Question

I have a Perl question which should be fairly easy but can’t quite seem to get it correct I have a file that contains 2 columns of numbers, what I need to is to take the second column and add the first value in this column to the second, the second to the third the third to the fourth etc. I don’t know how many numbers will be in column 2 so I need to do this until the end of the file, I also want to be able to use these values further on in my program. I will give a dummy example below along with the code I have tried so far.

Example file tab delimited file data.txt

1   29
2   26
3   24
4   28

Example of desired out put

55

50

52

Code as is:

#!/usr/bin/perl -w
# use and library files
use strict;
use warnings;


my $line;
my $Val;
my $sum;
open(FH, "data.txt") or die $!;
while (my $line = <FH>){
my @val = split("\t", $line);
my $Val = $val[1];
my $sum = $Val+$Val;
print "$sum\n";
}
close FH;

The line $sum = $Val+$Val; needs to have some sort of loop so the correct values are being added but not sure how to get it right, and then I need to somehow assign the summed values variable names to use later.

Any help would be greatly appreciated

Thanks in advance
Sinead

Answer 1

You got it almost in your example. $Val is your previous value. So you must not overwrite it before you used it in the $sum. This means swap these two lines and add the previous and the current value.

my $sum = $Val+$val[1];
my $Val = $val[1];

One advice, try to give distinct names to variables. Differences in case only easily lead to confusion.

Here’s my version

my $prev = 0;
while (<>) {
    my(@cols) = split;
    my $sum = $prev + $cols[1];
    $prev = $cols[1];
    print "$sum\n" if ($. > 1); # skip first line
}

and call it with

perl sum.pl data.txt