Why is Perl script asking for 'global declaration' when I declare something in for loop?

Question

I am reading $digit from infile and want to print it to outfile. But the Perl script is giving an error saying 'Global symbol "$digit" requires explicit package name'. But, if I do declare it globally , $digit='', then the this value is printed to the outfile instead of the value extracted/read from infile. Any suggestions as to what should be done?

This how I am doing it:

    my $digit=''; 
    open (DATA, "</usr/infile") || die "cant open infile\n"; #file from digit has to read

    while (<DATA>){
        ($digit)= $_=~ /\s9\s(\d+)/; #regex to capture digit '234' from ' 9 234'
        if ($digit ne ""){
        print "digit is $digit\n"; # this prints fine
        }
    }

    open (FILE, ">/usr/outfile") || die "cant open outfile\n"; #file to which digit has to be finally written
    print FILE "9   $digit"; #$digit takes in the value declared globally i.e. ''

    close(DATA);
    close (FILE);

Answer 1

Your $digit is scoped within the while loop and then you are trying to access it outside it with the line:

print FILE "9   $digit";

You must declare it outside the while loop for this to be visible by that line.

Or better yet just open your outfile before you loop and write the digit to the outfile within the loop and keep your $digit variable scoped within the loop.

If you only want the last instance that matches your regex then:

#!/usr/bin/perl
use strict;

open (DATA, "infile.txt")  || die("cant open infile\n");   

my $digit = "";
while (<DATA>) {
    chomp;
    if (/\s9\s(\d+)/) #regex to capture digit '234' from ' 9 234' 
    {
       print "Found digit is $1\n"; # 
       $digit = $1;
    }
}
close(DATA);

if ($digit ne "") {
open (FILE, ">outfile.txt") || die("cant open outfile\n"); 
print FILE "9   $digit\n";
close(FILE);
}
else
{
    print "No digit found!\n";
}
exit;

If you want to get all instances in the infile where your regex matches, the following should do the trick.

#!/usr/bin/perl
use strict;

open (DATA, "</usr/infile")  || die("cant open infile\n");   
open (FILE, ">/usr/outfile") || die("cant open outfile\n"); 

while (<DATA>) {
    chomp;
    if (/\s9\s(\d+)/) #regex to capture digit '234' from ' 9 234' 
    {
       print "Found digit is $1\n"; # 
       print FILE "9   $1\n";
    }
}


close(DATA);
close (FILE);
exit;

Answer 2

Don't declare $digit with my inside your while() loop; declare it just outside:

my $digit;
while( <DATA> ) {
    # stuff that sets $digit... 
}

open( FILE , '>out' );
print FILE $digit;

Answer 3

You have declared $digits to be local to the scope of the while loop by using my. The $digit in the print statement is out of scope and that is what is causing the message.

To correct this, declare my $digit before the while loop or declare it with our instead of my to make it global.

I would do the following:

use strict;
use warnings;

my $digit = "None";  # None if there are no digits found

open (DATA, "</usr/infile") || die "cant open infile\n"; #file from digit has to read
while (<DATA>){    
    ($digit) = $_=~ /\s9\s(\d+)/; #regex to capture digit " 9 234"    
    if ($digit ne ""){    
        print "digit is $digit\n"; # this prints fine    
    }
}
open (FILE, ">/usr/outfile") || die "cant open outfile\n"; #file to which digit has to be finally written
print FILE "9   $digit";
close(DATA);
close(FILE);

If you did not get a digit, you would get None for the $digit.

Hope this helps.

Answer 4

while (<DATA>){
    my ($digit)= $_=~ /\s9\s(\d+)/; #regex to capture digit '234' from ' 9 234'
    if ($digit ne ""){
    print "digit is $digit\n"; # this prints fine
    }
}

Lexical variables (the ones created with my) exist within the scope they are declared, so $digit is created and destroyed each time through the loop.