Posts Tagged ‘map’

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)) }

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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