5. Perl Functions/Subroutines

5.1 Perl User Defined Functions

Subroutine:

• A block of code module that are given a name for support modular programming
• In Perl, it is referred as subroutine
• It is called a procedure or function in other programming languages
• Keywords related to subroutines are: sub, return
• Call a subroutine with input parameter(s) and return value
• Call a subroutine with no return values

User Defined Functions or Subroutines

Define a Subroutine and give it a name:

sub monitor

{

statements;

}

Call or use a subroutine with its name and a & symbol:

&monitor;

Parameter passing

• @_ is a defined symbol for referring to the entire parameter array
• To access each element of the parameter array we use the notation as
• $_[0], $_[1], .., and $_[n]
• Perl parameters are called by reference
• To pass more than two parameters (including one array parameter) without an error, we would need to place the scalar in the first position of the parameter list

Scope control

• my(), the built-in function, can be used to create a local scope variable (tightly control)
• my($thisValue) = $_[0];
• local() function, less strict that my(), can be used to assign @_ array elements to scalar

5.2 Examples of Perl User Defined Functions

Example 5-1: An example program illustrates how to define and use functions.

#!/usr/bin/perl

#func1.pl

#

# OUTPUT:

# Area of a Circle = 314.159

# Area of a Square = 100

#

# Define Variables

$radius = 10; $length = 10;

$width = 10; $height = 20;

#

# Function Calls

#

$areaC = areaCircle($radius);

$areaSq = areaSquare($length);

$areaRect = areaRectangle($width, $height);

print(" Area of a Circle = $areaC\n");

print(" Area of a Square = $areaSq\n");

print(" Area of a Rectangle = $areaRect\n");

#

# Functions

# Parameter array @_ is used to store all parameters to a function.

# The first element of the parameter array can be retrieved with $_[0].

# The second element of the parameter array can be retrieved with

# $_[1].

sub areaCircle {

$rad = $_[0];

return(3.14159 * ($rad ** 2));

}

sub areaSquare{

$len = $_[0];

return($len ** 2);

}

sub areaRectangle {

($w, $h) = @_; # $w = $_[0]; $h = $_[1];

return($w * $h);

}

Example 5-2: Understanding function calls and parameter passing

#!/usr/bin/perl

#funct2.pl

# Passing a list and a scalar

# OUTPUT:

# D:\Perl\Exs>perl funct2.pl

# bString = bbbbb

# nArray = 0 1 2 3 4 5 6 7 8 9 10

# Array: = 0 1 2 3 4 5 6 7 8 9 10

# String: = AAAA

$bString = "bbbbb";

@nArray = (0..10);

print ("bString = $bString\n");

print ("nArray = @nArray\n");

thisSub("AAAA", (0..10));

sub thisSub{

local ($para1, @array) = @_;

print("Array: = @array\n");

print("String: = $para1\n\n");

}

5.3 Perl Built-In Functions

Array Manipulations

pop(@array)

# Return the last element of the @array, and removes the last

# element of the array. It also reduce the size of array by 1

# Example:

$last_element = pop(@array);

push(@array, $thisval)

# add a scalar element $thisvalue to the end of the array

shift(@array)

Return the first value of the @array, and reduce the size of

@array

@number1 = (0, 1, 2, 3);

shift(@number1); # the result of the array is (1, 2, 3) and

# the value 0 is shifted out

unshift(@array1, @array2)

# Add the elements of @array2 to the front of @array1

split(PATTERN, @STRING, LIMIT)

# Breaks up a string based on some delimiters

splice(@ARRAY1, OFFSET, LENGTH, @ARRAY2)

# Replace elements of @ARRAY1 with elements of @ARRAY2

Associative Array Manipulations

each(@ASSOC_ARRAY)

# return a two-element list (key,value) from a given associative

# array

String Manipulation

Substring Function

substr($original_str, n, m);

# Extract a sub-string from the original string, beginning at the

# n-th character position, for m number of characters

Example 5-3:

$institution = 'Indiana University - Purdue University Fort Wayne';

$iu = substr($institution, 0, 18); # Indiana University

$pu = substr($institution, 22, 28); # Purdue University Fort Wayne

Number to ASCII Character Conversion

chr(NUMBER);

$num = chr(66); # Yields ASCII character A in the $num

$string = "AABB";

lc("AABB"); # Convert to lower case string and return "aabb"

lcfirst("AABB"); # Convert the first char of the string to lower case

# and return "aABB"

length("AABB"); # Return the length of the string: 4

uc("aabb"); # Return an upper case string "AABB"

ucfirst("aabb"); # Convert the first char to upper case and return

# "Aabb"

chomp(STRING)

chomp(ARRAY)

Local Time Function

localtime(time) returns an array of nine elements of the local time through the following calling syntax:

($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) ($sec, $min, $hour, $day, $month, $year, $day_of_week, $day_of_year, $flag_day_light_saving) = localtime(time);

where

the array index is ranged from 0 through 8,

$mday is the day of the month,

$mon is the month,

$wday is the day of the week is ranged from 0 = Sunday, 1 =

Monday, ..,

6 = Saturday

$yday is the the day of the year (0-364), and

$isdst is the flag for showing day light saving time

Random Number Generation

srand() #Initialize random number generator

rand() #Generate a random number between 0 and ?

System Pre-defined Variables

$ENV('REMOTE_HOST')

# a associative array that filled with shell variables

$ENV('PATH') = '/bin:/usr/bin';

# Setup the $PATH for program execution