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

Hi Monks 
#!/user/bin/perl -w
upper($1,$2);
sub upper 
{
  for(@_)
   {
     tr/a-z/A-Z/;
    }
}

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I am trying to modify the nameless farmal arguments to actual arguments ,the above code is giving an error as "modification to readonly value attempted".so,suggest me so,that i can trap my mistake.

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Re: How to modify the actual arguments
by Zaxo (Archbishop) on Jun 17, 2006 at 19:01 UTC

    $1 and $2 are read-only, set internally by perl as a result of regex matching. If you call upper() on $foo and $bar, your function will do what you intend.

    You can guard against fatal errors by wrapping in eval{}. You can catch some such errors at compile time with the (\$) prototype, but that wouldn't catch an attempt to modify $1.

    Your function can be made more independent of language by writing it with uc, 

    sub upper {
    eval {
        $_ = uc for @_;
        1;
    }
}

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    After Compline,
    Zaxo

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Re: How to modify the actual arguments
by GrandFather (Saint) on Jun 17, 2006 at 19:07 UTC

    There is something you are not telling us.

    Update: your function is fine, it is your call that is the issue as Zaxo pointed out.

    use strict;
use warnings;

my $str1 = "This is string 1.";
my $str2 = "String 2 this one is.";

upper ($str1, $str2);
print "$str1 $str2";

sub upper {
  tr/a-z/A-Z/ for @_;
}

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    Prints:

    THIS IS STRING 1. STRING 2 THIS ONE IS.

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    DWIM is Perl's answer to Gödel
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Re: How to modify the actual arguments
by sgifford (Prior) on Jun 17, 2006 at 19:44 UTC
    If that's your whole program, I suspect you mean $ARGV[0] and $ARGV[1], which refer to the command-line arguments to your program, and not $1 and $2, which refer to the last two captures from a regular expression match. Making that change, your program doesn't give an error.

    --
    sgifford's Web page
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      HI Monk, thankx for suggation,i am trying to modify the constants in this program,so,we can't use the subscript on constant items.
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