interleaving lists

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

What is the most efficient (or most elegant, for that matter) way of interleaving lists in Perl? For example, let's say I have two lists:

( 'blorke', 'gromble', 'poomba' ) and ( 'snork', 'dumble', 'gronke' ). I'd like to end up with:

( 'blorke', 'snork', 'gromble', 'dumble', 'poomba', 'gronke' )

In my case, I'm filling a hash, but I can think of other reasons to do this as well.

I'd prefer if it were self-contained (i.e. had no need for temporary or index variables).

One obvious solution:

my @list1 = ( 'blorke', 'gromble', 'poomba' );
my @list2 = ( 'snork', 'dumble', 'gronke' );
my %index = map { $_ => pop @list2 } (@list1);
[download]

...but that's destructive of @list2 (and I rather feel like it violates the spirit of map ... should probably have used for). Are there better (maybe non-destructive) ways?

Comment on interleaving lists Download Code

Replies are listed 'Best First'.
Re: interleaving lists by chipmunk (Parson) on Jan 23, 2001 at 02:52 UTC
For filling a hash, assignment to a hash slice is probably the best way: `my %index; @index{@list1} = @list2;` [download] If you actually want a list as the result, rather than a hash, a for loop would work: `for (my $i = 0; $i <= $#list1; $i++) { push @result, $list1[$i], $list2[$i]; }` [download] The push could also be done as a nested loop, if there are lots of arrays to interleave. Anyway, that's a very basic solution; much fancier solutions could be written to satisfy more requirements for the interleaving.	[reply] [d/l] [select]
Re (tilly) 1: interleaving lists by tilly (Archbishop) on Jan 23, 2001 at 05:34 UTC
This will handle any number of lists at once without destroying the originals. Pass them in by reference and it will return by reference or value, your choice. `sub interleave { my @lists = map [@$_], @_; my @res; while (my $list = shift @lists) { if (@$list) { push @res, shift @$list; push @lists, $list; } } wantarray ? @res : \@res; }` [download]	[reply] [d/l]
(tye)Re: interleaving lists by tye (Sage) on Jan 23, 2001 at 04:32 UTC
`my @combined= mapcar {@_} \@list1,\@list2;` [download] See mapcar -- map for more than one list for a version of mapcar in Perl. - tye (but my friends call me "Tye")	[reply] [d/l]
Re: interleaving lists by Fastolfe (Vicar) on Jan 23, 2001 at 05:15 UTC
I might do it: `@combined = map { $list1[$_], $list2[$_] } 0 .. ($#list1 > $#list2 ? $#list1 : $#list2);` [download] This inserts 'undef's in the event one array is longer than the other.	[reply] [d/l]
oooh by Anonymous Monk on Jan 23, 2001 at 22:31 UTC
Ahhh, elegant. Thanks, this is exactly what I was looking for.	[reply]
Re: oooh by Fastolfe (Vicar) on Jan 23, 2001 at 22:48 UTC
Of course if you're filling a hash, it's already been mentioned that this is probably the best solution: `my %hash; @hash{@keys} = @values;` [download]	[reply] [d/l]
Re: Re: interleaving lists by rmgiroux (Initiate) on Jan 23, 2001 at 18:32 UTC
Nice!	[reply]
Re: interleaving lists by Coyote (Deacon) on Jan 23, 2001 at 03:13 UTC
I'm sure there better ways of doing this, but ... `use strict; use warnings; my @array_1 = qw(blorke gromble poomba); my @array_2 = qw(snork dumble gronke); my (@array_3); foreach (@array_1){ push @array_3, $_, shift(@array_2); } foreach (@array_3){ print "$_\n"; }` [download] This yields: blorke snork gromble dumble poomba gronke I am assuming here that your arrays are going to be the same size. You might want to check the size first. This also kills `@array_2`, so work on a copy if you need to keep the original. ---- Coyote	[reply] [d/l] [select]
Re: interleaving lists by InfiniteSilence (Curate) on Jan 23, 2001 at 19:44 UTC
Hey, how about this: `perl -e "@m=(1,2,3); @s=(4,5,6,7,8); for (0 ..scalar(@m + @s)) {print +$m[$_], $s[$_] };"` [download] Prints out: 14253678 Celebrate Intellectual Diversity	[reply] [d/l]