Variable Interpolation Interpolation

[dpatrick]

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 };
    }

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

[broquaint]

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::);

For further info on why symbolic references are generally a no-no check out Dominus' enlightening articles here, here and here.
HTH

broquaint

[dragonchild]

It's naughty to use symbolic refrences, but the way to do it is:

if ${"fromQuery::$f"};

------
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.

[dpatrick]

Dragonchild, why is it naughty to use symbolic references?

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

[dragonchild]

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.

[knobunc]

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);
}

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);
}

-ben