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$ 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

cat jackkinfe.pl

...
  printf "%e %e ", $g_jack_av[$j], $g_jack_err[$j];
}
printf "\n";

$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.

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