colink has asked for the wisdom of the Perl Monks concerning the following question:

Running this code with -w, use strict or use diagnostics doesn't reveal anything useful, except that when the variables are lexically scoped with my that the shell escapes seem to have different precedence, being interpreted as an array (which doesn't exist) instead of a scalar. Does anyone have any ideas why? 
#!/usr/bin/perl

$a = 'name';
$b = 5;

my $c = 'name';
my $d = 5;

print "${a}[$b]" eq "${c}[$d]" ? "ok\n" : "booger\n";

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Re: variable scope affects variables in crace shell escapes
by DaveH (Monk) on Mar 30, 2003 at 11:03 UTC

    Hi.

    I did a double take on this one, because at first it wasn't at all clear to me what was happening. I think that most of this confusion was because you called "" (double quotes) shell escapes. What you are referring to is not "shell escaping", it is "double quote interpolation".

    Anyway, the problem you are having seems to be related to Perl's DWIM ("Do What I Mean") attitude to string interpolation, and it auto-vivifying arrays when you don't want them. In your example code, you are auto-vivifying two arrays: @a and @c. If you want to know more details, I suggest inserting the following at the top of your script:

    use strict;
use warnings;
use diagnostics;

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    Fortunately, there is a very simple solution - if you don't want Perl to treat square brackets ("[" and "]") as array subscripts, then escape them with a backslash ("\") in your strings.

    $a = 'name';
$b = 5;

my $c = 'name';
my $d = 5;

print "$a\[$b\]" eq "$c\[$d\]" ? "ok\n" : "booger\n";

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    (As an aside, you should really try to avoid $a and $b as variable names. Whilst I'm sure they were used just as an illustration, these are reserved variables which are used in the 'sort' built-in function. Although "use strict" won't complain about them, there is a good reason for this: they are always package global symbols. Moral: always use 'my' and stay away from $a and $b.)

    I hope that helps.

    Cheers,

    -- Dave :-)

    $q=[split+qr,,,q,~swmi,.$,],+s.$.Em~w^,,.,s,.,$&&$$q[pos],eg,print
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      Well, I think you missed the essence of my question. "shell escapes" are straight from the perldata manpage:

      As in some shells, you can enclose the variable name in braces to disambiguate it from following alphanumerics (and underscores). You must also do this when interpolating a variable into a string to separate the variable name from a following double‐colon or an apostrophe, since these would be otherwise treated as a package separator. 

      $a[$b]  is the 6th element of the @a array

${a}[$b] is supposed to be the same as

join '', $a, '[', $b, ']';

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      but if you declare the variable $a to be lexically scoped with my then perl seems to want to force them the construct to be considered as an array, which is wrong.

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Re: variable scope affects variables in crace shell escapes
by robartes (Priest) on Mar 31, 2003 at 05:51 UTC
    This is a bug in Perl. It has recently been fixed (patch 18687, if you're interested :) ). It is currently working in bleadperl (the 'head' of the current development version) and will be fixed in 5.8.1. For more information, see this thread on perl5porters.

    If you're critically dependent on this being fixed, you will have to roll your own Perl for now, I'm afraid.

    CU
    Robartes-

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      Cool! But could I have saved a silly bug report by some more careful searching? I tried perlbug at bugs6.perl.org, but nothing turned up, but didn't think of trying the p5p archives. I will for my next bug report, but maybe I'll try posting here first. Thank you very much!
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