PHP How to use array filter to filter array keys

Question

The callback function in array filter   only passes in the array s values  not the keys   If I have    my array   array  foo    gt  1   hello    gt   world      allowed   array  foo    bar      What s the best way to delete all keys in  my array that are not in the  allowed array   Desired output    my array   array  foo    gt  1

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With this function you can filter a multidimensional array  function filter array keys  array  filter keys array           l array  amp  array        c 1        This first loop will loop until the count var is stable       for  r 0  r lt  c  r               This loop will loop thru the child element list            keys   array keys  l  r             for  z 0  z lt count  l  r    z                  object    amp  l  r   keys  z                 if is array  object                     i 0                   keys on array array keys  object                    object array filter  object function  el  use  amp  i  keys on array  filter keys                        key    keys on array  i                        i                         if in array  key  filter keys     is int  key  return false                                      return true                                                                         if is array  l  r   keys  z                       l      amp  l  r   keys  z                     c                   IF                       FOR        FOR        return  l 0

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array filter function from php   array filter    array   callback function   flag      array - It is the input array   callback function - The callback function to use  If the callback function returns true  the current value from array is returned into the result array    flag - It is optional parameter  it will determine what arguments are sent to callback function  If this parameter empty then callback function will take array values as argument  If you want to send array key as argument then use  flag as ARRAY FILTER USE KEY  If you want to send both keys and values you should use  flag as ARRAY FILTER USE BOTH    For Example   Consider simple array   array   array  a   gt 1   b   gt 2   c   gt 3   d   gt 4   e   gt 5     If you want to filter array based on the array key  We need to use ARRAY FILTER USE KEY as third parameter of array function array filter    get key res   array filter  array  get key  ARRAY FILTER USE KEY      If you want to filter array based on the array key and array value  We need to use ARRAY FILTER USE BOTH as third parameter of array function array filter    get both   array filter  array  get both  ARRAY FILTER USE BOTH      Sample Callback functions    function get key  key         if  key     a                 return true        else           return false          function get both  val  key        if  key     a   amp  amp   val    1                return true          else           return false            It will output   Output of  get key is  Array    a    gt  1    Output of  get both is  Array    a    gt  1

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Naive and ugly  but seems to be faster  solution   Only tried this in php 7 3 11 but an ugly loop seems to execute in about a third of the time  Similar results on an array with a few hundred keys  Micro-optimization  probably not useful in RW  but found it surprising and interesting    time   microtime true    i   100000  while  i         my array     foo    gt  1   hello    gt   world         allowed      foo    bar         filtered   array filter           my array          function   key  use   allowed                return in array  key   allowed                      ARRAY FILTER USE KEY             i--    print r  filtered   echo microtime true  -  time     on array filter       0 40600109100342 on array filter  time2   microtime true    i2   100000  while  i2         my array2     foo    gt  1   hello    gt   world         allowed2      foo    bar         filtered2           foreach   my array2 as  k   gt   v            if  in array  k   allowed2    filtered2  k     v             i2--    print r  filtered2   echo microtime true  -  time2     on ugly loop      0 15677785873413 on ugly loop

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I needed to do same  but with a more complex array filter on the keys    Here s how I did it  using a similar method      Filter out array elements with keys shorter than 4 characters  a   array    0        gt   val 0       one     gt   val one       two     gt   val two       three   gt   val three       four    gt   val four       five    gt   val five       6       gt   val 6        f   array filter array keys  a   function   k   return strlen  k  gt  4        b   array intersect key  a  array flip  f    print r  b     This outputs the result   Array        three    gt  val three      four    gt  val four      five    gt  val five

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With array intersect key and array flip   var dump array intersect key  my array  array flip  allowed      array 1        foo    gt    int 1

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PHP 5 6 introduced a third parameter to array filter    flag  that you can set to ARRAY FILTER USE KEY to filter by key instead of value    my array     foo    gt  1   hello    gt   world     allowed      foo    bar     filtered   array filter       my array      function   key  use   allowed            return in array  key   allowed              ARRAY FILTER USE KEY      Clearly this isn t as elegant as array intersect key  my array  array flip  allowed    but it does offer the additional flexibility of performing an arbitrary test against the key  e g   allowed could contain regex patterns instead of plain strings   You can also use ARRAY FILTER USE BOTH to have both the value and the key passed to your filter function  Here s a contrived example based upon the first  but note that I d not recommend encoding filtering rules using  allowed this way    my array     foo    gt  1   bar    gt   baz    hello    gt   wld     allowed      foo    gt  true   bar    gt  true   hello    gt   world     filtered   array filter       my array      function   val   key  use   allowed       N b   val   key not  key   val         return isset  allowed  key    amp  amp                 allowed  key      true     allowed  key       val                       ARRAY FILTER USE BOTH         foo    gt  1   bar    gt   baz

