Self-writing code revisited

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

Hello Perl Monks:

I was wondering if there was a way to do this (you will get the idea when you see the code - if not, I'll explain below):

#!/usr/bin/perl -w
use strict "vars" ;
my $data ;
my @splitter ;
$data = "testval,1" ;
@splitter = split( "," , $data ) ;
my $$splitter[ 0 ] = $splitter[ 1 ] ;
[download]

I know you can do something like this with hashes, but I don't want to use hashes. Here's the gist of what I am trying to do with this code. I have data (defined here as $data, but will be in a file). It is comma-separated. The first value in this data is the actual variable name I want to define. The second value is the value I want to assign to it. Obviously, assigning the value is easy, but creating the variable is not so easy. The $$ makes sense in terms of real-time substitution, however it doesn't work. Perl is interpreting it to mean something other than a string declaration. The error message given is:

Can't declare scalar dereference in "my" at srtest line 7, near "$spli
+tter["
syntax error at srtest line 7, near "$splitter["
srtest had compilation errors.
[download]

Any ideas of how to make this work?

Statue

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Re: Self-writing code revisited by kennethk (Abbot) on Sep 05, 2013 at 23:44 UTC
I know you can do something like this with hashes, but I don't want to use hashes. Requisite warnings: Why it's stupid to use a variable as a variable name. Seriously, read all three pages. But since you asked, you are looking for Symbolic references. Your code would be written as `#!/usr/bin/perl -w use strict "vars" ; my $data ; my @splitter ; $data = "testval,1" ; @splitter = split( "," , $data ) ; ${$splitter[ 0 ]} = $splitter[ 1 ] ; print ${$splitter[ 0 ]}, "\n";` [download] where I added a print for fun. This creates a maintenance headache, and note that your `$testval` variable is never associated with a declaration (my or our), so if the last line were `print $testval, "\n";` you'd get a compile time error from strict 'vars'. Also note that what I've written creates a package variable, not a lexical variable. This means you might have weird scoping issues pop up depending on what you are actually doing. #11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.	[reply] [d/l] [select]
Re: Self-writing code revisited by kcott (Archbishop) on Sep 06, 2013 at 01:02 UTC
G'day Statue, Welcome to the monastery. That's illegal; it's got nothing to do with the array: `$ perl -E ' my $x = "z"; my $y = 1; my $$x = $y; say $z; ' Can't declare scalar dereference in "my" at -e line 4, near "$x =" Execution of -e aborted due to compilation errors.` [download] If you didn't want a my variable, you could do this: `$ perl -E ' my $x = "z"; my $y = 1; $$x = $y; say $z; ' 1` [download] You could eval a string containing that code but the scope of the lexical variable so declared would only be that string: `$ perl -E ' my $x = "z"; my $y = 1; eval "my \$$x = \$y; say \">>>\$z<<<\""; say ">>>$z<<<"; ' >>>1<<< >>><<<` [download] If you're actually trying to write code that writes code, your best bet might be to output the generated code to a file (e.g. `script_my_script_wrote.pl`) which you could subsequently run in the normal fashion. For example, this code in your generating script: `print $out_script_fh "my \$$splitter[0] = $splitter[1];";` [download] Would write this line into `script_my_script_wrote.pl` (or whatever you call it): `my $testval = 1;` [download] [Aside: this is your first post so the title "Self-writing code revisited" doesn't make much sense. Perhaps a link to whatever you're revisiting would help to clarify what you mean.] -- Ken	[reply] [d/l] [select]