Mastering Perl for Bioinformatics [Electronic resources] نسخه متنی

2.1 Basic Perl Data Types

Before tackling
references, let's review the basic Perl data types:

Scalar

A scalar value is a
string or any one of several kinds of numbers such as integers,
floating-point (decimal) numbers, or numbers in scientific notation
such as 2.3E23. A scalar variable begins with the dollar sign $, as in
$dna.

Array

An array is an
ordered collection of scalar values. An array variable begins with an
at sign @, as in @peptides. An
array can be initialized by a list such as
@peptides =
('zeroth', 'first',
'second'). Individual scalar elements of an array
are referred to by first preceding the array name with a dollar sign
(an individual element of an array is a scalar value) and then
following the array name with the position of the desired element in
square brackets. Thus the first element of the
@peptides array is referenced by
$peptides[0] and has the value
'zeroth'. (Note that array elements are given the
positions 0, 1, 2, ..., n-1, where
n is the number of elements in the array.)

Recall that printing an array within double quotes causes the
elements to be separated by spaces; without the double quotes, the
elements are printed one after the other without separations. This
snippet:

@pentamers = ('cggca', 'tgatc', 'ttggc');
print "@pentamers", "\n";
print @pentamers, "\n";

produces the output:

cggca tgatc ttggc
cggcatgatcttggc

Hash

A hash is an
unordered collection of key value pairs of scalar values. Each scalar
key is associated with a scalar value. A hash
variable begins with the percent sign %, as in
%geneticmarkers. A hash can be initialized like an
array, except that each pair of scalars are taken as a key with its
value, as in:

The => symbol is just a synonym for a comma
that makes it easier to see the key/value pairs in such
lists.[1] An individual scalar value is
retrieved by preceding the hash name with a dollar sign (an
individual value is a scalar value) and following the hash name with
the key in curly braces, as in
$geneticmarkers{'hairless'}, which, because of how
it's initialized, has the value
'no'.

[1] It also forces the left side to be interpreted
as a string.