7
User-Defined
Functions
CGI/Perl
Programming
By Diane Zak
1
7
Objectives
• In this chapter, you will:
• Create and call a user-defined function
• Exit a script using the Perl exit
statement
• Pass information to and receive
information from a user-defined function
2
7
Objectives
• In this chapter, you will:
• Access the contents of an environment
variable
• Learn how to code the repetition
structure using the Perl while and
until statements
3
7
Introduction
• Several built-in Perl functions have
been used in the previous chapters:
– print, push, chomp
– length
• Used to return the number of characters
stored in a scalar variable
• Syntax: length (variable);
• Have also used a function from the
CGI.pm module
– param
• Can also create your own functions
– User-defined functions
4
7
User-defined Functions
• Reasons to create user-defined
functions:
– 1. Allow a programmer to avoid writing
and testing the same code more than
once
– 2. Allow large and complex scripts to be
broken into small and manageable tasks
• A team of programmers can work on a script,
each programmer assigned a task which can
be coded as a function
5
7
Functions
• Function (or subroutine)
– Block of code that begins with
keyword sub
– Code for a function can be in the
same script or can be in a
separate file
• Examples:
– Code for param is stored in CGI.pm file
– Code for push is stored in perl/perl.exe
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7
Functions
• Code for a function is processed
when the function is called or
invoked
– Invoke a function by including its
name and optional argument(s) in
a statement
• Example:
– $game = param (‘Game’);
– param function is called and the argument
is the Game key
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7
Functions
• Each function has a specific task
– After the function has completed, a
value is returned to the statement that
called it
– The statement does not have to use
the return value of the function
• function:
– Syntax:
8
7
Functions
• function:
– function header:
• Begins with sub keyword, space, function name,
space, opening brace ({)
– Same rules for naming functions as naming
variables
– Variable names should be easy to understand
– function footer:
• Closing brace (})
– return statement:
• Optional
– If there is no return statement, the function returns
the value of the last statement it processes
• Can return multiple value(s)
• The function is terminated after the value(s) are9
returned
The Dombrowski
Company Script
7
– 2 calls to userdefined
functions:
• display_error_pag
e();
• display_acknowle
dgement();
• If there are no
arguments to
pass to a
function,
following the
function name is
an empty set of
parentheses.
10
7
The exit; statement
• exit; terminates the script
– Stops the script from processing
further instructions
– Not required, but a good
programming practice
• The script will terminate after the
statements have completed
11
The Temp-Employment
Script
7
– Depending on the
value of $hourly_pay,
a dynamic web page
will be created
• The display_msg
function is passed a
string which will be
written to the page
• @_ is a special array
that contains
arguments passed to
that function
– For example, if one
argument is passed,
can access it with
$_[0]
12
O’Rourke Sales Script
– Version 1
7
– If a variable is
declared within a
function, it is only
available to that
function
– Many programmers
consider it poor
programming to have
a function assign
values to variables
declared in the main
part of the script
• More difficult to debug
and troubleshoot
13
The O’Rourke Sales
Script – Version 2
7
• Variables
are assigned
values in
main section
of script
14
7
International Coffees Form
and Dynamic Web Pages
15
7
International Coffees Form
and Dynamic Web Pages
– TEXTAREA tag is used to create a large
text box, typically for comments
16
7
Planning the International
Coffees Script
17
7
Environment Variables
• Series of hidden keys and values
that the Web server sends to a CGI
script when the script is run
• Can find out more information about
the web server, user, and the user’s
browser through environment
variables
• Stored in the %ENV hash
18
7
Environment Variables
19
Coding the International
Coffees Script
7
– The REQUEST_METHOD
key of the %ENV hash
is checked to see if it is
equal to “POST”
• The user clicked on the
Submit Form button on
the International Coffees
form to get to the script.
20
7
Coding the
International
Coffees Script
21
7
while and until statements
• while statement:
– Used to repeat a block of instructions while a
condition is true
– Syntax:
while (condition) {
statement(s) to process while condition is true
}
Result:
– Example:
my ($x, @nums);
@nums = (5000, 200, 100, 3);
$x = 0;
while ($x < 4) {
print “$nums[$x]\n”;
$x = $x + 1;
}
5000
200
100
3
22
7
while and until statements
• until statement:
– Used to repeat a block of instructions until a
condition becomes true
– Syntax:
until (condition) {
}
statement(s) to process until condition is true
– Example:
my ($x, @nums);
@nums = (5000, 200, 100, 3);
$x = 0;
until ($x == 4) {
}
print “$nums[$x]\n”;
$x = $x + 1;
Result:
5000
200
100
3
23
7
Using while in the
International Coffees Script
– If a file is large, it can be a problem to use the
angle operator (<>) to read a file into an array
• Each time the angle operator reads a record, it
temporarily stores it in the $_ special variable.
– Instead, can use while statement to read each
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record one at a time
7
Summary
• User-defined functions allow a programmer
to avoid writing and testing the same code
more than once.
– Large and complex scripts can be broken down
into smaller, more manageable tasks.
• A function is a block of code beginning with
the Perl keyword sub.
• A function’s code is processed only when
the function is called/invoked.
– A function is invoked by including the function’s
name and arguments in a statement.
25
Summary
7
• Every function performs a specific task and
returns a value(s) to the calling statement.
– The calling statement can use or ignore the
return value(s).
• The length function can be used to determine
the number of characters in a scalar variable.
– Syntax: length (variable);
• A function definition is composed of: function
header, function body, function footer.
• The exit statement can be used to terminate
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a script.
7
Summary
• The return statement is used in a function
body to return a value(s) to the calling
statement.
– Syntax: return expression;
– If there is no return statement, the function
returns the value of the last statement it
processes.
• Perl stores arguments passed to user-defined
functions in the @_ array.
• It is considered poor programming to have a
user-defined function assign a value to a 27
variable declared in the main section.
Summary
7
• Environment variables are hidden keys and
values that the Web server sends to a CGI
script when the script is run
– Stored in the %ENV hash
• The while statement is used to repeat one or
more instructions while a condition is true.
• The until statement it used to repeat one or
more instructions until a condition becomes
true.
• The angle operator temporarily stores each
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record it reads in the $_ variable.