1. Introduction to Programming in
Perl

2. 2
Outline
1.1 History of Perl
2.1 Introduction
2.2 Simple Programs that Display lines of Text
2.2.1 A Simple Program: Displaying a Line of Text
2.2.2 Executing the Program
2.2.3 A Simple Program: Printing Several Lines of Text
2.3 Another Simple Program: Adding Two Integers
2.4 Memory Concepts
2.5 Scalar Values and Scalar Variables
2.6 Arithmetic Operators
2.7 Assignment Operators
2.8 Increment and Decrement Operators
2.9 Decision Making: Equality and Relational Operators
2.10 Confusing Equality (==) and Assignment (=) Operators
2.11 String Operators
2.12 Numeric and String Context
2.13 Internet and world wide Web Resources

3. 2001 Prentice Hall, Inc. All
rights reserved.
1.1 History of Perl
• Developed by Larry Wall in 1987
– Aimed to integrate features of the UNIX languages awk
and sed with framework provided by shell
• Gained support in open-source community
• Versions of Perl
– Perl 3
• Adopted GNU General Public License
– Perl 4
• Adopted Artistic License to attract more users
– Perl 5
• Major reorganization of Perl language

4. 2001 Prentice Hall, Inc. All
rights reserved.
1.1 History of Perl
– Perl 6
• Development announced on July 18, 2000
• Will be a complete rewrite of internals and externals of Perl

5. 2001 Prentice Hall, Inc. All
rights reserved.
1.1 Perl Library
• Perl modules
– Reusable pieces of software
– Defined in library files ending in .pm
– Primary distribution center is the Comprehensive Perl
Archive Network (CPAN)

6. 2001 Prentice Hall, Inc. All
rights reserved.
1.1 General Notes About Perl
• Perl approach to programming
– Qualities of a good programmer
• Laziness
– Reusable code
• Impatience
– Functional programs
• Hubris
– Easy to understand code
– Programs can be written in many different ways
– Highly portable

7. 7
1 #!/usr/bin/perl
2 # Fig. 2.1: fig02_01.pl
3 # A first program in Perl
4
5 print "Welcome to Perl!n"; # print greeting
Welcome to Perl!
The “shebang” construct (#!)
indicates the path to the Perl
interpreter on Unix and Linux
systems.
The Perl comment character (#) indicates that
everything on the current line following the # is a
comment.
Function print outputs the string “Welcome to
Perl!n” to the screen.
The escape sequence n represents newline and
positions the screen cursor at the beginning of the
next line.

8. 8
Escape
Sequence
Description
n Newline. Position the screen cursor
at the beginning of the next line.
t Horizontal tab. Move the screen
cursor to the next tab stop.
r Carriage return. Position the screen
cursor at the beginning of the
current line; do not advance to the
next line.
$ Dollar sign. Used to insert a
dollar-sign character in a string.
Backslash. Used to insert a
backslash character in a string.
" Double quote. Inserts a double-quote
character in a double-quoted string.
' Single quote. Inserts a single-quote
character in a single-quoted string.
Fig. 2.2 Some common escape sequences.

9. 9
1 #!/usr/bin/perl
2 # Fig. 2.3: fig02_03.pl
3 # Prints a welcome statement in a variety of ways
4
5 print ( "1. Welcome to Perl!n" );
6 print "2. Welcome to Perl!n" ;
7 print "3. Welcome ", "to ", "Perl!n";
8 print "4. Welcome ";
9 print "to Perl!n";
10 print "5. Welcome to Perl!n";
11 print "6. Welcomen tonn Perl!n";
1. Welcome to Perl!
2. Welcome to Perl!
3. Welcome to Perl!
4. Welcome to Perl!
5. Welcome to Perl!
6. Welcome
to
Perl!
Arguments to built-in functions like
print can be placed in parentheses.
Whitespace characters are ignored by
Perl.
More than one argument may be passed to
function print.
Although these statements are
on separate lines, the string
Welcome to Perl! is
displayed on one line.
Newline characters may be used to print
multiple lines of text using one line of code.

