Perl: Looping over input lines with an index-based approach

Question

This is a beginner-best-practice question in perl. I'm new to this language. The question is:

If I want to process the output lines from a program, how can I format THE FIRST LINE in a special way?

I think of two possibilities:

1) A flag variable, once the loop is executed first time is set. But it will be evaluated for each cycle. BAD solution

2) An index-based loop (like a "for"). Then I would start the loop in i=1. This solution is far better. The problem is HOW CAN I DO IT?

I just found the code for looping over with the while ( <> ) construct.

Here you can see better:

$command_string = "par-format 70j p0 s0 < " . $ARGV[0] . "|\n";                                                                                

open DATA, $command_string  or die "Couldn't execute program: $!";

print "\t    <div>&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;|-- <strong>Description</strong></div>\n";
while ( defined( my $line = <DATA> )  ) {
   chomp($line);
   # print "$line\n";
   print "\t    <div>&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;&‎nbsp;|&‎nbsp;&‎nbsp;&‎nbsp;-- " . $line  . "</div>\n";
}

close DATA;

Please also don't hesitate in correcting any code in here, this is my first perl poem.

Thanks!

Answer 1

To handle the first line differently, you could just put

$line = <DATA>;

above your loop.

With proper checking for read problems (empty file, etc.) this should be

if ($line = <DATA>) {
    ...do special things...
}

while (my $line = <DATA>) {
    ...do regular things...
}

I'm not sure about the defined() call. You might not need it, since an empty string has a false truth value.

Answer 2

You can always use $. or the English name $INPUT_LINE_NUMBER to control the logic in your loop with:

while (my $line = <>) {
    if ($. == 1) {
        # do cool stuff here
    }
    # do normal stuff here
}

Answer 3

From a 'best practices' perspective there is much wrong with that code sample:

open DATA, $command_string  or die "Couldn't execute program: $!";

• Security hole, please exploit me.
• DATA is a magical value that points to a __DATA__ section at the end of the current file.

• You should use

  open my $fh
  

  Which uses a lexical variable for a file handle instead of a global.

• You should use 3 arg open, ie:

  open my $fh, '<'  , $filename
  open my $fh, '-|' , $command
  open my $fh, '-|' , $command, @args
  

  sadly I have yet to work out how 3-arg works with dual-pipes. theres' this IPC::Open2 thing, but I haven't worked out how to use that effectively yet. Suggestions welcome .