Re: Stumped with $_ being modified

Although using local will solve your problem, I'd stay away from it. While it does "fix" that one problem, you still read the stylesheet file each time you encounter a tag to substitute. Since you open the stylesheet file each time, reading exactly the same data, it would make a lot more sense to read it once, like jdporter suggests.

Here's an example of my take:

use strict;
use warnings;

my $stylesheet_file = "stylesheet.txt";

sub read_stylesheet
{
     open(STYLESHEET, "<", $stylesheet_file)
        || die "Cannot read stylesheet_file\n";

     my (%tag, $header);
     while (<STYLESHEET>)
     {
          if (/^HEADER\s+(\d+):/)
          {
               $header = $1;
          }
          elsif (/^nm\s+:\s+(\w+)/
          {
               # Make a note of what header this tag
               # appeared in.
               $tag{$1} = $header;
          }
     }
     close(STYLESHEET);

     return \%tag;    # Returns a reference to the hash
}

# ... (Main routine)

my $input_file    = "taggedfile.txt";
my ($output_file) = @ARGV;

my $tag = read_stylesheet();

open(FILE, "<", $input_file)
     || die "Could not read $input_file\n";
open(OUT, ">", $output_file)
     || die "Could not write $output_file\n";

while (<FILE>)
{
     # Perform substitutions
     s/^\{(\w+)\}/\{$1:$tag->{$1}\}/g;

     # Write line ($_) to OUT
     print OUT;
}

close(FILE);
close(OUT);
[download]

This variation has a routine which reads in the stylesheet and returns a hash reference to the data that was read. Theoretically, then, you can read in more than one stylesheet and choose which one you look up from, something that a global variable doesn't really permit.

You shouldn't have to localise $_ if you're careful about what's going on. Having nested file reads is one way to cause trouble, which is what you had there.

If you're not sure where $_ has been, the safe thing to do is used a named variable, such as this:

while (my $line = <FILE>)
{
     # ... Use $line where you would normally use $_
}
[download]

I'd really suggest steering away from using local declared variables.

Comment on Re: Stumped with $_ being modified Select or Download Code

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Re: Re: Stumped with $_ being modified by jdporter (Paladin) on Jan 09, 2003 at 15:30 UTC
While tadman gives a perfectly fine solution, I'd pick a nit with certain of his advice: Although using local will solve your problem, I'd stay away from it. I'd really suggest steering away from using local declared variables. That is really not good advice. `local` does not "declare" a variable, it localizes its value. This is a critically important capability -- as young_david's problem clearly demonstrates. In many cases we don't have to worry about variables being localized, because the ops we use do it implicitly. (I'm thinking specifically of `foreach`.) But others, such as `while`, do not, and so we have to localize the variable explicitly -- if we want to be safe. So rather than say "Don't use `local`", I'd say "`use strict`." That is a practice which will encourage good programming habits. jdporter The 6th Rule of Perl Club is -- There is no Rule #6.	[reply] [d/l] [select]
Re: Re: Stumped with $_ being modified by ihb (Deacon) on Jan 09, 2003 at 15:24 UTC
I too am stumped... by your reply. I don't know where to begin. How can you say that he shouldn't localize? What's wrong with localizing? I even find it hard to argue with you since it all boils down to "why are you so anti localization?" and all my arguments seem so obvious that they shouldn't have to be mentioned. I just don't see it. How far do you take this? Should you localize filehandles? Or should you use "good names" so you don't have to localize? (In perl5.6 you're saved by the `open my $fh, $file` syntax, but the issue is still very interesting.) If you steer away from using "`local` declared variables", do you also always use your own named variable in `foreach` loops? `foreach my $foo (LIST) { ... }` I just wrote a note about localization and while loops; the relevant thread is here. As you see, I have the very opposite opinion of you. So I'm very curious about what I've overlooked. Curiously, `ihb`	[reply] [d/l] [select]
Re: Stumped with $_ being modified by tadman (Prior) on Jan 10, 2003 at 03:18 UTC
There's a difference between lexically scoping with `my` and localising using `local`, although the terminology is one that seems rather contentious. Like the difference between arrays and lists. There are only a few things you still need to localise with `local`, such as the `$"` variable. Ever since 5.0 came out, a lot of effort has been put into moving people away from `local` and to `my`, for example, the way you can use lexically scoped filehandles. Where possible, it's good to use filehandles like that since they can be passed around easily from subroutine to subroutine, or stored in an object's hash easily. In this example, though, I'm using Old-School handles because it's not really an issue. As for loops, I won't hesitate to use `$_` as long as it doesn't get too complicated. Where it's obvious what's being iterated, that is. For example: `$_->iron() foreach (@shirts);` You could say `my $shirt`, but it would be redundant. On the other hand, where there's no hint as to what you're using, a simple `my` declaration acts as documentation. `foreach my $shirt ($laundry->contents()) ...` You have to be careful with `$_`, just like with `$1` and its relatives. Where there's risk of contamination, I use alternate names or copies. If there's no risk, then it's a matter of preference. I think my point is really this: Don't use `local $_`, instead, use a named lexical variable such as `my $line`.	[reply] [d/l] [select]


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