Most efficient way to swap two array values in Perl

Question

I was curious what the most efficient way to swap two elements in an array in Perl would be.

Say I want to switch the elements with indexes of 3 and 5 respectively, what would be the fastest way to do that?

My thoughts were something like:

@array[3,5] = @array[5,3];

Is this very efficient? My hope is that it wouldn't make a difference if those two array values contained large amounts of data. I would expect the back-end to change the pointers around, without doing any actual copying. Is that correct?

Answer 1

What do you mean "large amounts of data"? If you mean references to large data structures, it doesn't matter how you do it; it will be crazy fast. (Less than one nanosecond per swap.) So I'm going to assume you meant "large strings".

---

Before 5.20, the code you posted would copy the strings. You can avoid that using Data::Alias.

use Data::Alias;
alias @array[3,5] = @array[5,3];

Or you can do the same with a little XS if you want to avoid the syntax magic of Data::Alias:

void swap_array_elements(AV * av, IV ix1, IV ix2) {
   SV ** p_ele1 = av_fetch(av, ix1, 1);
   SV ** p_ele2 = av_fetch(av, ix2, 1);
   SV * sv = *p_ele1;
   *p_ele1 = *p_ele2;
   *p_ele2 = sv;
}

---

Since 5.20, the string copies were replaced with a simple pointer swap.

$ 5.18t/bin/perl -MDevel::Peek -e'
   my @a = qw( a b c d e f );
   Dump($a[3]);
   Dump($a[5]);
   @a[3,5] = @a[5,3];
   Dump($a[3]);
   Dump($a[5]);
' 2>&1 | grep '^  PV'
  PV = 0x353fba0 "d"\0
  PV = 0x357abb0 "f"\0
  PV = 0x3541ff0 "f"\0
  PV = 0x3537940 "d"\0

$ 5.20t/bin/perl -MDevel::Peek -e'
   my @a = qw( a b c d e f );
   Dump($a[3]);
   Dump($a[5]);
   @a[3,5] = @a[5,3];
   Dump($a[3]);
   Dump($a[5]);
' 2>&1 | grep '^  PV'
  PV = 0xe9ace0 "d"\0
  PV = 0xe8f230 "f"\0
  PV = 0xe8f230 "f"\0
  PV = 0xe9ace0 "d"\0

This means that the straightforward @array[3,5] = @array[5,3]; will do just fine.

Answer 2

Running bvr's benchmark script with a modification:

my @array = ('thing' x 1E5, 'stuff' x 5E5, 'that' x 1E6, 'joe' x 5E6,
             'sofa' x 1E7, 'jim' x 5E7);

Results:

           Rate      slice       temp      alias
slice    1.98/s         --       -69%      -100%
temp     6.47/s       227%         --      -100%
alias 4218715/s 213045016%  65227727%         --