$ 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

##</code><code>##

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";

##</code><code>##

$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.

##</code><code>##

$x[$n[$j]][$j] = $column[$j];
$x_tot[$j] += $x[$n[$j]][$j];