eval function

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

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Re: eval function by graff (Chancellor) on Aug 18, 2005 at 05:47 UTC
You're missing a couple of things: <code> and </code> around your script, so we can read it more easily as code (you've been here long enough now that you should know this) a simple, concise statement about what you actually want, and how that's different from what you're getting. Oh, and maybe you're missing the difference between single quotes and double quotes? I'm not sure, but if you wanted to get "$data{value} abcd.1000.xyz", then you should use double quotes instead of single quotes in the 3rd line. ~~Apart from that, I don't understand what you expect to accomplish with that eval statement.~~ Sorry -- I think I get it now. You want to put a variable name into a string assignment, then later on, you want to assign a value to a variable having that name, and have it interpolated into the string. There's a better way to do that: `$data{value} = 'abc.VALUE.xyz'; sub process_value { my $digit = 1000; $data{value} =~ s/VALUE/$digit/; print "\$data{value} $data{value}\n" } process_value();` [download]	[reply] [d/l]
Re: eval function by Anonymous Monk on Aug 18, 2005 at 10:19 UTC
`eval { }`, that is, the block form of eval, just executes the code in the block, trapping any `die`s. An eval block where the code just contains a variable (as in your case), only makes sense if the variable is tied - with it isn't in your case. What you probably want is: `$data{value} = eval "qq{$data{value}}";` [download] Note the double double quoting of the variable. And note that the double double in neither the previous sentence, nor the current one are typos.	[reply] [d/l] [select]
Re: eval function by anonymized user 468275 (Curate) on Aug 18, 2005 at 09:31 UTC
Rather than find an alternative with s/, it seems more appropriate to address the actual mistake, which was to use single quotes (causing the contents to be interpreted literally) where double quotes were required i.e. `$data{value} = "abcd.$digit.xyz";` [download] in which perl will indeed apply the required substitution. One world, one people	[reply] [d/l]
Re^2: eval function by swaroop (Beadle) on Aug 18, 2005 at 09:51 UTC
Hi, If we give double quotes , The output will be "abcd..xyz". Its because of perl assignes the value of $digit to $data{value} in the begining itself. Anyways , thanks for the reply. - swaroop	[reply]
Re^3: eval function by anonymized user 468275 (Curate) on Aug 18, 2005 at 10:54 UTC
Perhaps so, but there's no quick fix then, because $data{ value } is trying to do too many jobs -- it seems to contain a formula for setting itself which then destroys the formula if needed for subsequent iterations. I think at this point a functional description of the requirement is needed before diving into any solution. One world, one people	[reply]
Re: eval function by Tanktalus (Canon) on Aug 18, 2005 at 20:41 UTC
It's not entirely clear to me what you're trying to accomplish, nor to anyone else, it seems. Perhaps it's this: `use strict; use vars qw ( %data $digit ); $data{value} = 'abcd.$digit.xyz'; sub process_value { $digit = 1000; $data{value} = eval qq["$data{value}"]; print "\$data{value} $data{value} \n"; } &process_value;` [download] Output is: `$data{value} abcd.1000.xyz` [download]	[reply] [d/l] [select]
Re: eval function by japhy (Canon) on Aug 18, 2005 at 13:35 UTC
You can't just eval() like you did. First, you can't use the BLOCK form of eval. Secondly, you'll need multiple levels of evaluation. You could use my DynScalar module on CPAN if you'd like to do this seamlessly. Jeff `japhy` Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and `perl` hacker How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart	[reply]