FAQ 4.39 What is the difference between a list and an array?

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4.39: What is the difference between a list and an array?

    (contributed by brian d foy)

    A list is a fixed collection of scalars. An array is a variable that
    holds a variable collection of scalars. An array can supply its
    collection for list operations, so list operations also work on arrays:

            # slices
            ( 'dog', 'cat', 'bird' )[2,3];

            # iteration
            foreach ( qw( dog cat bird ) ) { ... }
            foreach ( @animals ) { ... }

            my @three = grep { length == 3 } qw( dog cat bird );
            my @three = grep { length == 3 } @animals;

            # supply an argument list
            wash_animals( qw( dog cat bird ) );
            wash_animals( @animals );

    Array operations, which change the scalars, reaaranges them, or adds or
    subtracts some scalars, only work on arrays. These can't work on a list,
    which is fixed. Array operations include "shift", "unshift", "push",
    "pop", and "splice".

    An array can also change its length:

            $#animals = 1;  # truncate to two elements
            $#animals = 10000; # pre-extend to 10,001 elements

    You can change an array element, but you can't change a list element:

            $animals[0] = 'Rottweiler';
            qw( dog cat bird )[0] = 'Rottweiler'; # syntax error!

            foreach ( @animals ) {
                    s/^d/fr/;  # works fine

            foreach ( qw( dog cat bird ) ) {
                    s/^d/fr/;  # Error! Modification of read only value!

    However, if the list element is itself a variable, it appears that you
    can change a list element. However, the list element is the variable,
    not the data. You're not changing the list element, but something the
    list element refers to. The list element itself doesn't change: it's
    still the same variable.

    You also have to be careful about context. You can assign an array to a
    scalar to get the number of elements in the array. This only works for
    arrays, though:

            my $count = @animals;  # only works with arrays

    If you try to do the same thing with what you think is a list, you get a
    quite different result. Although it looks like you have a list on the
    righthand side, Perl actually sees a bunch of scalars separated by a

            my $scalar = ( 'dog', 'cat', 'bird' );  # $scalar gets bird

    Since you're assigning to a scalar, the righthand side is in scalar
    context. The comma operator (yes, it's an operator!) in scalar context
    evaluates its lefthand side, throws away the result, and evaluates it's
    righthand side and returns the result. In effect, that list-lookalike
    assigns to $scalar it's rightmost value. Many people mess this up
    becuase they choose a list-lookalike whose last element is also the
    count they expect:

            my $scalar = ( 1, 2, 3 );  # $scalar gets 3, accidentally


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