$_ confusion with loops in loops

stanley82 has asked for the wisdom of the Perl Monks concerning the following question:

oops I've maybe got the post right this time I'm a bit messed up with $_ The code below failed to work til I was specific as what to write. In the IF block "print NEW;" worked but in the ELSE block it failed, but once I explicitly used $fields[0] it was fine. $_ has me a bit frustrated with loops in loops as to which $_ I'm using. Regards Ian.

    while (<UNMATCHED>)
        {
        @fields = split(/:/, $_);
        $wd = substr($fields[0], 0, 1);
        if($wd eq "/")
            {
                my $rtn = search ($fields[0], UNUSED);    #Returns 1 i
+f #fields[0] is found in UNUSED.
print "Return is = $rtn \n";
                if($rtn == 1)
                {
print "Writing to CHANGED. \n";
                    print CHANGED "$fields[0] \n";
#                    print NEW;                #This works in IF but n
+ot in ELSE ?
                }
                else
                {
print "Writing to NEW \n";
                    print NEW "$fields[0] \n"; #This works
#                    print NEW;                    #This does not
                }
            }

        }
[download]

Comment on $_ confusion with loops in loops Download Code

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Re: $_ confusion with loops in loops by AnomalousMonk (Archbishop) on Mar 06, 2019 at 20:30 UTC
`while (<UNMATCHED>) { ... }` It's a little difficult for me to understand just what you expect from the posted code and what "failed to work," but one thing I suspect is that you may be expecting the value of `$_` set in the `while`-loop quoted above (a block read from a file) to be unchanged throughout the `while`-loop. But `while`-loops do not "topicalize" the value of `$_` as `for`-loops do. I also see that there is a `my $rtn = search ($fields[0], UNUSED);` statement in the quoted `while`-loop that seems to be searching a file with the handle `UNUSED`. If this search uses another `while (<UNUSED>) { ... }` `while`-loop structure, the value of `$_` will be changed and will not be restored upon return to the scope of the posted code unless `$_` was properly local-ized within the `search()` function. I think my advice would be to use lexical variables as the loop variable and in other cases within the block: `while (my $record = <UNMATCHED>) { my @fields = split(/:/, $record); ... }` [download] See my. Save your worries about `$_` for `for`-loops, map and grep built-ins, etc. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: $_ confusion with loops in loops by AnomalousMonk (Archbishop) on Mar 06, 2019 at 20:00 UTC
Rather than starting a new thread, the better approach would have been to have edited the previous post to simply fix the `<code> ... </code>` tags as needed. Please see How do I change/delete my post? for site etiquette and protocol regarding such changes. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: $_ confusion with loops in loops by stanley82 (Novice) on Mar 06, 2019 at 21:37 UTC
Many thank for the reply. my I understand and the rest makes good sense. I'll do what you demonstrated and not rely on the $_ as much, it's a bit terse anyways. Ian.	[reply]