symbol table vs. eval

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

I'm currently in a situation in which I need to create variables whose names are determined by other variable values. (this is to support older code which relies on those variables having those names while moving to a new architecture in which the info is stored in a db.

I have:

my $test="my_variable_name"; #imagine this is a hash key
my $default_value="the value I set"; #while this is the hash value
eval("\$$test = '$default_value';");
print "the value of \$my_variable_name = $my_variable_name\n";
$my_variable_name = "A new value";
print "the value of \$my_variable_name = $my_variable_name\n";
[download]

So far, so good. That works.

I'm fairly certain that there is a Better(tm) way to do this using symbol table manipulation, but I'm not sure how to go about it. I've tried all of:

my $test="my_variable_name";
my $default_value="the value I set";
*$test = $default_value;
#*{$test} = $default_value;
#*{"$test"} = $default_value;
#*{"$test"} = \$default_value;
#*{"$test"} = \&default_value;
print "the value of \$my_variable_name = $my_variable_name\n";
$my_variable_name = "A new value";
print "the value of \$my_variable_name = $my_variable_name\n";
[download]

none of which work. (all commented lines have been tried).

Any pointers would be appreciated. Thanks

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Re: symbol table vs. eval by chargrill (Parson) on Feb 23, 2006 at 18:30 UTC
This will do what you want, I think: `my $test="my_variable_name"; my $default_value="the value I set"; $$test = "old value"; print "the value of \$my_variable_name = $my_variable_name\n"; $my_variable_name = "A new value"; print "the value of \$my_variable_name = $my_variable_name\n";` [download] ... but don't do that :) Update: Perhaps I read & posted too quickly, does this do what you want? `my $test="my_variable_name"; my $default_value="the value I set"; $default_value = $test; $$default_value = "old value"; print "the value of \$my_variable_name = $my_variable_name\n"; $my_variable_name = "A new value"; print "the value of \$my_variable_name = $my_variable_name\n";` [download] Update2:* Thanks ikegami. You'll notice I omitted the obligatory "`use strict;`" and "`use warnings;`", because when they're not omitted: `$ perl symbol.pl Global symbol "$my_variable_name" requires explicit package name at sy +mbol.pl line 12. Global symbol "$my_variable_name" requires explicit package name at sy +mbol.pl line 13. Global symbol "$my_variable_name" requires explicit package name at sy +mbol.pl line 14. Execution of symbol.pl aborted due to compilation errors.` [download] --chargrill `$/ = q#(\w)# ; sub sig { print scalar reverse join ' ', @_ } + sig map { s$\$/\$/$\$2\$1$g && $_ } split( ' ', ",erckha rlPe erthnoa stJu +" );` [download]	[reply] [d/l] [select]
Re^2: symbol table vs. eval by raflach (Pilgrim) on Feb 23, 2006 at 18:49 UTC
A clear case of my overthinking the problem. Your first example does what I am needing. However I wonder if you can explain why the following occurs: Code: `package apackage; my $test="my_variable_name"; my $default_value="the value I set"; $$test = $default_value; print "the value of \$my_variable_name = $my_variable_name\n"; package main; $my_variable_name = "A new value"; print "the value of \$my_variable_name = $my_variable_name\n"; print "the value of the package variable = $apackage::my_variable_name +\n";` [download] Output: `the value of $my_variable_name = the value I set the value of $my_variable_name = A new value the value of the package variable = the value I set` [download] which works and is what I need, but... Code: `package apackage; my $test="my_variable_name"; my $default_value="the value I set"; our $$test = $default_value; print "the value of \$my_variable_name = $my_variable_name\n"; package main; $my_variable_name = "A new value"; print "the value of \$my_variable_name = $my_variable_name\n"; print "the value of the package variable = $apackage::my_variable_name +\n";` [download] Output: `Can't declare scalar dereference in our at -e line 5, near "$test =" Execution of -e aborted due to compilation errors.` [download]	[reply] [d/l] [select]
