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

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).

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

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

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

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

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

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