Perl Tutorial

Creating the Program

A Perl program consists of an ordinary text file containing a series of Perl statements. Statements are written in what looks like an amalgam of C, UNIX shell script, and English.Perl code can be quite free-flowing. The broad syntactic rules governing where a statement starts and ends are:

• Leading spaces on a line are ignored. You can start a Perl statement anywhere you want: at the beginning of the line, indented for clarity (recommended) or even right-justified (definitely frowned on because the code would be difficult to understand) if you like.
• Statements are terminated with a semicolon.
• Spaces, tabs, and blank lines outside of strings are irrelevant-one space is as good as a hundred. That means you can split statements over several lines for clarity. A string is basically a series of characters enclosed in quotes.
• Anything after a hash sign (#) is ignored except in strings. Use this fact to pepper your code with useful comments.

Comments

In most programming languages you can add comments to the code which are ignored by the compiler/interpreter. This is used to increase the readability of the code. • In Perl, everything between a # and the end of a line is treated as a comment.

print "Hello World!"; # prints Hello World! to the screen.

Datatypes

Perl has three basic data types − scalars, arrays of scalars, and hashes of scalars, also known as associative arrays. Here is a little detail about these data types.

S.N.	Types and Description
1	Scalar − Scalars are simple variables. They are preceded by a dollar sign ($). A scalar is either a number, a string, or a reference. A reference is actually an address of a variable, which we will see in the upcoming chapters.
2	Arrays − Arrays are ordered lists of scalars that you access with a numeric index which starts with 0. They are preceded by an "at" sign (@).
3	Hashes − Hashes are unordered sets of key/value pairs that you access using the keys as subscripts. They are preceded by a percent sign (%).

Variable

Creating Variables

Perl variables do not have to be explicitly declared to reserve memory space. The declaration happens automatically when you assign a value to a variable. The equal sign (=) is used to assign values to variables.

$age = 25;             # An integer assignment
$name = "John Paul";   # A string
$salary = 1445.50;     # A floating point

Array

An array, in Perl, is an ordered list of scalar data.

Each element of an array is an separate scalar variable with a independent scalar value -- unlike PASCAL or C.

@ages = (25, 30, 40);            
@names = ("John Paul", "Lisa", "Kumar");

conditional statements

If

if (expression)
  {  true_statement_1;
     .....
     true_statement_n;
  }

 

If..else

if (expression)
  {  true_statement_1;
     .....
     true_statement_n;
  }
else
  {  false_statement_1;
     .....
     false_statement_n;
  }

 

If..elseif..else

if (expression)
  {  true_statement_1;
     .....
     true_statement_n;
  }
else
  {  false_statement_1;
     .....
     false_statement_n;
  }

Loop

For loop

A for loop counts through a range of numbers, running a block of code each time it iterates through the loop.

for ( initialise_expr; test_expr; increment_expr )
  {
     statement(s);
  }

while loop

While loops continually iterate as long as the conditional statement remains true.

while (expression)
  { # while expression is true execute this block

statement(s);
  }

Until loop

Repeats a statement or group of statements until a given condition becomes true. It tests the condition before executing the loop body.

Do..while loop

Like a while statement, except that it tests the condition at the end of the loop body

Function

A function definition is very simple. It consists of:

sub functionName {

}

perl - hashes

Hashes are complex list data, like arrays except they link a key to a value. To define a hash, we use the percent (%) symbol before the name.

String Functions

Perl provides a very useful set of string handling functions:

lc, uc, length

There are a number of simple functions such as lc and uc to return the lower case and upper case versions of the original string respectively. Then there islength to return the number of characters in the given string.

Index

This function will get two strings and return the location of the second string within the first string.

substr

substr will give you the substring at a given locations. Normally substr gets 3 parameters. The first one is the string. The second is a 0-based location, also called the offset, and the third is the length of the substring we would like to get.

Menu Parser Example