Hello Dear Monks I am a perl newbie. I am trying to calculate the jacknife average and error of each column in a multi-column file. My example data file look like this: 
$ cat data.HW2
1.1 2.1 3.1 4.1
1.2 2.2 3.2 4.2
1.3 2.3 3.3 4.3
1.4 2.4 3.4 4.4

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My attempted solution is to define arrays that will eventually be the size same as the number of columns (in this case 4) and iterate over them line by line: 
cat jackkinfe.pl

#! /usr/bin/perl
use warnings; use strict;
my @n=0;
my @x;
my $j;
my $i;
my $dg;
my @x_jack;
my @x_tot=0;
my $cols;
my $col_start=0;

# read in the data
while(<>)
{
  my @column = split();
  $cols=@column;
  foreach my $j ($col_start .. $#column) {
    $x[$n[$j]][$j] = $column[$j];
    $x_tot[$j] += $x[$n[$j]][$j];
    $n[$j]++;
  }
}
# Do the jackknife estimates
for ($j=$col_start; $j<$cols; $j++)
{
  for ($i = 0; $i < $n[$j]; $i++)
  {
    $x_jack[$i][$j] = ($x_tot[$j] - $x[$i][$j]) / ($n[$j] - 1);
  }
# Do the final jackknife estimate
  my @g_jack_av=0;
  my @g_jack_err=0;
  for ($i = 0; $i < $n[$j]; $i++)
  {
    $dg = $x_jack[$i][$j];
    $g_jack_av[$j] += $dg;
    $g_jack_err[$j] += $dg**2;
  }
  $g_jack_av[$j] /= $n[$j];
  $g_jack_err[$j] /= $n[$j];
  $g_jack_err[$j] = sqrt(($n[$j] - 1) * abs($g_jack_err[$j] - $g_jack_
+av[$j]**2));
  printf "%e %e ", $g_jack_av[$j], $g_jack_err[$j];
}
printf "\n";

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It gives me the following two warnings: 
$cat data.HW2 | perl jackknife.pl
Use of uninitialized value within @n in array element at cols_jacknife
+.pl line 19, <> line 1.
Use of uninitialized value within @n in array element at cols_jacknife
+.pl line 20, <> line 1.

[download]
It is complaining at the following two lines: 
$x[$n[$j]][$j] = $column[$j];
$x_tot[$j] += $x[$n[$j]][$j];

[download]
But I want to set the size of @n dynamically depending on the size of the data file. How do I remove this warning? Any other suggestions on my perl usage are also welcome and much appreciated since I am trying to learn the best practices.

In reply to Calculate jackknife error from of each column of a multi-column file by pyari_billi

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