Difference between array map array walk and array filter

Question

What exactly is the difference between array map  array walk and array filter  What I could see from documentation is that you could pass a callback function to perform an action on the supplied array  But I don t seem to find any particular difference between them  Do they perform the same thing  Can they be used interchangeably  I would appreciate your help with illustrative example if they are different at all

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From the documentation      bool array walk   array  amp  array   callback  funcname    mixed  userdata      lt -return bool   array walk takes an array and a function F and modifies it by replacing every element x with F x         array array map   callback  callback     array  arr1    array          lt -return array   array map does the exact same thing except that instead of modifying in-place it will return a new array with the transformed elements       array array filter   array  input      callback  callback     lt -return array    array filter with function F  instead of transforming the elements  will remove any elements for which F x  is not true

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Changing Values    array map cannot change the values inside input array s  while array walk can  in particular  array map never changes its arguments   Array Keys Access    array map cannot operate with the array keys  array walk can   Return Value    array map returns a new array  array walk only returns true  Hence  if you don t want to create an array as a result of traversing one array  you should use array walk   Iterating Multiple Arrays    array map also can receive an arbitrary number of arrays and it can iterate over them in parallel  while array walk operates only on one   Passing Arbitrary Data to Callback    array walk can receive an extra arbitrary parameter to pass to the callback  This mostly irrelevant since PHP 5 3  when anonymous functions were introduced    Length of Returned Array    The resulting array of array map has the same length as that of the largest input array  array walk does not return an array but at the same time it cannot alter the number of elements of original array  array filter picks only a subset of the elements of the array according to a filtering function  It does preserve the keys     Example    lt pre gt   lt  php   origarray1   array 2 4  2 6  3 5    origarray2   array 2 4  2 6  3 5    print r array map  floor    origarray1        origarray1 stays the same     changes  origarray2 array walk  origarray2  function   amp  v   k     v   floor  v        print r  origarray2       this is a more proper use of array walk array walk  origarray1  function   v   k    echo   k   gt   v     n           array map accepts several arrays print r      array map function   a   b    return  a    b      origarray1   origarray2         select only elements that are  gt  2 5 print r      array filter  origarray1  function   a    return  a  gt  2 5           gt   lt  pre gt    Result   Array        0    gt  2      1    gt  2      2    gt  3   Array        0    gt  2      1    gt  2      2    gt  3   0   gt  2 4 1   gt  2 6 2   gt  3 5 Array        0    gt  4 8      1    gt  5 2      2    gt  10 5   Array        1    gt  2 6      2    gt  3 5

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The other answers demonstrate the difference between array walk  in-place modification  and array map  return modified copy  quite well  However  they don t really mention array reduce  which is an illuminating way to understand array map and array filter   The array reduce function takes an array  a two-argument function and an  accumulator   like this   array reduce array  a    b    c    d                  my function                 accumulator    The array s elements are combined with the accumulator one at a time  using the given function  The result of the above call is the same as doing this   my function    my function      my function        my function           accumulator           a           b         c       d     If you prefer to think in terms of loops  it s like doing the following  I ve actually used this as a fallback when array reduce wasn t available    function array reduce  array   function   accumulator      foreach   array as  element         accumulator    function  accumulator   element         return  accumulator      This looping version makes it clear why I ve called the third argument an  accumulator   we can use it to accumulate results through each iteration   So what does this have to do with array map and array filter  It turns out that they re both a particular kind of array reduce  We can implement them like this   array map  function   array         array reduce  array   MAP     array    array filter  array   function      array reduce  array   FILTER  array      Ignore the fact that array map and array filter take their arguments in a different order  that s just another quirk of PHP  The important point is that the right-hand-side is identical except for the functions I ve called  MAP and  FILTER  So  what do they look like    MAP   function  accumulator   element       accumulator      function  element     return  accumulator       FILTER   function  accumulator   element      if   function  element    accumulator      element    return  accumulator       As you can see  both functions take in the  accumulator and return it again  There are two differences in these functions     MAP will always append to  accumulator  but  FILTER will only do so if  function  element  is TRUE   FILTER appends the original element  but  MAP appends  function  element     Note that this is far from useless trivia  we can use it to make our algorithms more efficient   We can often see code like these two examples      Transform the valid inputs array map  transform   array filter  inputs   valid        Get all numeric IDs array filter array map  get id    inputs    is numeric     Using array map and array filter instead of loops makes these examples look quite nice  However  it can be very inefficient if  inputs is large  since the first call  map or filter  will traverse  inputs and build an intermediate array  This intermediate array is passed straight into the second call  which will traverse the whole thing again  then the intermediate array will need to be garbage collected   We can get rid of this intermediate array by exploiting the fact that array map and array filter are both examples of array reduce  By combining them  we only have to traverse  inputs once in each example      Transform valid inputs array reduce  inputs               function  accumulator   element                   if  valid  element    accumulator     transform  element                  return  accumulator                               array        Get all numeric IDs array reduce  inputs               function  accumulator   element                    id   get id  element                  if  is numeric  id    accumulator      id                 return  accumulator                               array      NOTE  My implementations of array map and array filter above won t behave exactly like PHP s  since my array map can only handle one array at a time and my array filter won t use  empty  as its default  function  Also  neither will preserve keys   It s not difficult to make them behave like PHP s  but I felt that these complications would make the core idea harder to spot

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The following revision seeks to more clearly delineate PHP s array filer    array map    and array walk    all of which originate from functional programming     array filter   filters out data  producing as a result a new array holding only the desired items of the former array  as follows    lt  php  array   array 1   apples  2   oranges  3   plums      filtered   array filter   array   ctype alpha    var dump  filtered     gt    live code here  All numeric values are filtered out of  array  leaving  filtered with only types of fruit   array map   also creates a new array but unlike array filter   the resulting  array contains every element of the input  filtered but with altered values  owing to applying a callback to each element  as follows      lt  php   nu   array map   strtoupper    filtered   var dump  nu     gt    live code here  The code in this case applies a callback using the built-in strtoupper   but a user-defined function is another viable option  too   The callback applies to every item of  filtered and thereby engenders  nu whose elements contain uppercase values   In the next snippet  array walk   traverses  nu and makes changes to each element vis a vis the reference operator   amp    The changes occur without creating an additional array  Every element s value changes in place into a more informative string specifying its key  category and value    lt  php   f   function  amp  item  key  prefix         item     key   prefix   item       array walk  nu   f  fruit    var dump  nu         gt        See demo  Note  the callback function with respect to array walk   takes two parameters which will automatically acquire an element s value and its key and in that order  too when invoked by array walk     See more here

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The idea of mapping an function to array of data comes from functional programming  You shouldn t think about array map as a foreach loop that calls a function on each element of the array  even though that s how it s implemented   It should be thought of as applying the function to each element in the array independently   In theory such things as function mapping can be done in parallel since the function being applied to the data should ONLY affect the data and NOT the global state  This is because an array map could choose any order in which to apply the function to the items in  even though in PHP it doesn t    array walk on the other hand it the exact opposite approach to handling arrays of data  Instead of handling each item separately  it uses a state   amp  userdata  and can edit the item in place  much like a foreach loop   Since each time an item has the  funcname applied to it  it could change the global state of the program and therefor requires a single correct way of processing the items   Back in PHP land  array map and array walk are almost identical except array walk gives you more control over the iteration of data and is normally used to  change  the data in-place vs returning a new  changed  array   array filter is really an application of array walk  or array reduce  and it more-or-less just provided for convenience

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