10. 10
1 #!/usr/bin/perl
2 # Fig. 2.4: fig02_04.pl
3 # A simple addition program
4
5 # prompt user for first number
6 print "Please enter first number:n";
7 $number1 = <STDIN>;
8 chomp $number1; # remove "n" from input
9
10 print "Please enter second number:n";
11 $number2 = <STDIN>;
12 chomp $number2; # remove "n" from input
13
14 $sum = $number1 + $number2;
15
16 print "The sum is $sum.n";
Please enter first number:
45
Please enter second number:
72
The sum is 117.
Prompt to tell the user to
enter the first number.
Declare scalar variable $number1.
Assignment operator =.
The expression <STDIN> causes program
execution to pause while the computer
waits for the user to enter data.
Function chomp removes the newline
character from the end of the line.
The assignment statement calculates the sum of
the variables $number1 and $number2 and
assigns the result to variable $sum.

11. 11
$number1 45n
$number1 45

12. 12
$number1
$number2 72
45

13. 13
$number1 45
$number2 72
$sum 117

14. 14
Operation Arithmet
ic
operator
Algebraic
expression
Perl
expression
Addition + x + y $x + $y
Subtraction - x – y $x - $y
Multiplication * xy $x * $y
Division / x / y $x / $y
Modulus % x mod y $x % $y
Exponentiation ** x y $x ** $y
Fig. 2.9 Arithmetic op era tors.

15. 15
Operator(s) Operation(s) Order of evaluation (precedence)
( ) Parentheses Evaluated first. If the parentheses
are nested, the expression in the
innermost pair is evaluated first. If
there are several pairs of parentheses
“on the same level” (i.e., not
nested), they are evaluated left to
right.
** Exponentiation Evaluated second. If there is more
than one, the operators are evaluated
from right to left.
*, / or % Multiplication
Division
Modulus
Evaluated third. If there is more than
one, the operators are evaluated from
left to right.
+ or - Addition
Subtraction
Evaluated last. If there is more than
one, the operators are evaluated from
left to right.
Fig. 2.10 Precedence of arithmetic operators.

16. 16
Assignment operator Sample
expression
Explanation Assigns
Assume that: $c = 3, $d = 5, $e
= 4, $f = 6, $g = 12 and $h = 5.
+= $c += 7 $c = $c + 7 10 to $c
-= $d -= 4 $d = $d - 4 1 to $d
*= $e *= 5 $e = $e * 5 20 to $e
/= $f /= 3 $f = $f / 3 2 to $f
%= $g %= 9 $g = $g % 9 3 to $g
**= $h **= 2 $h = $h **
2
25 to $h
Fig. 2.12 Arithmetic assignment operators.

17. 17
Operat
or
Name Sample
expression
Explanation
++ preincrement ++$c Increment $c by 1, then use the
new value of $c in the expression
in which $c resides.
++ postincrement $c++ Use the current value of $c in
the expression in which $c
resides, then increment $c by 1.
-- predecrement --$d Decrement $d by 1, then use the
new value of $d in the expression
in which $d resides.
-- postdecrement $d-- Use the current value of $d in
the expression in which $d
resides, then decrement $d by 1.
Fig. 2.13 Increment and decrement operators.

18. 18
1 #!/usr/bin/perl
2 # Fig. 2.14: fig02_14.pl
3 # Demonstrates the difference between pre- and postincrement
4
5 $c = 5;
6 $d = 5;
7
8 print $c, " "; # print 5
9 print $c++, " "; # print 5 then postincrement
10 print $c, "n"; # print 6
11
12 print $d, " "; # print 5
13 print ++$d, " "; # preincrement then print 6
14 print $d, "n"; # print 6
5 5 6
5 6 6
Print variable $c.
Use the postincrement operator to use the
current value of $c in the expression and then
increment $c by 1.
Print the new value of variable $c.
Use the preincrement operator to increment $c by
1 and then use the new value of $c in the
expression.

19. 19
Operator(s) Associativity Called
( ) left to right parentheses
++ or -- none increment and decrement
** right to left exponentiation
*, / or % left to right multiplicative
+ or - left to right additive
=, +=, -=, *=,
/=, %= or **=
right to left assignment
Fig. 2.15 Precedence and associativity of operators
discussed so far.

20. 20
Standard algebraic
equality operator or
relational operator
Perl
equality
or
relational
operator
Example
of Perl
condition
Meaning of
Perl condition
Relational operators
> > $x > $y $x is greater than
$y
< < $x < $y $x is less than $y
> >= $x >= $y $x is greater than
or equal to $y
< <= $x <= $y $x is less than or
equal to $y
Equality operators
= == $x == $y $x is equal to $y
= != $x != $y $x is not equal to
$y
Fig. 2.16 Equality and relational operators.

