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up vote 5 down vote favorite
4

What's the performance difference (if there is any) between these three approaches, both used to transform an array to another array?

  1. Using foreach
  2. Using array_map with lambda/closure function
  3. Using array_map with 'static' function/method
  4. Is there any other approach?

To make myself clear, let's have look at the examples, all doing the same - multiplying the array of numbers by 10:

$numbers = range(0, 1000);

Foreach

$result = array();
foreach ($numbers as $number) {
    $result[] = $number * 10;
}
return $result;

Map with lambda

return array_map(function($number) {
    return $number * 10;
}, $numbers);

Map with 'static' function, passed as string reference

function tenTimes($number) {
    return $number * 10;
}
return array_map('tenTimes', $numbers);

Is there any other approach? I will be happy to hear actually all differences between the cases from above, and any inputs why one should be used instead of others.

share|improve this question
4  
Why don't you just benchmark and see what happens? –  Jon Aug 9 '13 at 10:38
2  
Well, I may make a benchmark. But I still do not know how it internally works. Even if I find out one is faster, I still do not know why. Is it because of the PHP version? Does it depend on the data? Is there a difference between associative and ordinary arrays? Of course I can make whole suite of benchmarks but getting some theory saves here a lot of time. I hope you understand... –  Pavel S. Aug 9 '13 at 14:21

1 Answer 1

up vote 8 down vote accepted

FWIW, I just did the benchmark since poster didn't do it. Running on PHP 5.3.10 + XDebug.


function lap($func) {
  $t0 = microtime(1);
  $numbers = range(0, 1000000);
  $ret = $func($numbers);
  $t1 = microtime(1);
  return array($t1 - $t0, $ret);
}

function useArray($numbers)  {
  $result = array();
  foreach ($numbers as $number) {
      $result[] = $number * 10;
  }
  return $result;
}

function useMapClosure($numbers) {
  return array_map(function($number) {
      return $number * 10;
  }, $numbers);
}

function _tenTimes($number) {
    return $number * 10;
}

function useMapNamed($numbers) {
  return array_map('_tenTimes', $numbers);
}

foreach (array('Array', 'MapClosure', 'MapNamed') as $callback) {
  list($delay,) = lap("use$callback");
  echo "$callback: $delay\n";
}

I get pretty consistent results with 1M numbers across a dozen attempts:

  • Array: 0.7 sec
  • Map on closure: 3.4 sec
  • Map on function name: 1.2 sec.

Supposing the lackluster speed of the map on closure was caused by the closure possibly being evaluated each time, I also tested like this:


function useMapClosure($numbers) {
  $closure = function($number) {
    return $number * 10;
  };

  return array_map($closure, $numbers);
}

But the results are identical, confirming that the closure is only evaluated once.

2014-02-02 UPDATE: opcodes dump

Here are the opcode dumps for the three callbacks. First useArray():



compiled vars:  !0 = $numbers, !1 = $result, !2 = $number
line     # *  op                           fetch          ext  return  operands
---------------------------------------------------------------------------------
  10     0  >   EXT_NOP                                                  
         1      RECV                                                     1
  11     2      EXT_STMT                                                 
         3      INIT_ARRAY                                       ~0      
         4      ASSIGN                                                   !1, ~0
  12     5      EXT_STMT                                                 
         6    > FE_RESET                                         $2      !0, ->15
         7  > > FE_FETCH                                         $3      $2, ->15
         8  >   OP_DATA                                                  
         9      ASSIGN                                                   !2, $3
  13    10      EXT_STMT                                                 
        11      MUL                                              ~6      !2, 10
        12      ASSIGN_DIM                                               !1
        13      OP_DATA                                                  ~6, $7
  14    14    > JMP                                                      ->7
        15  >   SWITCH_FREE                                              $2
  15    16      EXT_STMT                                                 
        17    > RETURN                                                   !1
  16    18*     EXT_STMT                                                 
        19*   > RETURN                                                   null

Then the useMapClosure()


compiled vars:  !0 = $numbers
line     # *  op                           fetch          ext  return  operands
---------------------------------------------------------------------------------
  18     0  >   EXT_NOP                                                  
         1      RECV                                                     1
  19     2      EXT_STMT                                                 
         3      EXT_FCALL_BEGIN                                          
         4      DECLARE_LAMBDA_FUNCTION                                  '%00%7Bclosure%7D%2Ftmp%2Flap.php0x7f7fc1424173'
  21     5      SEND_VAL                                                 ~0
         6      SEND_VAR                                                 !0
         7      DO_FCALL                                      2  $1      'array_map'
         8      EXT_FCALL_END                                            
         9    > RETURN                                                   $1
  22    10*     EXT_STMT                                                 
        11*   > RETURN                                                   null

and the closure it calls:


compiled vars:  !0 = $number
line     # *  op                           fetch          ext  return  operands
---------------------------------------------------------------------------------
  19     0  >   EXT_NOP                                                  
         1      RECV                                                     1
  20     2      EXT_STMT                                                 
         3      MUL                                              ~0      !0, 10
         4    > RETURN                                                   ~0
  21     5*     EXT_STMT                                                 
         6*   > RETURN                                                   null

then the useMapNamed() function:


compiled vars:  !0 = $numbers
line     # *  op                           fetch          ext  return  operands
---------------------------------------------------------------------------------
  28     0  >   EXT_NOP                                                  
         1      RECV                                                     1
  29     2      EXT_STMT                                                 
         3      EXT_FCALL_BEGIN                                          
         4      SEND_VAL                                                 '_tenTimes'
         5      SEND_VAR                                                 !0
         6      DO_FCALL                                      2  $0      'array_map'
         7      EXT_FCALL_END                                            
         8    > RETURN                                                   $0
  30     9*     EXT_STMT                                                 
        10*   > RETURN                                                   null

and the named function it calls, _tenTimes():


compiled vars:  !0 = $number
line     # *  op                           fetch          ext  return  operands
---------------------------------------------------------------------------------
  24     0  >   EXT_NOP                                                  
         1      RECV                                                     1
  25     2      EXT_STMT                                                 
         3      MUL                                              ~0      !0, 10
         4    > RETURN                                                   ~0
  26     5*     EXT_STMT                                                 
         6*   > RETURN                                                   null
share|improve this answer
    
Thanks for the benchmarks. However, I would like to know why there is such difference. Is it because of a function call overhead? –  Pavel S. Jan 25 at 13:06
    
I added the opcode dumps in the issue. First thing we can see is that the named function and closure have exactly the same dump, and they are called via array_map in much the same way, with just one exception: the closure call includes one more opcode DECLARE_LAMBDA_FUNCTION, which explains why using it is a bit slower than using the named function. Now, comparing the array loop vs array_map calls, everything in the array loop is interpreted inline, without any call to a function, meaning no context to push/pop, just a JMP at the end of the loop, which likely explains the big difference. –  FGM Feb 2 at 17:35

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