Hello Monks, trying to combine two .csv files into one resulting file with 3 average values and 1 difference for plotting, I do not seem to accomplish that. The problem differs to multiplication a column of data in csv file by a factor. Appreciate your help! 
#!/usr/bin/perl -w

use File::Basename;

# filename
my ($nameA,$pathA,$suffixA) = fileparse($ARGV[0],'\.[^\.]*');
my ($nameB,$pathB,$suffixB) = fileparse($ARGV[1],'\.[^\.]*');
my $nameC = $nameA.'dif';

# read data
open(dataA, $nameA)   or die " cannot open/read file:$!\n";
my @headerA; push @headerA, $_ = <dataA> for 1 .. 2;
my @multi_arrayA; push @multi_arrayA, [split(' ', $_)] for <dataA>;
pop @multi_arrayA; pop @multi_arrayA; # trash header
close dataA;
open(dataB, $nameB)   or die " cannot open/read file:$!\n";
my @headerB; push @headerB, $_ = <dataB> for 1 .. 2;
my @multi_arrayB; push @multi_arrayB, [split(' ', $_)] for <dataB>;
pop @multi_arrayB; pop @multi_arrayB; # trash header
close dataB;

# modify data
my @multi_arrayC;
my ($avgX,$avgY,$avgZ,$difF); #u_cor_T pi_T_ts TFP eta_is_T_ts
for my $i (0..$#multi_arrayA) {

    $avgX = ( $multi_arrayA[$i]->[0] + $multi_arrayB[$i]->[0] ) / 2.;
    $avgY = ( $multi_arrayA[$i]->[1] + $multi_arrayB[$i]->[1] ) / 2.;
    $avgZ = ( $multi_arrayA[$i]->[2] + $multi_arrayB[$i]->[2] ) / 2.;
    $difF = $multi_arrayA[$i]->[3] - $multi_arrayB[$i]->[3];

    push( $multi_arrayC[$i]->[0], $avgX );
    push( $multi_arrayC[$i]->[1], $avgY );
    push( $multi_arrayC[$i]->[2], $avgZ );
    push( $multi_arrayC[$i]->[3], $difF );
    
}

# write data
open(dataC, ">".$nameC)   or die " cannot open/read file:$!\n";
print dataC join(",", map { sprintf "%E", $_ } @{$_}),"\n" for (@multi
+_arrayC);
close dataC;

# input
__dataA__
variables
units
1.0 1.0 1.0 4.0 9.99999
2.0 2.0 2.0 4.0 9.99999
3.0 3.0 3.0 4.0 9.99999

__dataB__
variables
units
3.0 3.0 3.0 5.0 9.99999
2.0 2.0 2.0 5.0 9.99999
1.0 1.0 1.0 5.0 9.99999

# output (expected
__dataC__
variables
units
2.00000E0 2.00000E0 2.00000E0 1.00000E0
2.00000E0 2.00000E0 2.00000E0 1.00000E0
2.00000E0 2.00000E0 2.00000E0 1.00000E0

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In reply to combining two csv files by using math operations by ng0177

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