Best way to iterate through a Perl array

Question

Which is the best implementation in terms of speed and memory usage  for iterating through a Perl array  Is there any better way    Array need not be retained    Implementation 1  foreach   Array          SubRoutine          Implementation 2  while  Element shift  Array           SubRoutine  Element       Implementation 3  while scalar  Array    0           Element shift  Array         SubRoutine  Element       Implementation 4  for my  i  0      Array          SubRoutine  Array  i        Implementation 5  map   SubRoutine        Array

User · Answer

In single line to print the element or array.

print $_ for (@array);

NOTE: remember that $_ is internally referring to the element of @array in loop. Any changes made in $_ will reflect in @array; ex.

my @array = qw( 1 2 3 );
for (@array) {
        $_ = $_ *2 ;
}
print "@array";

output: 2 4 6

User · Answer

The best way to decide questions like this to benchmark them   use strict  use warnings  use Benchmark qw  all    our  input array    0  1000    my  a   sub       my  array        input array         my  index   0      foreach my  element   array           die unless  index     element          index              my  b   sub       my  array        input array         my  index   0      while  defined my  element   shift  array            die unless  index     element          index              my  c   sub       my  array        input array         my  index   0      while  scalar  array    0           my  element   shift  array          die unless  index     element          index              my  d   sub       my  array        input array         foreach my  index  0     array           my  element    array  index          die unless  index     element            my  e   sub       my  array        input array         for  my  index   0   index  lt     array   index             my  element    array  index          die unless  index     element            my  f   sub       my  array        input array         while  my   index   element    each  array           die unless  index     element            my  count  timethese  count        1    gt   a      2    gt   b      3    gt   c      4    gt   d      5    gt   e      6    gt   f        And running this on perl 5  version 24  subversion 1  v5 24 1  built for x86 64-linux-gnu-thread-multi  I get   Benchmark  running 1  2  3  4  5  6 for at least 3 CPU seconds             1   3 wallclock secs   3 16 usr    0 00 sys    3 16 CPU    12560 13 s  n 39690           2   3 wallclock secs   3 18 usr    0 00 sys    3 18 CPU    7828 30 s  n 24894           3   3 wallclock secs   3 23 usr    0 00 sys    3 23 CPU    6763 47 s  n 21846           4   4 wallclock secs   3 15 usr    0 00 sys    3 15 CPU    9596 83 s  n 30230           5   4 wallclock secs   3 20 usr    0 00 sys    3 20 CPU    6826 88 s  n 21846           6   3 wallclock secs   3 12 usr    0 00 sys    3 12 CPU    5653 53 s  n 17639    So the  foreach   Array   is about twice as fast as the others   All the others are very similar    ikegami also points out that there are quite a few differences in these implimentations other than speed

User · Answer

If you only care about the elements of  Array  use   for my  el   Array              or   If the indices matter  use   for my  i  0      Array              Or  as of perl 5 12 1  you can use   while  my   i   el    each  Array              If you need both the element and its index in the body of the loop  I would expect using each to be the fastest  but then you ll be giving up compatibility with pre-5 12 1 perls   Some other pattern than these might be appropriate under certain circumstances

User · Answer

IMO  implementation  1 is typical and being short and idiomatic for Perl trumps the others for that alone   A benchmark of the three choices might offer you insight into speed  at least

User · Answer

In terms of speed   1 and  4  but not by much in most instances   You could write a benchmark to confirm  but I suspect you ll find  1 and  4 to be slightly faster because the iteration work is done in C instead of Perl  and no needless copying of the array elements occurs      is aliased to the element in  1  but  2 and  3 actually copy the scalars from the array      35 5 might be similar  In terms memory usage  They re all the same except for  5   for   a  is special-cased to avoid flattening the array  The loop iterates over the indexes of the array  In terms of readability   1  In terms of flexibility   1  4 and  5     35 2 does not support elements that are false   2 and  3 are destructive

User · Answer

1 is substantially different from 2 and 3  since it leaves the array in tact  whereas the other two leave it empty   I d say  3 is pretty wacky and probably less efficient  so forget that   Which leaves you with  1 and  2  and they do not do the same thing  so one cannot be  better  than the other   If the array is large and you don t need to keep it  generally scope will deal with it  but see NOTE   so generally   1 is still the clearest and simplest method   Shifting each element off will not speed anything up  Even if there is a need to free the array from the reference  I d just go   undef  Array    when done    NOTE  The subroutine containing the scope of the array actually keeps the array and re-uses the space next time  Generally  that should be fine  see comments

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