Introduction to Perl

Day 1

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What is Perl? Perl is a Portable Scripting Language

No compiling is needed. Runs on Windows, UNIX, LINUX and cygwin

Fast and easy text processing capability Fast and easy file handling capability Written by Larry Wall “Perl is the language for getting your job done.”

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How to Access Perl

To install at home Perl Comes by Default on Linux, Cygwin, MacOSX www.perl.com Has rpm's for Linux www.activestate.com Has binaries for Windows

Latest Version is 5.18 To check if Perl is working and the version number

% perl -v

4

Resources For Perl Books:

Learning Perl By Larry Wall Published by O'Reilly

Programming Perl By Larry Wall,Tom Christiansen and Jon Orwant Published by O'Reilly

Web Site http://safari.oreilly.com

Contains both Learning Perl and Programming Perl in ebook form

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Web Sources for Perl Web

www.perl.com www.perldoc.com www.perl.org www.perlmonks.org

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The Basic Hello World Program

which perl Program:

#! /…path…/perl -wprint “Hello World!\n”;

Save this as “hello.pl” Give it executable permissions

chmod a+x hello.pl Run it as follows:

./hello.pl or perl hello.pl

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“Hello World” Observations

“.pl” extension is optional but is commonly used The first line “#!/usr/local/bin/perl” tells UNIX where

to find Perl “-w” switches on warning : not required but a really

good idea

Variables and Their Content

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Numerical Literals Numerical Literals

6 Integer 12.6 Floating Point 1e10 Scientific Notation 6.4E-33 Scientific Notation 4_348_348 Underscores instead of

commas for long numbers

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String Literals String Literals

“There is more than one way to do it!” 'Just don't create a file called -rf.‘ “Real programmers can write assembly in any

language.”

Quotes from Larry Wall

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Types of Variables

Types of variables: Scalar variables : $a, $b, $c Array variables : @array Hash variables : %hash File handles : STDIN, STDOUT, STDERR

Variables do not need to be declared Variable type (int, char, ...) is decided at run time

$a = 5; # now an integer $a = “perl”; # now a string

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Operators on Scalar Variables

Numeric and Logic Operators Typical : +, -, *, /, %, ++, --, +=, -=, *=, /=, ||, &&, ! etc

…

String Operators Concatenation: “.” - similar to strcat

$first_name = “Larry”; $last_name = “Wall”;$full_name = $first_name . “ “ . $last_name;

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Equality Operators for Strings Equality/ Inequality : eq and ne

$language = “Perl”;if ($language == “Perl”) ... # Wrong!if ($language eq “Perl”) ... #Correct

Use eq / ne rather than == / != for strings

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Relational Operators for Strings

Greater than Numeric : > String : gt

Greater than or equal to Numeric : >= String : ge

Less than Numeric : < String : lt

Less than or equal to Numeric : <= String : le

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String Functions Convert to upper case

$name = uc($name); Convert only the first char to upper case

$name = ucfirst($name);

Convert to lower case $name = lc($name);

Convert only the first char to lower case $name = lcfirst($name);

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A String Example Program Convert to upper case

$name = uc($name); Convert only the first char to upper case

$name = ucfirst($name);

Convert to lower case $name = lc($name);

Convert only the first char to lower case $name = lcfirst($name);#!/usr/bin/perl$var1 = “larry”;$var2 = “moe”;$var3 = “shemp”;……Output: Larry, MOE, sHEMP

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A String Example Program

#!/usr/local/bin/perl$var1 = “larry”;$var2 = “moe”;$var3 = “shemp”;

print ucfirst($var1); # Prints 'Larry'print uc($var2); # Prints 'MOE'print lcfirst(uc($var3)); # Prints 'sHEMP'

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Variable Interpolation

Perl looks for variables inside strings and replaces them with their value

$stooge = “Larry”print “$stooge is one of the three stooges.\n”;

Produces the output:Larry is one of the three stooges.

This does not happen when you use single quotesprint '$stooge is one of the three stooges.\n’;

Produces the output:$stooge is one of the three stooges.\n

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Character Interpolation

List of character escapes that are recognized when using double quoted strings \n newline \t tab \r carriage return

Common Example :

print “Hello\n”; # prints Hello and then a return

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Numbers and Strings are Interchangeable

If a scalar variable looks like a number and Perl needs a number, it will use it as a number

$a = 4; # a numberprint $a + 18; # prints 22$b = “50”; # looks like a string, but ...print $b – 10; # will print 40!

Control Structures: Loops and Conditions

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If ... else ... statements

if ( $weather eq “Rain” ) {

print “Umbrella!\n”; }

elsif ( $weather eq “Sun” ) { print “Sunglasses!\n”;}else { print “Anti Radiation Armor!\n”;}

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Unless ... else Statements

Unless Statements are the opposite of if ... else statements.

unless ($weather eq “Rain”) { print “Dress as you wish!\n”;}else { print “Umbrella!\n”;}

And again remember the braces are required!

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While Loop Example :

$i = 0;while ( $i <= 1000 )

{ print “$i\n”; $i++;}

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Until Loop

The until function evaluates an expression repeatedly until a specific condition is met.

Example: $i = 0; until ($i == 1000) { print “$i\n”;

$i++;}

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For Loops

Syntax 1: for ( $i = 0; $i <= 1000; $i=$i+2 )

{ print “$i\n”; }

Syntax 2: for $i(0..1000)

{ print “$i\n”; }

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Moving around in a Loop

next: ignore the current iteration last: terminates the loop.

What is the output for the following code snippet:for ( $i = 0; $i < 10; $i++)

{ if ($i == 1 || $i == 3) { next; } elsif($i == 5) { last; }

else {print “$i\n”;} }

Answer

0 2 4

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Exercise Use a loop structure and code a program that

produces the following output:

AAAAAAAAABAAABAAAABAAAAABAAAAAABAAAB

…..

TIP: $chain = $chain . “A”;

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Exercise

#! /usr/bin/perl

for ($i=0, $j=0; $i<100; $i++){

if ( $j==3){$chain.=“B”;$j=0;}else {$chain.=“A”; $j++;}print “$chain\n”;

}

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Exercise: Generating a Random Sample

A study yields an outcome between 0 and 100 for every patient. You want to generate an artificial random study for 100 patients:

Patient 1 99Patient 2 65Patient 3 89….

Tip:-use rand 100 to generate values between 0 and 100 :

rand 100

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Exercise

for ($i=0; $i<100; $i++){$v=rand 100;#print “Patient $i $v\n”;printf “Patient %d %.2f\n\n”, $i, $v;#%s : chaines, strings#%d : integer#%f : floating points}