What is the Perl context with range operator?

Question

I'm a Perl newbie. I want to understand Perl context in conjunction with range operator. This is my code.

use strict;
use warnings;

my $asc = ( 10 .. 50 );
print "$asc\n";

I have two doubts.

1. If the expression ( 10 .. 50 ) returns an array, then, as it's a scalar context, "asc" variable should be assigned the length of the array, that is, 41.

2. If expression ( 10 ..50 ) returns a list, then, as it's a scalar context, "asc" variable should be assigned the last item from the list, that is, 50.

But, I get the following shout ..

Use of uninitialized value in range (or flip) at main.pl line ..

Appreciate and welcome any guide.

Answer 1

You're working with the Range Operator .. in an scalar context, which is otherwise known as the flip-flop operator.

You should read the entire documentation, but the following excerpts are relevant to your situation:

In scalar context, ".." returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of sed, awk, and various editors.

...

If either operand of scalar ".." is a constant expression, that operand is considered true if it is equal (==) to the current input line number (the $. variable).

The “exact“ error message explains what's going on:

Use of uninitialized value $. in range (or flip)

Basically, Perl interprets this usage as a flip/flop test.

It's testing if the current line number $. is equal to the integer values you specified:

my $asc = ( $. == 10 .. $. == 50 );

However, because you haven't read from a file handle, the $. variable is uninitialized and throws a warning.

Achieving a List Context

It is possible to get the list context behavior that you described, but you'll need to adjust the code to make your intent more explicit:

my $count = () = (10..50);          # Forces a list context
my $last_element = (10..50)[-1];    # Also forces a list context

print "$count\n";
print "$last_element\n";

Outputs:

41
50