Variable Names and References

chakram88 has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: Variable Names and References by Joost (Canon) on Mar 09, 2007 at 15:59 UTC
${"str$_"} is not a "hard" reference, it's a symbolic reference. i.e. it just refers to the name of a variable. You can't use symbolic refences while you have "strict refs" in use (as the error correctly states). You can use symbolic references if you switch off "strict refs": `for (1..3) { no strict 'refs'; my $ref = \${'str'.$_}; # this creates a hard reference VIA a symb +olic reference # or: my $ref = "str$_"; # this is a purely symbolic reference, +access is the same as above print "The string is $$ref"; }` [download] update: you can only refer to GLOBALs with symbolic references. (update2: here's the relevant documentation in perlref) That's why that code still won't work. Here's a working version: Read more... (591 Bytes) Note that in your example using a simple array would be much easier, less error-prone and more efficient. Symbolic references are almost only useful these days for creating subroutines / packages at runtime (basically, wherever the only alternative is to use eval STRING). updated again, added some comments "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l] [select]
Re^2: Variable Names and References by chakram88 (Pilgrim) on Mar 09, 2007 at 16:13 UTC
huh. Thanks. I understand what you mean about the symbolic reference. I just was not seeing it that way. (hence, while I understood the error message, I thought my problem was in the syntax, when all along it was improper referencing.) Thanks. And re your suggestion, I was going to go the route of an array -- but got hung up on why I couldn't make this work. Thank you, I can now stop floundering and get back to being productive.	[reply]
Re^2: Variable Names and References by rvosa (Curate) on Mar 12, 2007 at 00:52 UTC
`# or: my $ref = "str$_"; # this is a purely symbolic reference, +access is the same as above` [download] I don't see how that is a symbolic reference? I think this is (had to think a while to come up with something contrived, here): `my $class = 'CGI'; my $version; { no strict 'refs'; $version = ${"${class}::VERSION"}; use strict; } print $version # e.g. 3.05 on my machine` [download] In other words, we construct a variable's name ($CGI::VERSION) out of expanded variables ($class), and expand (it's almost like a string eval()) the result in turn to return the value stored under the constructed variable's name. ...mmmm still not really put elegantly, sorry.	[reply] [d/l] [select]
Re^3: Variable Names and References by Joost (Canon) on Mar 12, 2007 at 10:25 UTC
Symbolic references are just names of global variables. That means "str2" is a symolic reference, just like "CGI::VERSION" is. In other words; `"${class}::VERSION";` is a symbolic reference, and `${"${class}::VERSION"};` derefences the reference into the value of $CGI::VERSION. Note that "hard" references use exactly the same syntax to dereference. "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l] [select]
Re: Variable Names and References by dragonchild (Archbishop) on Mar 09, 2007 at 18:13 UTC
Any time you find yourself wanting to create variables using a string as the name, you want a hash. My criteria for good software: Does it work? Can someone else come in, make a change, and be reasonably certain no bugs were introduced?	[reply]
Re^2: Variable Names and References by Anonymous Monk on Mar 11, 2007 at 19:09 UTC
Any time you find yourself wanting to create variables using a string as the name, you want a hash. Except when you want an array...	[reply]
Re: Variable Names and References by bart (Canon) on Mar 10, 2007 at 12:02 UTC
I understand hard references wrt using a variable name as a variable. But, if I want to create the variable name on the fly, I can't seem to get the refernce right Uh oh. Why on earth would you want to do that? Usually, when people think they want to do this, they don't have a really good reason. But there are plenty of good reasons for not wanting it.	[reply]
Re: Variable Names and References by Sartan (Pilgrim) on Mar 09, 2007 at 20:25 UTC
Aren't we all missing the obvious answer given to us by the compiler? You just need to use `no strict 'refs';`. `... { no stict 'refs'; for (1..3) { my $ref = \${'str'.$_}; print "The string is $$ref"; } } ...` [download] or, as other people suggested, just use a hash.	[reply] [d/l] [select]
Re^2: Variable Names and References by Joost (Canon) on Mar 10, 2007 at 16:19 UTC
He also needs to use globals, since you can't refer to lexicals via symbolic refs. As I posted above. "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply]
Re: Variable Names and References by DrHyde (Prior) on Mar 12, 2007 at 10:26 UTC
Rule 1: Every time you want to create a variable whose name is decided at runtime, you really want to use a hash; Rule 2: If you think you know better, you are wrong; Rule 3: No, really	[reply]
Re^2: Variable Names and References by Sartan (Pilgrim) on Mar 12, 2007 at 15:55 UTC
Against rules 1-3, I always sorta thought of a pointer/reference to a hash object as basically a hash(with certain perks). There don't really seem to be void-ish references. Perl creates a runtime error if you take `$hash = {}` and try to dereference it as an array or anything else(er well hashes and arrays are about it) such as `@$hash`. Perl seems to think a reference to a hash is still a hash but wants to protect the user a bit in terms of nasty memory issues. Doing $hash->{ $key } basically acts just like a hash object with slightly different syntax. So `$hash->{ 'runtime' . $key }` seems like a normal progression. D	[reply] [d/l] [select]