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Here is a more flexible solution using a closure    my array   array  foo    gt  1   hello    gt   world     allowed   array  foo    bar     result   array flip array filter array flip  my array   function   key  use   allowed        return in array  key   allowed        var dump  result     Outputs   array 1       foo    gt    int 1      So in the function  you can do other specific tests

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Here s a less flexible alternative using unset      array   array      1   gt   one       2   gt   two       3   gt   three      disallowed   array 1 3   foreach  disallowed as  key       unset  array  key        The result of print r  array  being   Array        2    gt  two     This is not applicable if you want to keep the filtered values for later use but tidier  if you re certain that you don t

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Perhaps an overkill if you need it just once  but you can use YaLinqo library  to filter collections  and perform any other transformations   This library allows peforming SQL-like queries on objects with fluent syntax  Its where function accepts a calback with two arguments  a value and a key  For example    filtered   from  array      - gt where function   v   k  use   allowed            return in array  k   allowed              - gt toArray       The where function returns an iterator  so if you only need to iterate with foreach over the resulting sequence once  - gt toArray   can be removed      developed by me

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If you are looking for a method to filter an array by a string occurring in keys  you can use    mArray array  foo   gt  bar   foo2   gt  bar2   fooToo   gt  bar3   baz   gt  nope     mSearch  foo    allowed array filter      array keys  mArray       function  key  use   mSearch           return stristr  key  mSearch            mResult array intersect key  mArray array flip  allowed      The result of print r  mResult  is   Array    foo    gt  bar  foo2    gt  bar2  fooToo    gt  bar3       An adaption of this answer that supports regular expressions  function array preg filter keys  arr   regexp       keys   array keys  arr      match   array filter  keys  function  k  use  regexp        return preg match  regexp   k      1          return array intersect key  arr  array flip  match        mArray   array  foo   gt  yes    foo2   gt  yes    FooToo   gt  yes    baz   gt  nope     print r array preg filter keys  mArray     foo i       Output  Array        foo    gt  yes      foo2    gt  yes      FooToo    gt  yes

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elements array     first    second         function to remove some array elements   function remove  arr   data        return array filter  arr  function   element  use   data            return  element     data                 call and print   print r remove  elements array   second          the result   Array    0    gt  first

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How to get the current key of an array when using array filter  Regardless of how I like Vincent s solution for Macek s problem  it doesn t actually use array filter  If you came here from a search engine you maybe where looking for something like this  PHP    5 3     array     apple    gt   red    pear    gt   green    reset  array      Unimportant here  but make sure your array is reset   apples   array filter  array  function  color  use    amp array       key   key  array     next  array      advance array pointer    return key  array       apple       It passes the array you re filtering as a reference to the callback  As array filter doesn t conventionally iterate over the array by increasing it s public internal pointer you have to advance it by yourself   What s important here is that you need to make sure your array is reset  otherwise you might start right in the middle of it   In PHP    5 4 you could make the callback even shorter    apples   array filter  array  function  color  use    amp array      return each  array   key        apple

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Starting from PHP 5 6  you can use the ARRAY FILTER USE KEY flag in array filter    result   array filter  my array  function   k  use   allowed        return in array  k   allowed      ARRAY FILTER USE KEY      Otherwise  you can use this function  from TestDummy    function filter array keys array  array   callback         matchedKeys   array filter array keys  array    callback        return array intersect key  array  array flip  matchedKeys        result   filter array keys  my array  function   k  use   allowed        return in array  k   allowed          And here is an augmented version of mine  which accepts a callback or directly the keys   function filter array keys array  array   keys        if  is callable  keys              keys   array filter array keys  array    keys              return array intersect key  array  array flip  keys          using a callback  like array filter   result   filter array keys  my array  function   k  use   allowed        return in array  k   allowed           or  if you already have the keys   result   filter array keys  my array   allowed       Last but not least  you may also use a simple foreach    result       foreach   my array as  key   gt   value        if  in array  key   allowed              result  key     value

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Filter out array elements with keys shorter than 4 characters    By using Anonymous function with  Closure    function comparison  min       return function  item  use   min           return strlen  item   gt    min                input   array    0        gt   quot val 0 quot      quot one quot     gt   quot val one quot      quot two quot     gt   quot val two quot      quot three quot   gt   quot val three quot      quot four quot    gt   quot val four quot        quot five quot    gt   quot val five quot          quot 6 quot       gt   quot val 6 quot           output   array filter array keys  input   comparison 4    print r  output

[php] PHP: How to use array_filter() to filter array keys?

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