Category: perl

I recently updated perlfaq4‘s answer to “What’s the difference between a list and an array?”. The difference between data and variables is often lost of the person who starts their programming career in a high level language.

We hit this subject pretty hard in the first chapter of Effective Perl Programming, 2nd Edition in at least three Items:

• Item 9: Know the difference between lists and arrays.
• Item 10: Don’t assign undef when you want an empty array.
• Item 12: Understand context and how it affects operations.

Here’s the current answer in perlfaq4:

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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];
@animals[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 comma:

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