1 *Copyright © 2002 Pearson Education, Inc.. 2 Web Wizard’s Guide to CGI/Perl David Lash Chapter...


Web Wizard’s Guide to CGI/Perl

David Lash

Chapter 3

Perl Basics


Chapter Objectives

Describe Perl scalar variables for numerical and string data

Describe Arithmetic Operations Describe Perl conditional statements


Using Scalar Variables

Variables allow you to store and access data in computer memory.

Two primary types: » Scalar variables - hold a singular item such

as a number (for example, 1, 1239.12, or –123) or a character string (for example, “apple,” “ John Smith,” or “address”).

» List variables - hold a set of items (such as a set of numbers). (More in Chapter 5).


Assigning Values to Variables

Place the variable’s name on the left side of an equals sign (=) and the value on the right side of the equals sign.

The following Perl statements use two variables: $x and $months

$x = 3; $months = 12;

Name of variable. Variable's new value


$X = 60; $Weeks = 4; $X = $Weeks;

Assigns 4 to $X and $Weeks. Note: Perl variables are case sensitive.

» $x and $X are considered different variable names.

Assigning New Values to Variables


Selecting Variable Names

Perl variable rules

» Perl variable names must have a dollar sign ($) as the first character.

» The second character must be a letter or underscore (_).

» Less than 251 characters.» Valid: $baseball, $_sum, $X, $Numb_of_bricks, $num_houses, and $counter1.

» Not Valid: $123go, $1counter, and counter.


Variables and the print Place a variable name inside the double quotes of the print statement. to print out the value. E.g.,

» print “The value of x= $x”;

1. #!/usr/bin/perl2. print “Content-type: text/html\n\n”;3. $x = 3;4. $y = 5;5. print “The value of x is $x. ”;6. print “The value of y= $y.”;

Assign 3 to $x

Assign 5 to $x


Would Output The Following:


Operating on Variables

Perl expressions are used to manipulate data values. » Use operators such as a plus sign (+) for addition and a

minus sign (–) for subtraction. For example, 1. #!/usr/bin/perl2. print “Content-type: text/html\n\n”;3. $x = 3 + 4; 4. $y = 5 + $x; 5. print “The value of x is $x but y = $y”;

Assign 7 to $xAssign 12 to $y


Would Output The following:


Some Key Perl Operators

Operator Effect Example Result

+ Adds two

items

$x = 2 + 2; $x is assigned 4

- Subtraction $y = 3; $y = $y – 1; $y is assigned 2

/ Division $y = 14 / 2; $y is assigned 7

* Multiplication $z = 4; $y = $z * 4; $y is assigned 16

** Exponent $y = 2; $z = 4 ** $y;

$z is assign 16

% Remainder $y = 14 % 3; $y is assigned 2


Example Program

1. #!/usr/bin/perl2. print “Content-type: text/html\n\n”;3. $cubed = 3 ** 3; 4. $onemore = $cubed + 1;5. $cubed = $cubed + $onemore;6. $remain = $onemore % 3;7. print “The value of cubed is $cubed onemore= $onemore ”;

8. print “The value of remain= $remain”;

Assign 27 to $cubedAssign 55 to $cubed

$remain is remainder of 28 / 3 or 1


Would Output The Following ...


Writing Complex Expressions

Operator precedence rules define the order in which the operators are evaluated.

For example, consider the following expression:

» $x = 5 + 2 * 6; » $x could = 56 or 17 depending on evaluation

order


Perl Precedence Rules

1. Operators within parentheses.

2. Exponential operators.

3. Multiplication and division operators.

4. Addition and subtraction operators.

Evaluates to 82

Evaluates to 19

Consider the following$X = 100 – 3 ** 2 * 2; $Y = 100 – ((3 ** 2) * 2);$Z = 100 – ( 3 ** (2 * 2) );

Variables with HTML Output - II

1. #!/usr/bin/perl

2. print “Content-type: text/html\n\n”;

3. print “<HTML> <HEAD> <TITLE> Example </TITLE></HEAD>”;

4. print “<BODY> ”;

5. $num_week = 8;

6. $total_day = $num_week * 7;

7. $num_months = $num_week / 4;

8. print “Number of days are $total_day ”;

9. print “<HR>The total number of months=$num_months”;

10. print “</BODY></HTML>”;

Assign 28Assign 2

Set bluefont, size 5

Horizontalrule followed by black font.


