Prev: FAQ 8.27 What's wrong with using backticks in a void context?
Next: FAQ 2.15 Where can I buy a commercial version of perl?
From: PerlFAQ Server on 30 Jul 2010 18:00 This is an excerpt from the latest version perlfaq7.pod, which comes with the standard Perl distribution. These postings aim to reduce the number of repeated questions as well as allow the community to review and update the answers. The latest version of the complete perlfaq is at http://faq.perl.org . -------------------------------------------------------------------- 7.29: How can I use a variable as a variable name? Beginners often think they want to have a variable contain the name of a variable. $fred = 23; $varname = "fred"; ++$$varname; # $fred now 24 This works *sometimes*, but it is a very bad idea for two reasons. The first reason is that this technique *only works on global variables*. That means that if $fred is a lexical variable created with my() in the above example, the code wouldn't work at all: you'd accidentally access the global and skip right over the private lexical altogether. Global variables are bad because they can easily collide accidentally and in general make for non-scalable and confusing code. Symbolic references are forbidden under the "use strict" pragma. They are not true references and consequently are not reference counted or garbage collected. The other reason why using a variable to hold the name of another variable is a bad idea is that the question often stems from a lack of understanding of Perl data structures, particularly hashes. By using symbolic references, you are just using the package's symbol-table hash (like %main::) instead of a user-defined hash. The solution is to use your own hash or a real reference instead. $USER_VARS{"fred"} = 23; $varname = "fred"; $USER_VARS{$varname}++; # not $$varname++ There we're using the %USER_VARS hash instead of symbolic references. Sometimes this comes up in reading strings from the user with variable references and wanting to expand them to the values of your perl program's variables. This is also a bad idea because it conflates the program-addressable namespace and the user-addressable one. Instead of reading a string and expanding it to the actual contents of your program's own variables: $str = 'this has a $fred and $barney in it'; $str =~ s/(\$\w+)/$1/eeg; # need double eval it would be better to keep a hash around like %USER_VARS and have variable references actually refer to entries in that hash: $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all That's faster, cleaner, and safer than the previous approach. Of course, you don't need to use a dollar sign. You could use your own scheme to make it less confusing, like bracketed percent symbols, etc. $str = 'this has a %fred% and %barney% in it'; $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all Another reason that folks sometimes think they want a variable to contain the name of a variable is because they don't know how to build proper data structures using hashes. For example, let's say they wanted two hashes in their program: %fred and %barney, and that they wanted to use another scalar variable to refer to those by name. $name = "fred"; $$name{WIFE} = "wilma"; # set %fred $name = "barney"; $$name{WIFE} = "betty"; # set %barney This is still a symbolic reference, and is still saddled with the problems enumerated above. It would be far better to write: $folks{"fred"}{WIFE} = "wilma"; $folks{"barney"}{WIFE} = "betty"; And just use a multilevel hash to start with. The only times that you absolutely *must* use symbolic references are when you really must refer to the symbol table. This may be because it's something that can't take a real reference to, such as a format name. Doing so may also be important for method calls, since these always go through the symbol table for resolution. In those cases, you would turn off "strict 'refs'" temporarily so you can play around with the symbol table. For example: @colors = qw(red blue green yellow orange purple violet); for my $name (@colors) { no strict 'refs'; # renege for the block *$name = sub { "<FONT COLOR='$name'>@_</FONT>" }; } All those functions (red(), blue(), green(), etc.) appear to be separate, but the real code in the closure actually was compiled only once. So, sometimes you might want to use symbolic references to directly manipulate the symbol table. This doesn't matter for formats, handles, and subroutines, because they are always global--you can't use my() on them. For scalars, arrays, and hashes, though--and usually for subroutines-- you probably only want to use hard references. -------------------------------------------------------------------- The perlfaq-workers, a group of volunteers, maintain the perlfaq. They are not necessarily experts in every domain where Perl might show up, so please include as much information as possible and relevant in any corrections. The perlfaq-workers also don't have access to every operating system or platform, so please include relevant details for corrections to examples that do not work on particular platforms. Working code is greatly appreciated. If you'd like to help maintain the perlfaq, see the details in perlfaq.pod. |