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Scalar context with list and range operator

by vinoth.ree (Parson)
on May 28, 2013 at 04:38 UTC ( #1035518=perlquestion: print w/ replies, xml ) Need Help??
vinoth.ree has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks!
#!/usr/bin/env perl use strict; use warnings; my @array = ( 1..5 ); my $array_count = @array; my $scalar_context = ( 1..5 ); print "$array_count\n"; print "$scalar_context\n";

In the above code where I expected $array_count to be 5 and $scalar_context to be 5, the last element, but surprise, $scalar_context turns out to be an empty string. Instead of using range operator I tried this, my $scalar_context = ( 1, 2, 3, 4, 5); the result is 5 indeed.

Also I am getting the following error,

Use of uninitialized value $. in range (or flip) at pl2.pl line 8.

Is that error related to range operator in list context? Thanks in advance.


All is well

Comment on Scalar context with list and range operator
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Re: Scalar context with list and range operator (flip flop)
by Anonymous Monk on May 28, 2013 at 04:52 UTC
Re: Scalar context with list and range operator
by kcott (Abbot) on May 28, 2013 at 05:06 UTC

    G'day vinoth.ree,

    All of these issues are explained in perlop - Range Operators. Quickly pointing you to the relevant parts:

    • "Binary ".." is the range operator, which is really two different operators depending on the context. In list context, it returns ..." (doco mirrors your expectations)
    • Two paragraphs down: "In scalar context, ".." returns ..." (doco explains how a false value, i.e. your 'empty string', is returned)
    • Two more paragraphs down: "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)." (explanation for your "uninitialized value $." warning)

    -- Ken

      Thanks, kcott

      In scalar context where 1..5 is no longer a range, but a flip-flop operator?


      All is well

        Yes.

Re: Scalar context with list and range operator
by choroba (Abbot) on May 28, 2013 at 07:10 UTC

      Oh nice!,Its Coincidence I guess, Today I was explaining the list context and scalar context assignment to my colleague, I usually write the full list like this (1,2,3,4,5), but today i tried with range operator and got this issue. I have tried for something and got something, anyhow learned flip-flop operator.


      All is well
Re: Scalar context with list and range operator
by hdb (Prior) on May 28, 2013 at 08:13 UTC

    I have been trying a few variants and in addition to the "flip flop complication" I also got interference with print interpreting the parantheses as the function call operator.

    my $x = ( 1 .. 5 ); # scalar context, flip flop print "$x <= 1\n"; # " <= 1\n" as $x is empty string print ( 1 .. 5 ), " <= 2\n"; # "12345" but no " <= 2\n", ( ) consumed + by print print +( 1 .. 5 ), " <= 3\n"; # "12345 <= 3\n" my $y = () = ( 1 .. 5 ); # enforces list context but then assigne +d to scalar print "$y <= 4\n"; # "5 <= 4\n"

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