Learning Perl, Fourth Edition

Autoincrement and Autodecrement

You’ll often want a scalar variable to count up or down by one. Since these are frequent constructs, there are shortcuts for them like nearly everything else we do frequently.

The autoincrement operator (++) adds one to a scalar variable as the same operator in C and similar languages:

    my $bedrock = 42;
    $bedrock++;  # add one to $bedrock; it's now 43

Like other ways of adding one to a variable, the scalar will be created if necessary:

    my @people = qw{ fred barney fred wilma dino barney fred pebbles };
    my %count;                     # new empty hash
    $count{$_}++ foreach @people;  # creates new keys and values as needed

The first time through that foreach loop, $count{$_} is incremented. That’s $count{"fred"}, which goes from undef (since it didn’t previously exist in the hash) up to 1. The next time through the loop, $count{"barney"} becomes 1; after that, $count{"fred"} becomes 2. Each time through the loop, one element in %count is incremented and possibly created as well. After that loop is done, $count{"fred"} is 3. This provides a quick and easy way to see which items are in a list and how many times each one appears.

Similarly, the autodecrement operator (--) subtracts one from a scalar variable:

    $bedrock--;  # subtract one from $bedrock; it's 42 again

The Value of Autoincrement

You can fetch the value of a variable and change that value at the same time. Put the ++ operator in front of the variable name to increment the variable first and then fetch its value. This is ...