Need an $a and $b as in perl's sort routine

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

I'm working on a perl module here at work for a reporting system we're developing. I'd like to be able to pass in a text file for parsing and munging, and programatically specify what to do as follows:


# Handles initial parse (gets field
# names from first line of 'file.out'
my $file = ParseFile('file.out');    

# Specifies fields on which to subtotal
$file->keys = qw( Cust_Grp Order_Date Item Name );

# Adds a field
$file->field('Line_Total', sub { $row->{Unit_Price}*$row->{Qty} });

...

# Process file
$file->process();
[download]

The '$file->field(...)' line allows me to dynamically add columns to the report based on existing data. I can't hard code the new column definitions, as this code needs to be somewhat abstract. For that reason, I'm using an anonymous sub to define the value of the field. As the file processor runs, each line will add a column to the report by running that sub, pulling values from that report line. I'd like the '$row' variable to be a hash reference filled with key/value pairs from the current line. I'd like it to be autopopulated in the sub, much like sort {$a <=> $b} @list autopopulates the '$a' and '$b' in the code block there. Is there a way to do this? I've super searched on '$a $b', etc., but too many results are kicking out since those variables are often used as temp variables, etc. Any help would be appreciated. Thanks.

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Re: Need an $a and $b as in perl's sort routine by demerphq (Chancellor) on Jan 10, 2006 at 23:22 UTC
Use dynamic variables. And then localize them in the sub doing the calling. Using variables names that start with :: will force them into main. `sub exec_callback { my $code=shift; local $::hash={}; local $::row=1; $code->(); }` [download] This is basically all that happens with $_ or $a and $b in a sort routine or map or File::Find or Scalar::List::Utils. $a and $b are declared "special" so there shouldnt be any conflicts with them. Using $main::hash ($::hash) might have the potential for conflict. --- $world=~s/war/peace/g	[reply] [d/l]
Re^2: Need an $a and $b as in perl's sort routine by rational_icthus (Sexton) on Jan 10, 2006 at 23:57 UTC
Wouldn't doing a local $::row={} leave '$row' defined in main even outside the code block, however? I'd like $row to disappear once the instruction is complete.	[reply]
Re^3: Need an $a and $b as in perl's sort routine by demerphq (Chancellor) on Jan 11, 2006 at 00:53 UTC
No it wouldn't. Or rather $::row would revert to whatever it was before you localized it once you exited the sub that contains the localization. In the example below i use a bare block instead of sub to demonstrate but the same thing applies. `D:\dev\PM>perl -le"$::x='outer'; $c=sub{print $::x}; { local $::x='inn +er'; $c->() } $c->()" inner outer` [download] Thats the whole point of dynamic scoping... --- $world=~s/war/peace/g	[reply] [d/l]
Re^2: Need an $a and $b as in perl's sort routine by duelafn (Parson) on Jan 11, 2006 at 14:54 UTC
But then the code is not usable from a module, no? I have used things like the following in the past: `sub exec_callback { my $code = shift; my $caller = caller; no strict 'refs'; no warnings 'once'; local({$caller."::a"}) = \my $a; local({$caller."::b"}) = \my $b; $a = {}; $b = 1; $code->(); }` [download] Good Day, Dean	[reply] [d/l]
Re^3: Need an $a and $b as in perl's sort routine by demerphq (Chancellor) on Jan 11, 2006 at 16:12 UTC
But then the code is not usable from a module, no? A fully qualified global name is always accessable and usable from everywhere. The "problem" I think you are indirectly referring to is when you have a module X that uses globals that exist within the package that other code must then fully qualify using the full package name. For a variable named $X::x thats not so bad, but when the package is Some::Long::Package::Name it starts getting clumsy. The options are to export the globals in to the consumers package or to use "special globals" (like $a, $b, and $_) or to just use vars in package main. Assuming that you don't have a lot of globals in use in package main (IME a safe assumption) using such vars is IMO a reasonable choice. In other words typing `$::Hash` isn't so tough. Typing `$Some::Long::Package::Name::Hash` is. --- $world=~s/war/peace/g	[reply] [d/l] [select]
Re: Need an $a and $b as in perl's sort routine by ysth (Canon) on Jan 10, 2006 at 23:20 UTC
Perhaps Scalar-List-Utils will provide some examples? See first() and reduce().	[reply]
Re: Need an $a and $b as in perl's sort routine by simonm (Vicar) on Jan 11, 2006 at 00:57 UTC
If you only have one implicit target variable, use the variable that's already been set aside for implicit targets -- $_. It's easy to local $_ before calling the sub, and with $_ everyone will know what you're doing.	[reply]