String Variables

Variables can hold numerical or character string data» For example, to hold customer names,

addresses, product names, and descriptions. $letters=”abc”;

$fruit=”apple”;

Assign “abc”

Assign “apple”

Enclose in double quotes


String Variables String variables have their own operations.

» You cannot add, subtract, divide, or multiply string variables.

The concatenate operator joins two strings together and takes the form of a period (“.”).

The repeat operator is used when you want to repeat a string a specified number of times.


Concatentate Operator

Joins two strings together (Uses period (“.”)).

$FirstName = “Bull”;$LastName = “and Bear”;$FullName1 = $FirstName . $LastName;$FullName2 = $FirstName . “ “ . $LastName;

print “FullName1=$FullName1 and

Fullname2=$FullName2”;

Would output the following:

FullName1=Bulland Bear and FullName2=Bull and Bear


Repeat Operator Used to repeat a string a number of times. Specified by the

following sequence:

$varname x 3

For example,

$score = “Goal!”;$lots_of_scores = $score x 3;print “lots_of_scores=$lots_of_scores”;


lots_of_scores=Goal!Goal!Goal!

Repeat string value3 times.


Repeat W/o Repeat Operator You don’t have to use the repeat operator. Also common to see …

For example,

$score = “Goal!”;$lots_of_scores = “$score$score$score”print “lots_of_scores=$lots_of_scores”;


lots_of_scores=Goal!Goal!Goal!

Repeat string value3 times.

A Full Program Example Using Repeat Operator ...

1. #!/usr/bin/perl2. print "Content-type: text/html\n\n";3. print "<HTML> <HEAD><TITLE> String Example</TITLE></HEAD>";

4. print "<BODY>";5. $first = "John";6. $last = "Smith";7. $name = $first . $last;8. $triple = $name x 3;9. print " name=$name";10.print " triple = $triple";

11. print "</BODY></HTML>";

Concatenate

Repeat


Conditional Statements

Conditional statements enable programs to test for certain variable values and then react differently

Use conditionals in real life: » Get on Interstate 90 East at Elm Street and go east toward the city. If you encounter construction delays at mile marker 10, get off the expressway at this exit and take Roosevelt Road all the way into the city. Otherwise, stay on I-90 until you reach the city.


Conditional Statements Perl supports 3 conditional clauses:

» An if statement specifies a test condition and set of statements to execute when a test condition is true.

» An elsif clause is used with an if statement and specifies an additional test condition to check when the previous test conditions are false.

» An else clause is used with an if statement and possibly an elsif clause. It specifies a set of statements to execute when one or more test conditions are false.


The if Statement Uses a test condition and set of statements to

execute when the test condition is true.

» A test condition uses a test expression enclosed in parentheses within an if statement.

» When the test expression evaluates to true, then one or more additional statements within the required curly brackets ({ … }) are executed.

if ( $aver > 69 ) { $Grade="Pass"; print "Grade=$Grade";}print "Your average was $aver";

Statement(s) to executeregardless of test condition.

Execute these statementswhen $aver is greater than 69.

Numerical Test Operators

Numerical Test Operator

Effect Example Result

== Equal to if ($x == 6){ $x = $y + 1; $y = $x + 1; }

Execute the second and third statements if the value of $x is equal to 6.

!= Not equal to if ($x != $y) { $x = 5 + 1; }

Execute the second statement if the value of $x is not equal to the value of $y.

< Less than if ($x < 100) { $y = 5; }

Execute the second statement if the value of $x is less than 100.

> Greater than if ($x > 51) { print “OK”; }

Execute the second statement if the value of $x is greater than 51.

