Please explain me what is hapenning in line 3 of this code

Question

Please explain me what is hapenning in line 3 of this code.

 for my $i (0 .. $dim) {
    for my $j (0 .. $dim) {
        $adj->[$i][$j] = $adj->[$i][$j] ? [$j] : [];

Answer 1

The code loops over two dimensions in an array reference $adj. Presumably, $dim is the dimension, and $i and $j iterate over a list of numbers from 0 to $dim, e.g. 0,1,2,3,4,5.

For each combination of numbers, the value of that array element is checked for trueness, and is assigned a new value. If the value is false, it is assigned an array ref containing the index $j, otherwise an empty array ref [].

The conditional operator is used here, with the basic syntax

CONDITION ? FOO : BAR
if CONDITION then FOO else BAR

Presumably, the array ref $adj is supposed to contain array references, which is why it can simply check for trueness as a shortcut for defined $adj->[$i][$j].

Answer 2

This is the ternary operator, aka conditional operator.

If $adj->[$i][$j] is 0 (or undefined) then [] is assigned to $adj->[$i][$j], in the other cases $adj->[$i][$j] is assigened to $adj->[$i][$j].

perlop has this quote:

Ternary "?:" is the conditional operator, just as in C. It works much like an if-then-else. If the argument before the ? is true, the argument before the : is returned, otherwise the argument after the : is returned.

Answer 3

    for my $i (0 .. $dim) {
      for my $j (0 .. $dim) {

Above for loops will itrate through the arry with dimension $dim x $dim

    $adj->[$i][$j] = $adj->[$i][$j] ? [$j] : [];

if $adj->[$i][$j] is zero, then assign [] to $adj->[$i][$j] else assign $j (column value)