Question

Task: to build hash using map, where keys are the elements of the given array @a, and values are the first elements of the list returned by some function f($element_of_a):

my @a = (1, 2, 3);
my %h = map {$_ => (f($_))[0]} @a;

All the okay until f() returns an empty list (that’s absolutely correct for f(), and in that case I’d like to assign undef). The error could be reproduced with the following code:

my %h = map {$_ => ()[0]} @a;

the error itself sounds like “Odd number of elements in hash assignment”. When I rewrite the code such that:

my @a = (1, 2, 3);
my $s = ()[0];
my %h = map {$_ => $s} @a;

or

my @a = (1, 2, 3);
my %h = map {$_ => undef} @a;

Perl does not complain at all.

So how should I resolve this — get first elements of list returned by f(), when the returned list is empty?

Perl version is 5.12.3

Thanks.

Answer 1

I’ve just played around a bit, and it seems that ()[0], in list context, is interpreted as an empty list rather than as an undef scalar. For example, this:

my @arr = ()[0];
my $size = @arr;
print "$size\n";

prints 0. So $_ => ()[0] is roughly equivalent to just $_.

To fix it, you can use the scalar function to force scalar context:

my %h = map {$_ => scalar((f($_))[0])} @a;

or you can append an explicit undef to the end of the list:

my %h = map {$_ => (f($_), undef)[0]} @a;

or you can wrap your function’s return value in a true array (rather than just a flat list):

my %h = map {$_ => [f($_)]->[0]} @a;

(I like that last option best, personally.)

---

The special behavior of a slice of an empty list is documented under “Slices” in perldata:

A slice of an empty list is still an empty list. […] This makes it easy to write loops that terminate when a null list is returned:

while ( ($home, $user) = (getpwent)[7,0]) {
    printf "%-8s %s\n", $user, $home;
}