CSET4100 – Fall 2009 Perl Introduction Scalar Data, Operators & Control Blocks...

CSET4100 – Fall 2009

Perl IntroductionScalar Data, Operators & Control

Blocks

Acknowledgements: Slides adapted from NYU Computer Science course on UNIX Tools

CSET4100: Server-Side Programming - Perl & CGI 2

What is Perl?

• Practical Extraction and Report Language• Scripting language created by Larry Wall in the

mid-80s• Functionality and speed somewhere between

low-level languages (like C) and high-level ones (like shell)

• Influence from awk, sed, and C Shell• Easy to write (after you learn it), but sometimes

hard to read• Widely used in CGI scripting

Perl Textbook

Good Ways to Learn Perl

• Textbook– Example programs

• perldoc– Online Perl documentation$ perldoc perldoc perldoc man page

$ perldoc perlintro Perl introduction

$ perldoc -f sort Perl sort function man page

$ perldoc CGI CGI module man page

Modules

• Perl modules are libraries of reusable code with specific functionalities

• Standard modules are distributed with Perl, others can be obtained from

• Include modules in your program with use, e.g. use CGI incorporates the CGI module

A Simple Perl Script

hello:#!/usr/bin/perl -wprint “Hello, world!\n”;

$ chmod a+x hello$ ./helloHello, world!$ perl -e ‘print “Hello, world!\n”;’

Hello, world!

turns on warnings

Data Types

• Basic types: scalar, lists, hashes• Support OO programming and user-defined

Data Types

• Type of variable determined by special leading character

$foo scalar

@foo list

%foo hash

&foo function

Scalars

• Can be numbers$num = 100;$num = 223.45;$num = -1.3e38;

Scalars Cont.

• Can be strings$str = ’unix tools’;$str = ’Who\’s there?’;$str = ”good evening\n”;$str = ”one\ttwo”;

• Backslash escapes and variable names are interpreted inside double quotes

Special Scalar Variables

$0 Name of script

$_ Default variable

$$ Current PID

$? Status of last pipe or system call

$! System error message

$/ Input record separator

$. Input record number

undef Acts like 0 or empty string

undef and defined

$f = 1;while ($n < 10) { # $n is undef at 1st iteration $f *= ++$n;}

• Use defined to check if a value is undefif (defined($val)) { … }

Operators

• Numeric: + - * / % **• String concatenation: .

$state = “New” . “York”;

#Prints “NewYork”

Operators Cont.

• String repetition: xprint “bla” x 3;

#Prints blablabla

Operators Cont.

• Binary assignments:$val = 2; $val *= 3;# $val is now 6

$state .= “City”;# $state is now “NewYorkCity”

Comparison Operators

Comparison Numeric String

Equal == eq

Not Equal != ne

Greater than > gt

Less than < lt

Less than or equal to <= le

Greater than or equal to >= ge

Boolean “Values”

if ($ostype eq “unix”) { … }if ($val) { … }• No boolean data type• undef is false• 0 is false; Non-zero numbers are true• ‘’ and ‘0’ are false; other strings are true• The unary not (!) negates the boolean value

Conditional Blocks & Loops

if - elsif - else

• if … elsif … else …if ( $x > 0 ) {print “x is positive\n”;

}elsif ( $x < 0 ) {print “x is negative\n”;

}else {print “x is zero\n”;

Examples

$a = 3;

if ($a) {print "foo1\n";

} else {print "bar1\n"

#prints “foo1”

Examples

$a = 0;

#prints “bar2”

Examples

$a = -1;

#prints “foo3”

Examples

$a = 3;

if ($b) {print "foo4\n";

#prints “bar4”

unless

• Like the opposite of if

unless ($x < 0) {print “$x is non-negative\n”;

unlink $file unless -A $file < 100;

while and until

while ($x < 100) {$y += $x++;

• until is like the opposite of whileuntil ($x >= 100) {$y += $x++;

• for (init; test; incr) { … }

# sum of squares of 1 to 5for ($i = 1; $i <= 5; $i++) {$sum += $i*$i;

• next skips the remaining of the current iteration (like continue in C)

# only print non-blank lineswhile (<>) {if ( $_ eq “\n”) { next; }else { print; }

• last exits loop immediately (like break in C)

# print up to first blank linewhile (<>) {if ( $_ eq “\n”) { last; }else { print; }

Logical AND/OR

• Logical AND : &&if (($x > 0) && ($x < 10)) { … }

• Logical OR : ||if ($x < 0) || ($x > 0)) { … }

• Both are short-circuit — second expression evaluated only if necessary

Ternary Operator

• Same as the ternary operator (?:) in C• expr1 ? expr2 : expr3• Like if-then-else: If expr1 is true, expr2 is used;

otherwise expr3 is used

$weather=($temp>50)?“warm”:“cold”;

Assignment

• Learning Perl– Review Ch. 1 & 2 – Read Ch. 3 (Lists and Arrays) & Ch. 6 (Hashes)

• Homework– Check the website…

CSET4100 – Fall 2009 Perl Introduction Scalar Data, Operators & Control Blocks...

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