Arrays are ideal when you want to store lists which always retain their order, and/or you don't need access to a value through a name. Hashes are ideal when you don't care about the order, but you need to access the data through a known name (key).

In this example, we need to keep the three arrays in order to keep a sequence which is always consistent, so we use an AoA:

my @a = qw(1 2 3);
my @b = qw(4 5 6);
my @c = qw(7 8 9);

my @all = ( \@a, \@b, \@c, );

for my $aref ( @all ){
    print $_ for @{ $aref };
    print "\n";
}

[download]

Outputs:

123
456
789

[download]

A hash on the other hand means you want to be able to access the data by name, not by the order in which it is stored:

my %hash = ( a => 1, b => 2, c => 3 );
print "$hash{ a }, $hash{ b }, $hash{ c }\n";

#output: 1, 2, 3

[download]

A good example I've found is this... imagine you have a school with a bunch of classrooms that are full of students. If you just want to store a list of students names, put them into an array. However, to find the students within a class, you'll need to specify the name of the classroom to access it. An AoA wouldn't be very handy here, but a HoA sure would:

# define the list of students per each class

my @room1_pupils = qw( steve mike melissa );
my @room2_pupils = qw( meredith alexa chris );

# create the school, and assign the student lists to each
# hash key

my %school = (
        room1 => \@room1_pupils,
        room2 => \@room2_pupils,
);

# now you can get a list of students by room name (through
# the reference)

my @students = @{ $school{ room2 } };

[download]

note: I did the assignments the long way because I don't know if OP knows about anonymous vars yet.

In reply to Re: Array of array vs hash by stevieb
in thread Array of array vs hash by rakheek

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