>= Greater than or equal if (16 >= $x) {

print “x=$x”; }

Execute the second statement if 16 is greater than or equal to the value of $x.

>= Greater than or equal if (16 >= $x) {

print “x=$x”; }

Execute the second statement if 16 is greater than or equal to the value of $x.

A Sample Conditional Program

1. #!/usr/bin/perl2. print "Content-type: text/html\n\n";3. print "<HTML> <HEAD><TITLE> String Example </TITLE></HEAD>";

4. print "<BODY>";5. $grade = 92; 6. if ( $grade > 89 ) { 7. print “ Hey you got an A. ”; 8. } 9. print “Your actual score was $grade”;

Different Values of Line 5

Line 5 Output

$grade=92;

Hey you got an A.

Your actual score was 92

$grade=89;


$grade=40;


$grade=1100;

Hey you got an A.


$grade=-50;

Your actual score was –50

1. #!/usr/bin/perl2. print "Content-type: text/html\n\n";3. print "<HTML> <HEAD><TITLE> String Example

</TITLE></HEAD>";4. print "<BODY>";5. $grade = 92; 6. if ( $grade > 89 ) { 7. print “ Hey you got an A. ”; 8. } 9. print “Your actual score was $grade”;


String Test Operators Perl supports a set of string test operators that are based

on ASCII code values.

» Standard, numerical representation of characters.

» Every letter, number, and symbol translates into a code number.

– “A” is ASCII code 65, and “a” is ASCII code 97.

– Numbers are lower ASCII code values than letters, uppercase letters lower than lowercase letters.

– Letters and numbers are coded in order, so that the character “a” is less than “b”, “C” is less than “D”, and “1” is less than “9”.


String Test OperatorsTest Operator Effect Example Result

eq Equal to $name=”Nelson”; if ($name eq “Nelson”){ print “Got Nelson”; }

Prints Got Nelson because $name is equal to “Nelson”.

ne Not equal to $alert=”Smoke”; if ( $alert ne “fire” ){ print “$alert NE fire”; $ok = $ok + 1; }

Prints Smoke NE fire and then executes fourth line, because $alert is not equal to “fire”.

lt Less than if ( “auto” lt “car” ) { print “Before car”; }

Prints Before car because “auto” is less than “car”.

gt Greater than if (“Jones” gt “Smith” ){ print “After Smith”; }

Prints nothing because “Jones” is not greater than “Smith”.

ge Greater than or equal to

if ( “WSOX” ge “Cubs” ){ print “Sox Win”; }

Prints Sox Win because “WSOX” is greater than or equal to “Cubs”.

le Less than or equal to $Name = “Bob”; if (“Bob” le $Name ) { print “Got Bob”; }

Prints Got Bob because “Bob” is less than or equal to $Name.


The elsif Clause

Specifies an additional test condition to check when all previous test conditions are false. » Used only with if statement» When its condition is true, gives one or more

statements to execute $found = 0; if ( $name eq "Joe" ) {

print "Got Name of Joe";$found = 1;

} elsif ( $name eq "Jane" ) {

print "Got Name of Jane";$found = 1;

} print "Name=$name and found=$found\n";

Execute these whenif condition is true.

Execute these whenits condition is true

but previous is false.

Executetregardless of theprevioustest conditions.


The elsif Clause 1. #!/usr/bin/perl2. print “Content-type: text/html\n\n”;3.$grade = 92; 4.if ( $grade > 100 ) { 5. print “Illegal Grade > 100”;6.} 7.elsif ( $grade > 89 ){ 8. print “Hey you got an A ”; 9.}

1.0 print “Your actual grade was $grade”;

