Performance of FOR vs FOREACH in PHP

Question

First of all  I understand in 90  of applications the performance difference is completely irrelevant  but I just need to know which is the faster construct  That and     The information currently available on them on the net is confusing  A lot of people say foreach is bad  but technically it should be faster since it s suppose to simplify writing a array traversal using iterators  Iterators  which are again suppose to be faster  but in PHP are also apparently dead slow  or is this not a PHP thing    I m talking about the array functions  next   prev   reset   etc  well  if they are even functions and not one of those PHP language features that look like functions   To narrow this down a little  I m not interesting in traversing arrays in steps of anything more than 1  no negative steps either  ie  reverse iteration   I m also not interested in a traversal to and from arbitrary points  just 0 to length  I also don t see manipulating arrays with more than 1000 keys happening on a regular basis  but I do see a array being traversed multiple times in the logic of a application  Also as for operations  largely only string manipulation and echo ing   Here are few reference sites  http   www phpbench com  http   www php lt benchmark phpbench php  What I hear everywhere    foreach is slow  and thus for while is faster PHPs foreach copies the array it iterates over  to make it faster you need to use references code like this   key   array keys  aHash    size   sizeOf  key   for   i 0   i  lt   size   i    is faster than a foreach   Here s my problem  I wrote this test script  http   pastebin com 1ZgK07US and no matter how many times I run the script  I get something like this   foreach 1 1438131332397 foreach  using reference  1 2919359207153 for 1 4262869358063 foreach  hash table  1 5696921348572 for  hash table  2 4778981208801   In short    foreach is faster than foreach with reference foreach is faster than for foreach is faster than for for a hash table   Can someone explain    Am I doing something wrong  Is PHP foreach reference thing really making a difference  I mean why would it not copy it if you pass by reference  What s the equivalent iterator code for the foreach statement  I ve seen a few on the net but each time I test them the timing is way off  I ve also tested a few simple iterator constructs but never seem to get even decent results -- are the array iterators in PHP just awful  Are there faster ways methods constructs to iterate though a array other than FOR FOREACH  and WHILE     PHP Version 5 3 0  Edit  Answer With help from people here I was able to piece together the answers to all question  I ll summarize them here       Am I doing something wrong   The consensus seems to be  yes  I can t use echo in benchmarks  Personally  I still don t see how echo is some function with random time of execution or how any other function is somehow any different -- that and the ability of that script to just generate the exact same results of foreach better than everything is hard to explain though just  you re using echo   well what should I have been using   However  I concede the test should be done with something better  though a ideal compromise does not come to mind   Is PHP foreach reference thing really making a difference  I mean why would it not copy it if you pass by reference   ircmaxell shows that yes it is  further testing seems to prove in most cases reference should be faster -- though given my above snippet of code  most definitely doesn t mean all  I accept the issue is probably too non-intuitive to bother with at such a level and would require something extreme such as decompiling to actually determine which is better for each situation   What s the equivalent iterator code for the foreach statement  I ve seen a few on the net but each time I test them the timing is way off  I ve also tested a few simple iterator constructs but never seem to get even decent results -- are the array iterators in PHP just awful   ircmaxell provided the answer bellow  though the code might only be valid for PHP version    5  Are there faster ways methods constructs to iterate though a array other than FOR FOREACH  and WHILE    Thanks go to Gordon for the answer  Using new data types in PHP5 should give either a performance boost or memory boost  either of which might be desirable depending on your situation   While speed wise a lot of the new types of array don t seem to be better than array    the splpriorityqueue and splobjectstorage do seem to be substantially faster  Link provided by Gordon  http   matthewturland com 2010 05 20 new-spl-features-in-php-5-3    Thank you everyone who tried to help   I ll likely stick to foreach  the non-reference version  for any simple traversal

User · Accepted Answer

My personal opinion is to use what makes sense in the context. Personally I almost never use for for array traversal. I use it for other types of iteration, but foreach is just too easy... The time difference is going to be minimal in most cases.

The big thing to watch for is:

for ($i = 0; $i < count($array); $i++) {

That's an expensive loop, since it calls count on every single iteration. So long as you're not doing that, I don't think it really matters...

