为什么 Perl 在 boolean 上下文中评估具有单个 undef 值的数组为真？

Question

Why is this snippet of code doesn't prints "2"?

#!/usr/bin/perl

sub get_undef() {
        return undef;
}

my $test1 = get_undef;
my @test2 = get_undef;

print "1\n" unless ($test1);
print "2\n" unless (@test2);

Answer 1

Your @test2 array contains one value ( undef ).

The conditional puts the array in scalar context, which results in the size of the array (1).

If you want $test to be undefined and @test2 to be empty, you can just return; from your sub.

Answer 2

The array @test2 has one element: undef . In scalar context, this array is not empty, so evaluated to true.

Answer 3

Perl lets you have a list with the single value undef. It can be just as meaningful as any other value, although you have to decide what you want it to mean in your context.

In scalar context, the value of an array is the number of elements in that array. Note this is different than the idea of "list in scalar context" which isn't a thing. An array is a container that holds a list and has its own behavior. You can shift an array, but not a list, for instance.

If you wanted to check that an array had at least one defined value, you can use grep :

if( grep { defined } @array ) { ... }