Line 3 Output

$grade=92; Hey you got an A Your actual grade was 92

$grade=89; Your actual grade was 89


$grade=1100; Illegal Grade > 100 Your actual grade was 1100

$grade=-50; Your actual grade was –50

The elsif Clause

1. #!/usr/bin/perl2.print “Content-type: text/html\n\n”;3.$grade = 92; 4.if ( $grade >= 100 ) { 5. print “Illegal Grade > 100 ”;6.}7.elsif ( $grade < 0 ) { 8. print “illegal grade < 0 ”; 9.} 10.elsif ( $grade > 89 ){ 11. print “Hey you got an A ”; 12.}

13. print “Your actual grade was $grade”;

Some Sample Values for Line 3

Line 3 Output

$grade=92; Hey you got an A Your actual grade was 9



$grade=1100; Illegal Grade > 100 Your actual grade was 1100

$grade=-50; illegal grade < 0 Your actual grade was –50

1. #!/usr/bin/perl2.print “Content-type: text/html\n\n”;3.$grade = 92; 4.if ( $grade >= 100 ) { 5. print “Illegal Grade > 100 ”;6.}7.elsif ( $grade < 0 ) { 8. print “illegal grade < 0 ”; 9.} 10.elsif ( $grade > 89 ){ 11. print “Hey you got an A ”; 12.}

13. print “Your actual grade was $grade”;


The else Clause

Specifies a set of statements to execute when all other test conditions in an if block are false.

» It must be used with at least one if statement, (can also be used with an if followed by one or several elsif statements.

if ( $name eq "Joe" ) {print "Got Name of Joe";

} elsif ( $name eq "Jane" ) {

print "Got Name of Jane"; } else {

print "Could not validate Name=$name"; }

One or more statements to executewhen test condition is true.

One or more statements to executewhen its test condition is true but the previous testcondition is false.

One or more statements to executewhen the previous test condition(s) are false.

Using An Else Clause

1.#!/usr/bin/perl2.print “Content-type: text/html\n\n”;3.$grade = 92; 4.if ( $grade >= 100 ) {5. print “Illegal Grade > 100”;6.}7.elsif ( $grade < 0 ) { 8. print “illegal grade < 0”; 9.} 10.elsif ( $grade > 89 ){ 11. print “Hey you got an A”; 12.} 13.else { 14. print “Sorry you did not get an A”;

15.}

Sample Values of Line 3

Line 3 Output

$grade=92; Hey you got an A

$grade=89; Sorry you did not get an A

$grade=40; Sorry you did not get an A

$grade=1100; Illegal Grade > 100

$grade=-50; illegal grade < 0 1.#!/usr/bin/perl2.print “Content-type: text/html\n\n”;3.$grade = 92; 4.if ( $grade >= 100 ) {5. print “Illegal Grade > 100”;6.}7.elsif ( $grade < 0 ) { 8. print “illegal grade < 0”; 9.} 10.elsif ( $grade > 89 ){ 11. print “Hey you got an A”; 12.} 13.else { 14. print “Sorry you did not get an A”;

15.}


Summary Variables are used to store and access data in

computer memory. Numerical and string variables have unique

operations that can be used to manipulate their values.

Conditional statements are used to test for conditions and, based on the results of the test, execute specific program statements. Perl provides different test condition operators for string and numerical variables.


Summary The if statement can be used by itself

and is a basic way of testing conditions. The elsif clause can be used with the if statement to provide another test condition when the original if statement is false. An else clause provides statements that execute when all other if and elsif conditions are false.


Perl “Jeoprdy”

Answer: $1st_counter, $2ndctr, squared?

Question: Give an example of 3 illegal variable names.

Question: What are the Perl operator precedence order?

Answer: exponential operators, parenthesis, multiplication & division, addition & subtraction


So think your smart? Answer: (a) if, elsif, else (b) if

Question: Which three types of conditional clauses were described in this chapter. Which is the only one that can be used by itself?

Question: Which test operator is used to test whether one of two string variables is greater than the other? What test operator is used to test whether one of two numerical variables is greater than the other?

Answer: gt, >


Impressive but ... Answer: “Independence forever”

Question: What did John Adams say in 1826 when asked if he had a message to pass along to people on the 50th anniversary of the signing of the declaration of Independence.

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