How to evaluate a variable in a text string

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

What would be a good way to evaluate a variable in a text
string and store it into another variable? Couldn't get 'eval' to work.

I am trying to do something like -

#!/usr/local/bin/perl

$val = '123';

open (FILE), "input") || die "Can't open for reading: $!\n";
while ( defined ( $_ = <FILE> ) ) {
    $string = eval { $val };
    print "<$val>\n";
}
[download]

The input file has the following text string

input:

test_string_$val

Expecting $string to be - test_string_123

Any suggestion?

janitored by ybiC: Balanced <code> tags around code snippet

Comment on How to evaluate a variable in a text string Download Code

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Re: How to evaluate a variable in a text string by Roy Johnson (Monsignor) on Mar 28, 2005 at 21:01 UTC
The non-strict (using symbolic references -- not recommended) way: `$val = '123'; while (<DATA>) { s/\$(\w+)/$$1/g; print; } __DATA__ test_string_$val` [download] The strict-compliant (recommended) way: `use strict; use warnings; my %subs = (val => '123'); while (<DATA>) { s/\$(\w+)/$subs{$1}/g; print; } __DATA__ test_string_$val` [download] Please read Why it's stupid to use a variable as a variable name Caution: Contents may have been coded under pressure.	[reply] [d/l] [select]
Re^2: How to evaluate a variable in a text string by tariqahsan (Beadle) on Mar 28, 2005 at 21:12 UTC
Much appreciated!!!	[reply]
Re: How to evaluate a variable in a text string by RazorbladeBidet (Friar) on Mar 28, 2005 at 20:52 UTC
You are eval'ing $val instead of eval'ing $_ I'm assuming the rest isn't really your code (or it would be failing) so I won't say anything about that. HTH! RBB -------------- "But what of all those sweet words you spoke in private?" "Oh that's just what we call pillow talk, baby, that's all."	[reply]
Re^2: How to evaluate a variable in a text string by tariqahsan (Beadle) on Mar 28, 2005 at 20:58 UTC
Thanks for your reply My mistake in typing up the code snippet It is actually - $string = eval { $_ }; print "<$string>\n";	[reply]
Re^3: How to evaluate a variable in a text string by merlyn (Sage) on Mar 28, 2005 at 21:00 UTC
`$string = eval { $_ };` [download] Presuming $_ is not tied in such a way that it would trigger code that "die"-ed, your snippet is the same as: `$string = $_;` [download] -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply] [d/l] [select]
Re^3: How to evaluate a variable in a text string by tlm (Prior) on Mar 28, 2005 at 22:48 UTC
It looks like you are confusing the "string form" of eval with the "block form". Try `perldoc -f eval` to get the full story, and then merlyn's comment may be clearer. To do what you want to do, you need something like `$string = eval qq("$_");` [download] where $_ contains your 'test_string_$val'. But as Roy already pointed out, this may not be a very wise thing to do in the first place. the lowliest monk	[reply] [d/l] [select]
Re: How to evaluate a variable in a text string by dragonchild (Archbishop) on Mar 28, 2005 at 20:58 UTC
You want to learn about hashes - they will make your life a lot easier. Plus, this smells of a config file. Why not look at the many config options around. They will also make your life even easier than that. :-) Being right, does not endow the right to be rude; politeness costs nothing. Being unknowing, is not the same as being stupid. Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence. Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.	[reply]