Nested Maps in Perl

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Nested Maps in Perl

October 4, 2010 by Jared

I’m not really a fan of nested loops, so when I need to create a list of combinations based on two or three other lists, I really miss list comprehensions¹ such as those in Python.

l = [(x,y) for x in range(5) for y in range(5)]

Very elegant.

If I was using Lisp, I might use a nested mapcar.

(mapcar (lambda (out) (mapcar (lambda (in) (cons in out))
                              '(a b c d e)))
          '(1 2 3 4 5))

Perl map uses $_ so you don’t need to explicitly specify the name of the lambda parameter. How can I differentiate between the outer $_ and the inner $_?

my $outer;
my @x = map { $outer = $_;
              (map { $outer . $_ } qw(a b c d e)) }
            (1..5);

Note: if you are trying to do something as simple as this, take a look at Set::CrossProduct instead.

1. It’s available on the CPAN of course

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Posted in Perl | Tagged functional programming, map, nested loops | 11 Comments

11 Responses

on October 4, 2010 at 05:58 Naveed Massjouni

Same concept, but this perl example is slightly nicer in my opinion, and it’s closer to the original python.

my @l = map { my $x = $_; map [$x, $_], 1 .. 5 } 1 .. 5;
on October 4, 2010 at 07:49 zloyrusskiy

You can look at List::Gen module, it has mapkey() and cartesian() functions for that.

I think that Perl needs thats feature in future.
on October 4, 2010 at 15:17 Jared

@Naveed – oh yes, the inner map is inside the outer map’s lexical scope so it is possible to my declare the variable inside the map. That makes it a lot neater already.

@zloyrusskiy – The $_{first}, $_{second}, etc… from List::Gen looks like a good generalisation of the technique I presented in my post. It would be nice if it was available in core Perl.
on October 4, 2010 at 22:34 juster

I know this isn’t exactly what you asked for but I found this interesting when I discovered this feature of glob:

my @x = glob( ‘{a,b,c,d,e}{1,2,3,4,5}’ );

Hehe!
on October 5, 2010 at 03:29 Naveed Massjouni

@juster, that is very clever :)
on October 6, 2010 at 22:33 Jared

Hey juster, I knew about this trick in shell, but I didn’t know it worked in perl’s glob. And it’s portable too!
```
$ c:/strawberry/perl/bin/perl test-juster.pl 
a1a2a3a4a5b1b2b3b4b5c1c2c3c4c5d1d2d3d4d5e1e2e3e4e5
$ 
```
on October 18, 2010 at 22:00 Using the diamond operator to find the Cartesian product of two lists. « Discovering Perl

[...] should go where they are due: I discovered this construct while reading juster’s comment on [...]
on April 10, 2012 at 20:04 vol7ron

my $t = [['a'..'e'],[1..5]];
my @x = map { $t->[0]->[$_] => $t->[1]->[$_] } 0..$#{$t->[0]};

There might be a better way than the above, but this is an alternative.
on April 10, 2012 at 20:06 vol7ron

my $x = { map { $t->[0]->[$_] => $t->[1]->[$_] } 0..$#{$t->[0]} };
#hashref
on June 7, 2012 at 10:59 tajuma

You can pass the outer $_ as a parameter to the inner map put inside a sub, and then refer the outer $_ in the inner as $_[0]:

map { qw(a b c d e) } }($_) } (1..5)
on June 7, 2012 at 11:08 tajuma

Ouch, something ate half of the perl code in my previous comment, new try

map { &{ sub { map { $_[0] . $_ } qw(a b c d e) } }($_) } (1..5)