perl,reference,subroutine , Backslash before a subroutine call


Backslash before a subroutine call

Question:

Tag: perl,reference,subroutine

As I was understanding the difference between [] and \ in references,I used both on subroutine the former was fine but when I tried later I thought it should give error but the below program in perl

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my @b;
for my $i ( 0 .. 10 ) {
    $b[$i] = \somefunc($i);

}
print Dumper( \@b );

sub somefunc {
    my $n = shift;
    my ( @a, $k );
    for my $j ( 11 .. 13 ) {
        $k = $n * $j;
        push( @a, $k );
    }
    print "a: @a \n";
    return @a;
}

gives output as :

a: 0 0 0 
a: 11 12 13 
a: 22 24 26 
a: 33 36 39 
a: 44 48 52 
a: 55 60 65 
a: 66 72 78 
a: 77 84 91 
a: 88 96 104 
a: 99 108 117 
a: 110 120 130 
$VAR1 = [
      \0,
      \13,
      \26,
      \39,
      \52,
      \65,
      \78,
      \91,
      \104,
      \117,
      \130
    ];

I was unable to understand the output.Need explanation.


Answer:

What is happening here is:

You are returning an array from somefunc.

But you are assigning it to a scalar. What this is effectively doing therefore, is simply putting the last value in the array, into the scalar value.

my $value =  ( 110, 120, 130 );
print $value;

When you do this - $value is set to the last value in the array. This is what's actually happening in your code. See for example perldata:

List values are denoted by separating individual values by commas (and enclosing the list in parentheses where precedence requires it): (LIST)

In a context not requiring a list value, the value of what appears to be a list literal is simply the value of the final element, as with the C comma operator. For example,

@foo = ('cc', '-E', $bar);

assigns the entire list value to array @foo, but

foo = ('cc', '-E', $bar);

assigns the value of variable $bar to the scalar variable $foo. Note that the value of an actual array in scalar context is the length of the array; the following assigns the value 3 to $foo:

@foo = ('cc', '-E', $bar); $foo = @foo; # $foo gets 3

It's this latter case that's often the gotcha, because it's a list in a scalar context.

And in your example - the backslash prefix denotes 'reference to' - which is largely meaningless because it's a reference to a number.

But for a scalar, it might be more meaningful:

my $newvalue = "fish"; 
my $value =  ( 110, 120, 130, \$newvalue );
print Dumper $value;

$newvalue = 'barg'; 
print Dumper $value;

Gives:

$VAR1 = \'fish';
$VAR1 = \'barg';

That's why you're getting the results. Prefix with the slash indicates that you're getting a reference to the result, not a reference to the sub. Reference to 130 isn't actually all that meaningful.

Normally, when doing the assignment above - you'd get a warning about Useless use of a constant (110) in void context but this doesn't apply when you've got a subroutine return.

If you wanted to insert a sub reference, you'd need to add &, but if you just want to insert the returned array by reference - you either need to:

$b[$i] = [somefunc($i)]

Or:

return \@a;

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