array keys  InputArray        array true       Case 2  all keys are strings   Note  This function returns true for empty arrays too       Check whether the input is an array whose keys are all strings           param in   InputArray           array  Input array       return                          bool   b true iff the input is an array whose keys are all strings     function IsArrayAllKeyString  InputArray        if  is array  InputArray                 return false             if count  InputArray   lt   0                return true             return array unique array map  is string   array keys  InputArray        array true       Case 3  some keys are strings  some keys are numeric   integers   Note  This function returns true for empty arrays too       Check whether the input is an array with at least one key being an integer and at least one key being a string           param in   InputArray           array  Input array       return                          bool   b true iff the input is an array with at least one key being an integer and at least one key being a string     function IsArraySomeKeyIntAndSomeKeyString  InputArray        if  is array  InputArray                 return false             if count  InputArray   lt   0                return true             return count array unique array map  is string   array keys  InputArray      gt   2      It follows that    If the value is not an array  all 3 functions return false  If the value is an empty array  all 3 functions return true  which is by definition  as in  the empty set is a subset of any set A because all its elements belong to A    If the value is a non-empty array  exactly 1 function returns true      Now  for an array to be a  genuine  array that we are all accustomed to  meaning    Its keys are all numeric   integers  Its keys are sequential  i e  increasing by step 1   Its keys start from zero    We can check with the following function   Case 3a  keys are numeric   integers  sequential  and zero-based   Note  This function returns true for empty arrays too       Check whether the input is an array whose keys are numeric  sequential  and zero-based           param in   InputArray           array  Input array       return                          bool   b true iff the input is an array whose keys are numeric  sequential  and zero-based     function IsArrayKeyNumericSequentialZeroBased  InputArray        if  is array  InputArray                 return false             if count  InputArray   lt   0                return true             return array keys  InputArray      range 0  count  InputArray  - 1       Caveats   Pitfalls  or  even more peculiar facts about array keys in PHP   Integer keys  The keys for these arrays are integers   array 0   gt   b    array 13   gt   b    array -13   gt   b                Negative integers are also integers  array 0x1A   gt   b               Hexadecimal notation    String keys  The keys for these arrays are strings   array  fish and chips    gt   b    array      gt   b                                         An empty string is also a string  array  stackoverflow email example com    gt   b          Strings may contain non-alphanumeric characters  array  stack t  over   r nflow s cool    gt   b           Strings may contain special characters  array   ta   k   v rfl  w     gt   b                          Strings may contain all kinds of symbols  array  funct on    gt   b                                 You think this looks fine  Think again   see https   stackoverflow com q 9246051 1402846  array       L    gt   b                               How about Japanese Korean Chinese Russian Polish  array  fi x0sh    gt   b                                  Strings may contain null characters  array file get contents  https   www google com images nav logo114 png     gt   b         Strings may even be binary    Integer keys that look like strings  If you think the key in array  13    gt   b   is a string  you are wrong  From the doc here      Strings containing valid integers will be cast to the integer type  E g  the key  8  will actually be stored under 8  On the other hand  08  will not be cast  as it isn t a valid decimal integer    For example  the key for these arrays are integers   array  13    gt   b    array  -13    gt   b                              Negative  ok    But the key for these arrays are strings   array  13     gt   b    array   13    gt   b                              Positive  not ok  array  -013    gt   b    array  0x1A    gt   b                             Not converted to integers even though it s a valid hexadecimal number  array  013    gt   b                              Not converted to integers even though it s a valid octal number  array  18446744073709551616    gt   b             Not converted to integers as it can t fit into a 64-bit integer    What s more  according to the doc      The size of an integer is platform-dependent  although a maximum value of about two billion is the usual value  that s 32 bits signed   64-bit platforms usually have a maximum value of about 9E18  except for Windows  which is always 32 bit  PHP does not support unsigned integers    So the key for this array may or may not be an integer - it depends on your platform   array  60000000000    gt   b                      Array key could be integer or string  it can fit into a 64-bit  but not 32-bit  integer    Even worse  PHP tends to be buggy if the integer is near the 231   2 147 483 648 boundary  see bug 51430  bug 52899   For example  on my local environment  PHP 5 3 8 on XAMPP 1 7 7 on Windows 7   var dump array  2147483647    gt   b    gives  array 1         2147483647   gt      string 1   b         but on this live demo on codepad  PHP 5 2 5   the same expression gives  array 1          2147483647    gt      string 1   b      So the key is an integer in one environment but a string in another  even though 2147483647 is a valid signed 32-bit integer

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I think the definition of a scalar array will vary by application   That is  some applications will require a more strict sense of what qualifies as a scalar array  and some applications will require a more loose sense   Below I present 3 methods of varying strictness    lt  php        Since PHP stores all arrays as associative internally  there is no proper    definition of a scalar array         As such  developers are likely to have varying definitions of scalar array     based on their application needs         In this file  I present 3 increasingly strict methods of determining if an    array is scalar          author David Farrell  lt DavidPFarrell gmail com gt              isArrayWithOnlyIntKeys defines a scalar array as containing    only integer keys         If you are explicitly setting integer keys on an array  you    may need this function to determine scalar-ness          param array  a     return boolean      function isArrayWithOnlyIntKeys array  a        if   is array  a           return false      foreach   a as  k   gt   v          if   is int  k               return false      return true            isArrayWithOnlyAscendingIntKeys defines a scalar array as    containing only integer keys in ascending  but not necessarily    sequential  order         If you are performing pushes  pops  and unsets on your array     you may need this function to determine scalar-ness          param array  a     return boolean      function isArrayWithOnlyAscendingIntKeys array  a        if   is array  a           return false       prev   null      foreach   a as  k   gt   v                if   is int  k      null      prev  amp  amp   k  lt    prev               return false           prev    k            return true            isArrayWithOnlyZeroBasedSequentialIntKeys defines a scalar array    as containing only integer keys in sequential  ascending order     starting from 0         If you are only performing operations on your array that are    guaranteed to either maintain consistent key values  or that    re-base the keys for consistency  then you can use this function          param array  a     return boolean     function isArrayWithOnlyZeroBasedSequentialIntKeys array  a        if   is array  a           return false       i   0      foreach   a as  k   gt   v          if   i        k              return false      return true

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function is array assoc  foo        if  is array  foo             return  count array filter array keys  foo    is string     gt  0             return false

[php] How to check if PHP array is associative or sequential?

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