Programming Perl, 4th Edition

Growing Your Own

Those big list assignments are well and good for creating a fixed data structure, but what if you want to calculate each element on the fly, or otherwise build the structure piecemeal?

Let’s read in a data structure from a file. We’ll assume that it’s a plain text file, where each line is a row of the structure, and each line consists of elements delimited by whitespace. Here’s how to proceed:^[131]

while (<>) {
    @tmp = split;           # Split elements into an array.
    push @AoA, [ @tmp ];    # Add an anonymous array reference to @AoA.
}

Of course, you don’t need to name the temporary array, so you could also say:

while (<>) {
    push @AoA, [ split ];
}

If you want a reference to an array of arrays, you can do this:

while (<>) {
    push @$ref_to_AoA, [ split ];
}

Both of those examples add new rows to the array of arrays. What about adding new columns? If you’re just dealing with two-dimensional arrays, it’s often easiest to use simple assignment:^[132]

for $x (0 .. 9) {                       # For each row...
    for $y (0 .. 9) {                   # For each column...
        $AoA[$x][$y] = func($x, $y);    # ...set that cell
    }
}

for $x ( 0..9 ) {                       # For each row...
    $ref_to_AoA–>[$x][3] = func2($x);   # ...set the fourth column
}

It doesn’t matter in what order you assign the elements, nor does it matter whether the subscripted elements of @AoA are already there or not; Perl will gladly create them for you, setting intervening elements to the undefined value as need be. Perl will even create the original reference in $ref_to_AoA for you if it needs ...