Making perl evaluate a variable with a string

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

I'd like perl to replace $variables in a file. The vars are simply listed as $a, $a+50, $b etc in a file, which I read. I use the following:

$a=1;
$b=2;

....
    while ($line = <INFILE>)    {
        chomp($line);   # contains $a, $b
        print "$line\n"; #prints "$a" "$b"
[download]

However, perl ignores all the $variables and print them explicitly. How do I make perl specify the variable value when prining $line?

Thx,

Comment on Making perl evaluate a variable with a string Download Code

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Re: Making perl evaluate a variable with a string by jwkrahn (Abbot) on Apr 01, 2007 at 15:34 UTC
How can I expand variables in text strings?	[reply]
Re: Making perl evaluate a variable with a string by shmem (Chancellor) on Apr 01, 2007 at 15:35 UTC
What do you expect your code `$a=1; $b=2; .... while ($line = <INFILE>) { chomp($line); # contains $a, $b print "$line\n"; #prints "$a" "$b"` [download] to do? Please provide a sample of the output you're trying to get. It's not clear what you are trying to achieve. See I know what I mean. Why don't you? If you read text to print it later, you print text. If you want to replace text (eg. "$a" with "1"), do so with `s/\$a/1/` - see perlre. If you want perl to evaluate the file, say so with eval, do or require. Evaluating a file whilst reading it linewise is tricky and outright impossible if those lines aren't self-contained perl statements (e.g. if the file contains control structures spanning several lines). --shmem _($_=" "x(1<<5)."?\n".q·/)Oo. G°\ / /\_¯/(q / ---------------------------- \__(m.====·.(_("always off the crowd"))."· ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}	[reply] [d/l] [select]
Re^2: Making perl evaluate a variable with a string by moked (Beadle) on Apr 01, 2007 at 15:57 UTC
Hi again I have a template script for another tool. from the template I want generate many scripts with different numerical values (not only replace $a with 1, but also replace in the template script, $a+5 with 6...) So, I have a template script and a variable definintion file which I call with "require" How best to implement? Sure I can do it with =~s/.../ but if I can use perl to make the sub in a native manner ($var) it will be much stronger. Thx, Nir	[reply]
Re^3: Making perl evaluate a variable with a string by klekker (Pilgrim) on Apr 01, 2007 at 16:20 UTC
I am not quite sure if I understand your post right, but before you write your own template engine (I interpret your text that you are up to it), have a look at http://www.perl.com/pub/a/2001/08/21/templating.html and/ or http://perl.apache.org/docs/tutorials/tmpl/comparison/comparison.html k	[reply]
Re: Making perl evaluate a variable with a string by Anonymous Monk on Apr 02, 2007 at 02:57 UTC
Since '$line' is a variable, the compiler doesn't know what it contains : variable names, prayers, profanity, ... . You want your program to evaluate the contents of $line as Perl code. Look up the 'eval' function. Hope this helps.	[reply]