Re^3: symbol table vs. eval by ikegami (Patriarch) on Feb 23, 2006 at 18:59 UTC
You need to use `our $my_variable_name;` You may thing hardcoding the name is a problem, but it's not. The only reason you need `our` is because you hardcoded the name in the `print`. For example, `use strict; use warnings; package Hack; sub slash { my $var_ref = do { no strict 'refs'; \${'ExistingPackage::var'} }; $$var_ref = 'bar'; } package main; ExistingPackage->print_it(); Hack->slash(); ExistingPackage->print_it();` [download] whether ExistingPackage is `use strict; use warnings; package ExistingPackage; our $var = 'foo'; sub print_it { print("$var\n"); } 1;` [download] or `package ExistingPackage; $var = 'foo'; sub print_it { print("$var\n"); } 1;` [download]	[reply] [d/l] [select]
Re^4: symbol table vs. eval by raflach (Pilgrim) on Feb 23, 2006 at 19:23 UTC
Re^5: symbol table vs. eval by ikegami (Patriarch) on Feb 23, 2006 at 19:37 UTC
Re^3: symbol table vs. eval by chargrill (Parson) on Feb 23, 2006 at 19:03 UTC
Hrm. Note where I declare vars: `package apackage; my $test="my_variable_name"; my $default_value="the value I set"; $$test = $default_value; print "the value of \$my_variable_name = $my_variable_name\n"; package main; $my_variable_name = "A new value"; our $test; print "the value of \$my_variable_name = $my_variable_name\n"; print "the value of the package variable = $apackage::my_variable_name +\n";` [download] Yields: `the value of $my_variable_name = the value I set the value of $my_variable_name = A new value the value of the package variable = the value I set` [download] --chargrill `$/ = q#(\w)# ; sub sig { print scalar reverse join ' ', @_ } + sig map { s$\$/\$/$\$2\$1$g && $_ } split( ' ', ",erckha rlPe erthnoa stJu +" );` [download]	[reply] [d/l] [select]
Re^2: symbol table vs. eval by ikegami (Patriarch) on Feb 23, 2006 at 18:45 UTC
Caveat: Your solution won't work if `$my_variable_name` is a lexical (`my`) variable. It will only work if it's a package (`our`) variable.	[reply] [d/l] [select]
Re^3: symbol table vs. eval by raflach (Pilgrim) on Feb 23, 2006 at 18:51 UTC
I am working with package variables, so in this case I'm OK, but I do wonder why I can't declare it using 'our' (see my post below). Thanks!	[reply]
Re^2: symbol table vs. eval by ikegami (Patriarch) on Feb 23, 2006 at 18:53 UTC
Concerning your update, strict has nothing to do with it. If `use strict` is already present, so is either `our $my variable_name;` or `my $my variable_name;`. I was saying your code will only work with the former, not the latter. The OP's code (and the code in my reply) works with both.	[reply] [d/l] [select]
Re: symbol table vs. eval by Roy Johnson (Monsignor) on Feb 23, 2006 at 18:31 UTC
Lexical (`my`) variables are not stored in the symbol table. You could, however, make a hash of references to the variable names. Then, with some dereferencing, `my $test="my_variable_name"; #imagine this is a hash key my $default_value="the value I set"; #while this is the hash value my %hash = ($test => \$default_value); print "the value of \$$test = ${$hash{$test}}\n"; ${$hash{$test}} = "A new value"; print "the value of \$$test = ${$hash{$test}}\n";` [download] I'm not sure this demonstrates what you want to do, but hopefully it gives you an idea about how to approach it. Caution: Contents may have been coded under pressure.	[reply] [d/l] [select]
Re: symbol table vs. eval by ikegami (Patriarch) on Feb 23, 2006 at 18:48 UTC
You can make your code safer by replacing `eval("\$$test = '$default_value';");` with `${eval('\$' . $test)} = $default_value;` For example, it will work even if the value in `$default_value` contains the character `'`.	[reply] [d/l] [select]