As for the reference making a difference, PHP uses copy-on-write, so if you don't write to the array, there will be relatively little overhead while looping. However, if you start modifying the array within the array, that's where you'll start seeing differences between them (since one will need to copy the entire array, and the reference can just modify inline)...

As for the iterators, foreach is equivalent to:

$it->rewind();
while ($it->valid()) {
    $key = $it->key();     // If using the $key => $value syntax
    $value = $it->current();

    // Contents of loop in here

    $it->next();
}

As far as there being faster ways to iterate, it really depends on the problem. But I really need to ask, why? I understand wanting to make things more efficient, but I think you're wasting your time for a micro-optimization. Remember, Premature Optimization Is The Root Of All Evil...

Edit: Based upon the comment, I decided to do a quick benchmark run...

$a = array();
for ($i = 0; $i < 10000; $i++) {
    $a[] = $i;
}

$start = microtime(true);
foreach ($a as $k => $v) {
    $a[$k] = $v + 1;
}
echo "Completed in ", microtime(true) - $start, " Seconds\n";

$start = microtime(true);
foreach ($a as $k => &$v) {
    $v = $v + 1;
}
echo "Completed in ", microtime(true) - $start, " Seconds\n";

$start = microtime(true);
foreach ($a as $k => $v) {}
echo "Completed in ", microtime(true) - $start, " Seconds\n";

$start = microtime(true);
foreach ($a as $k => &$v) {}    
echo "Completed in ", microtime(true) - $start, " Seconds\n";

And the results:

Completed in 0.0073502063751221 Seconds
Completed in 0.0019769668579102 Seconds
Completed in 0.0011849403381348 Seconds
Completed in 0.00111985206604 Seconds

So if you're modifying the array in the loop, it's several times faster to use references...

And the overhead for just the reference is actually less than copying the array (this is on 5.3.2)... So it appears (on 5.3.2 at least) as if references are significantly faster...

User · Answer

One thing to watch out for in benchmarks  especially phpbench com   is even though the numbers are sound  the tests are not  Alot of the tests on phpbench com are doing things at are trivial and abuse PHP s ability to cache array lookups to skew benchmarks or in the case of iterating over an array doesn t actually test it in real world cases  no one writes empty for loops   I ve done my own benchmarks that I ve found are fairly reflective of the real world results and they always show the language s native iterating syntax foreach coming out on top  surprise  surprise      make a nicely random array  aHash1   range  0  999999     aHash2   range  0  999999    shuffle   aHash1    shuffle   aHash2     aHash   array combine   aHash1   aHash2       start1   microtime true   foreach  aHash as  key  gt  val   aHash  key      end1   microtime true     start2   microtime true   while list  key    each  aHash    aHash  key      end2   microtime true      start3   microtime true    key   array keys  aHash    size   sizeOf  key   for   i 0   i lt  size   i     aHash  key  i       end3   microtime true     start4   microtime true   foreach  aHash as  amp  val   val     end4   microtime true    echo  foreach     end1 -  start1    n     foreach 0 947947025299 echo  while     end2 -  start2    n     while 0 847212076187 echo  for     end3 -  start3    n     for 0 439476966858 echo  foreach ref     end4 -  start4    n     foreach ref 0 0886030197144    For these tests we MUST do an array lookup    since that is normally the  point  of iteration   i m also calling noop on it so that PHP doesn t   optimize out the loopup  function noop   value         Create an array of increasing indexes  w  random values  bHash   range  0  999999    shuffle   bHash      bstart1   microtime true   for  i   0   i  lt  1000000     i  noop   bHash  i      bend1   microtime true     bstart2   microtime true    i   0  while  i  lt  1000000    noop   bHash  i        i     bend2   microtime true      bstart3   microtime true   foreach   bHash as  value     noop   value       bend3   microtime true    echo  for     bend1 -  bstart1    n     for 0 397135972977 echo  while     bend2 -  bstart2    n     while 0 364789962769 echo  foreach     bend3 -  bstart3    n     foreach 0 346374034882

