Variable Interpolation Interpolation

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

Fellow Monks, I humbly bring before you a piece of code that I have been looking at for about the past hour:

    foreach my $f (@fields) {
        push(@where, qq($f='$fromQuery::{ $f }'))
            if $fromQuery::{ $f };
        push(@searchString, qq($f=).escape($fromQuery::{ $f }))
            if $fromQuery::{ $f };
    }
[download]

What I'm trying to do is interpolate $f into $fromQuery::$f so that we end up with $fromQuery:foo if $fromQuery::foo exists. Does anyone know how this could best be handled?

As an aside, the fromQuery namespace comes from the CGI::import_names object method but that's not really that important.

dpatrick
- I think scsh is cool.
Open Sourceror, Perlmonk
http://perlmonk.org/~dpatrick

Comment on Variable Interpolation Interpolation Download Code

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Re: Variable Interpolation Interpolation by broquaint (Abbot) on Mar 20, 2002 at 19:28 UTC
Ack, symbolic references! Perhaps a better way would be to look in the symbol table of `$fromQuery::` like so `print "found $f!\n" if grep($_ eq $f, keys %fromQuery::);` [download] For further info on why symbolic references are generally a no-no check out Dominus' enlightening articles here, here and here. HTH broquaint	[reply] [d/l]
Re: Variable Interpolation Interpolation by dragonchild (Archbishop) on Mar 20, 2002 at 19:15 UTC
It's naughty to use symbolic refrences, but the way to do it is: `if ${"fromQuery::$f"};` [download] ------ We are the carpenters and bricklayers of the Information Age. Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.	[reply] [d/l]
Re: Re: Variable Interpolation Interpolation by dpatrick (Scribe) on Mar 20, 2002 at 19:20 UTC
Dragonchild, why is it naughty to use symbolic references? dpatrick - I think scsh is cool. Open Sourceror, Perlmonk http://perlmonk.org/~dpatrick	[reply]
Re: Re: Re: Variable Interpolation Interpolation by dragonchild (Archbishop) on Mar 20, 2002 at 19:30 UTC
Doing a search (which I'm too lazy to do) would yield some dozen or so threads on the matter. A quick answer is this - symbolic references directly impact the symbol table. This could result in clobbering a variable you already have. As this is completely dependent on run-time activity, debugging this could be complex. ------ We are the carpenters and bricklayers of the Information Age. Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.	[reply]
Re: Variable Interpolation Interpolation by knobunc (Pilgrim) on Mar 20, 2002 at 19:42 UTC
You can just eval the bit where you want to get the variable's value: `foreach my $f (@fields) { my $val = eval qq{ \$fromQuery::$f }; next unless $val; push(\@where, $val); push(\@searchString, $val); }` [download] But I am sure that dragonchild's solution is faster. The only gotcha is that it won't run under strict but `no strict 'refs';` in the correct scope will turn off that check: `foreach my $f (@fields) { no strict 'refs'; # This will only last in this scope my $val = ${ "fromQuery::$f" }; next unless $val; push(\@where, $val); push(\@searchString, $val); }` [download] -ben	[reply] [d/l] [select]