Mastering Perl for Bioinformatics [Electronic resources] نسخه متنی

A.3 Scalar Values and Scalar Variables

A scalar value is a single item of data,
such as a string, a number, or a reference.

A.3.1 Strings

Strings are scalar values and
are written as text enclosed within
single quotes, like so:

'This is a string in single quotes.'

or double quotes, such as:

"This is a string in double quotes."

A single-quoted string prints
out exactly as written. With double quotes, you can include a
variable in the string, and its value will be inserted or
"interpolated." You can also
include commands such as \n to represent
Table A-3):

$aside = '(or so they say)';
$declaration = "Misery\n $aside \nloves company.";
print $declaration;

This snippet prints out:

Misery 
(or so they say) 
loves company.

A.3.2 Numbers

Numbers are scalar
values that can be:

Integers:

 3
-4
0

Floating-point (decimal):

4.5326

Scientific (exponential)
notation (3.13 x 1023 or 313000000000000000000000):

3.13E23

Hexadecimal (base 16):

Ox12bc3

Octal (base 8):

O5777

Binary (base 2):

0b10101011

Complex (or imaginary)
numbers, such as 3 +
i, and
fractions (or ratios, or rational numbers), such as
1/3, can be a little tricky. Perl can handle
fractions but converts them internally to floating-point numbers,
which can make certain operations go wrong (Perl is not alone among
computer languages in this regard):

if ( 10/3  == ( (1/3) * 10 ) {
print "Success!";
}else {
print "Failure!";
}

This prints:

Failure!

To properly handle rational arithmetic with fractions, complex
numbers, or many other mathematical constructs, there are mathematics
modules available, which aren't covered here.

A.3.3 References

Perl references are scalar values that contain
a location in memory where some other Perl data can be found. Think
of references as scalar values that point to other data, such as
scalars like strings or numbers, or arrays, hashes, or subroutines.

References are mainly used to:

Pass diverse types of arguments into a Perl subroutine

Avoid copying large amounts of data

Define the objects of object-oriented programming

Build complex data structures

To create a reference, prepend a Perl value, variable, or subroutine
with a backslash.

To save a reference, assign it to a scalar variable.

You can refer to a string by preceding it with a backslash:

\'A referenced string'

and you make a reference to a number similarly:

\3.1415925

These references may be saved in scalar variables, discussed next.

A.3.4 Scalar Variables

Scalar values can be stored in
scalar variables. A scalar
variable is indicated with a
$ before the variable's name. The
name begins with a letter or underscore and can have any number of
letters, underscores, or digits. A digit, however,
can't be the first character in a variable name.
Here are some examples of legal names of scalar variables:

$Var
$var_1

Here are some improper names for scalar variables:

$1var
$var!iable

Names are case-sensitive: $dna is different from
$DNA.

These rules for making proper variable names (apart from the
beginning $) also hold for the names of array and
hash variables and for subroutine names.

A scalar variable may hold any type of scalar value mentioned
previously, such as strings or the different types of numbers.