21. 21
1 #!/usr/bin/perl
2 # Fig. 2.17: fig02_17.pl
3 # Using if statements with relational and equality operators
4
5 print "Please enter first integer: ";
6 $number1 = <STDIN>;
7 chomp $number1;
8
9 print "Please enter second integer: ";
10 $number2 = <STDIN>;
11 chomp $number2;
12
13 print "The integers satisfy these relationships:n";
14
15 if ( $number1 == $number2 ) {
16 print "$number1 is equal to $number2.n";
17 }
18
19 if ( $number1 != $number2 ) {
20 print "$number1 is not equal to $number2.n";
21 }
22
23 if ( $number1 < $number2 ) {
24 print "$number1 is less than $number2.n";
25 }
26
27 if ( $number1 > $number2 ) {
28 print "$number1 is greater than $number2.n";
29 }
30
Prompt the user for two
numbers, read in the numbers,
and remove the newline
characters with chomp.
The if structure compares the values of
variable $number1 and variable
$number2 to test for equality.
The body of the if structure,
enclosed by a pair of braces ({}),
executes if the condition evaluates
to true.
The equality operator != tests
whether $number1 and
$number2 are not equal.
The relational operator < tests
whether $number1 is less than
$number2.
The relational operator > tests
whether $number1 is greater
than $number2.

22. 22
31 if ( $number1 <= $number2 ) {
32 print "$number1 is less than or equal to $number2.n";
33 }
34
35 if ( $number1 >= $number2 ) {
36 print "$number1 is greater than or equal to $number2.n";
37 }
Please enter first integer: 3
Please enter second integer: 7
The integers satisfy these relationships:
3 is not equal to 7.
3 is less than 7.
3 is less than or equal to 7.
Please enter first integer: 22
Please enter second integer: 12
The integers satisfy these relationships:
22 is not equal to 12.
22 is greater than 12.
22 is greater than or equal to 12.
Please enter first integer: 7
Please enter second integer: 7
The integers satisfy these relationships:
7 is equal to 7.
7 is less than or equal to 7.
7 is greater than or equal to 7.
The relational operator <=
tests whether $number1 is
less than or equal to
$number2.
The relational operator >=
tests whether $number1 is
greater than or equal to
$number2.

23. 23
Operators Associativity Type
() left to right parentheses
++ -- none increment and
decrement
** right to left exponential
* / % left to right multiplicativ
e
+ - left to right additive
< <= > >= none relational
== != none equality
= += -= *= /= %= **= right to left assignment
Fig. 2.18 Precedence and associativity of operators discussed
so far.

24. 24
1 #!/usr/bin/perl
2 # Fig. 2.19: fig02_19.pl
3 # Program to illustrate numeric and string context, and undef
4
5 $string = "Top 10";
6 $number = 10.0;
7 print "Number is 10.0 and string is 'Top 10'nn";
8
9 $add = $number + $string; # 10 (not 20)
10 print "Adding a number and a string: $addn";
11
12 $concatenate = $number . $string; # '10Top 10'
13 # (not '10.0Top 10')
14 print "Concatenating a number and a string: $concatenaten";
15
16 $add2 = $concatenate + $add; # 20 (not 30, not 1020)
17 print "Adding the previous two results: $add2nn";
18
19 $undefAdd = 10 + $undefNumber;
20 print "Adding 10 to an undefined variable: $undefAddn";
21
22 print "Printing an undefined variable:
$undefVariable(end)n";
Number is 10.0 and string is 'Top 10'
Adding a number and a string: 10
Concatenating a number and a string: 10Top 10
Adding the previous two results: 20
Adding 10 to an undefined variable: 10
Printing an undefined variable: (end)
The binary addition
operator evaluates strings in
numeric context. If nothing
can be interpreted as
numeric, the string
evaluates to 0.
The concatenation operator
evaluates $number in string
context, in this case “10”.
When using a string in
numeric context, Perl
stops conversion at the
first character that cannot
be used in numeric
context.
When an undefined variable is
found in numeric context, it
evaluates to 0.
When an undefined value is
interpreted in string context, it
evaluates to an empty string (“”).