User · Answer

It s 2020 and stuffs had greatly evolved with php 7 4 and opcache   Here is the OP  benchmark  ran as unix CLI  without the echo and html parts   Test ran locally on a regular computer   php -v  PHP 7 4 6  cli   built  May 14 2020 10 02 44    NTS       Modified benchmark script     lt  php      preperations  just a simple environment state     test iterations   100     test arr size   1000        a shared function that makes use of the loop  this should      ensure no funny business is happening to fool the test   function test  input            echo   lt  --   trim  input    -- gt              for each test we create a array this should avoid any of the      arrays internal representation or optimizations from getting      in the way        normal array    test arr1   array       test arr2   array       test arr3   array         hash tables    test arr4   array       test arr5   array       for   i   0   i  lt   test arr size     i          mt srand         hash   md5 mt rand          key   substr  hash  0  5   i        test arr1  i     test arr2  i     test arr3  i     test arr4  key     test arr5  key           hash            foreach     start   microtime true     for   j   0   j  lt   test iterations     j          foreach   test arr1 as  k   gt   v              test  v               echo  foreach    microtime true  -  start    n           foreach  using reference      start   microtime true     for   j   0   j  lt   test iterations     j          foreach   test arr2 as  amp  value              test  value               echo  foreach  using reference     microtime true  -  start    n         for     start   microtime true     for   j   0   j  lt   test iterations     j           size   count  test arr3       for   i   0   i  lt   size     i              test  test arr3  i                echo  for    microtime true  -  start    n           foreach  hash table      start   microtime true     for   j   0   j  lt   test iterations     j          foreach   test arr4 as  k   gt   v              test  v               echo  foreach  hash table     microtime true  -  start    n         for  hash table      start   microtime true     for   j   0   j  lt   test iterations     j           keys   array keys  test arr5        size   sizeOf  test arr5       for   i   0   i  lt   size     i              test  test arr5  keys  i                 echo  for  hash table     microtime true  -  start    n       Output   foreach 0 0032877922058105 foreach  using reference  0 0029420852661133 for 0 0025191307067871 foreach  hash table  0 0035080909729004 for  hash table  0 0061779022216797   As you can see the evolution is insane  about 560 time faster than reported in 2012   On my machines and servers  following my numerous experiments  basics for loops are the fastest  This is even clearer using nested loops   i  j  k     It is also the most flexible in usage  and has a better readability from my view

User · Answer

I think but I am not sure   the for loop takes two operations for checking and incrementing values  foreach loads the data in memory then it will iterate every values

User · Answer

I m not sure this is so surprising   Most people who code in PHP are not well versed in what PHP is actually doing at the bare metal   I ll state a few things  which will be true most of the time    If you re not modifying the variable  by-value is faster in PHP   This is because it s reference counted anyway and by-value gives it less to do   It knows the second you modify that ZVAL  PHP s internal data structure for most types   it will have to break it off in a straightforward way  copy it and forget about the other ZVAL    But you never modify it  so it doesn t matter   References make that more complicated with more bookkeeping it has to do to know what to do when you modify the variable   So if you re read-only  paradoxically it s better not the point that out with the  amp    I know  it s counter intuitive  but it s also true  Foreach isn t slow   And for simple iteration  the condition it s testing against      am I at the end of this array      is done using native code  not PHP opcodes   Even if it s APC cached opcodes  it s still slower than a bunch of native operations done at the bare metal  Using a for loop  for   i 0   i  lt  count  x    i    is slow because of the count    and the lack of PHP s ability  or really any interpreted language  to evaluate at parse time whether anything modifies the array   This prevents it from evaluating the count once  But even once you fix it with   c count  x   for   i 0   i lt  c   i    the  i lt  c is a bunch of Zend opcodes at best  as is the  i     In the course of 100000 iterations  this can matter   Foreach knows at the native level what to do   No PHP opcodes needed to test the  am I at the end of this array  condition  What about the old school  while list   stuff   Well  using each    current    etc  are all going to involve at least 1 function call  which isn t slow  but not free   Yes  those are PHP opcodes again   So while   list   each has its costs as well    For these reasons foreach is understandably the best option for simple iteration   And don t forget  it s also the easiest to read  so it s win-win

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