Programming Perl, 4th Edition

Access and Printing

Now let’s print the data structure. If you only want one element, this is sufficient:

print $AoA[3][2];

But if you want to print the whole thing, you can’t just say:

print @AoA;         # WRONG

It’s wrong because you’ll see stringified references instead of your data. Perl never automatically dereferences for you. Instead, you have to roll yourself a loop or two. The following code prints the whole structure, looping through the elements of @AoA and dereferencing each inside the say statement:

for $row ( @AoA ) {
    say "@$row";
}

If you want to keep track of subscripts, you might do this:

for $i ( 0 .. $#AoA ) {
    say "row $i is: @{$AoA[$i]}";
}

or maybe even this (notice the inner loop):

for $i ( 0 .. $#AoA ) {
    for $j ( 0 .. $#{$AoA[$i]} ) {
        say "element $i $j is $AoA[$i][$j]";
    }
}

As you can see, things are getting a bit complicated. That’s why sometimes it’s easier to use a temporary variable on your way through:

for $i ( 0 .. $#AoA ) {
    $row = $AoA[$i];
    for $j ( 0 .. $#{$row} ) {
        say "element $i $j is $row–>[$j]";
    }
}

When you get tired of writing a custom print for your data structures, you might look at the standard Dumpvalue or Data::Dumper modules. The former is what the Perl debugger uses, while the latter generates parsable Perl code. For example:

use v5.14; # using the + prototype, new to v5.14 sub show(+) { require Dumpvalue; state $prettily = new Dumpvalue:: tick => q("), compactDump => 1, # comment these two lines out veryCompact => 1, # if you want a bigger dump ; dumpValue ...