Introduce to perl programming

 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

Introduction to Perl

programming

By: Cédric Notredame (Adapted from BT McInnes)

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.”

 Too Slow For Number Crunching

 Ideal for Prototyping

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 5.8

 To check if Perl is working and the version number

 % perl -v

Resources For Perl

Web Sources for Perl

The Basic Hello World Program

 which perl

 pico hello.pl

 Program:

#! /…path…/perl -w

print “Hello World!\n”;

 Save this as “hello.pl”

 Give it executable permissions

 chmod a+x hello.pl

 Run it as follows:

./hello.pl

“Hello World” Observations

 “.pl” extension is optional but is commonly

Variables and Their Content

Numerical Literals

 Numerical Literals

 12.6 Floating Point

 1e10 Scientific Notation

 6.4E-33 Scientific Notation

 4_348_348 Underscores instead of

commas for long numbers

String Literals

 String Literals

 “There is more than one way to do it!”

 'Just don't create a file called -rf.'

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

Operators on Scalar Variables

 Numeric and Logic Operators

 Typical : +, -, *, /, %, ++, , +=, -=, *=, /=, ||, &&, ! ect

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

Relational Operators for Strings

A String Example Program

 Convert to upper case

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'

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 quotes

print '$stooge is one of the three stooges.\n’;

Produces the output:

$stooge is one of the three stooges.\n

Character Interpolation

 List of character escapes that are recognized when

using double quoted strings

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 number

print $a + 18; # prints 22

$b = “50”; # looks like a string, but

print $b – 10; # will print 40!

Control Structures: Loops and Conditions

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”;

}

Moving around in a Loop

 next: ignore the current iteration

 last: terminates the loop.

 What is the output for the following code

0 2 4

 Use a loop structure and code a program that

produces the following output:

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:

Collections Of Variables: Arrays

 @array = ( “Larry”, “Curly”, “Moe” );

array

 print @array; # prints : Larry Curly Moe

 Notice that you do not need to loop through the whole array to print it – Perl does this for you

Arrays cont…

 Array Indexes start at 0 !!!!!

 To access one element of the array : use $

 Why? Because every element in the array is scalar

 print “$array[0]\n”; # prints : Larry

 Question:

 What happens if we access $array[3] ?

elements in the array:

$array_size = @array;

 $array_size now has 3 in the above example because there are 3 elements in the array

Sorting Arrays

 Perl has a built in sort function

 Two ways to sort:

 Default : sorts in a standard string comparisons

order

 sort LIST

 Usersub: create your own subroutine that returns

an integer less than, equal to or greater than 0

 Sort USERSUB LIST

 The <=> and cmp operators make creating sorting subroutines very easy

Numerical Sorting Example

#!/usr/local/bin/perl -w

@unsortedArray = (3, 10, 76, 23, 1, 54);

@sortedArray = sort numeric @unsortedArray;

print “@unsortedArray\n”; # prints 3 10 76 23 1 54

print “@sortedArray\n”; # prints 1 3 10 23 54 76

sub numeric

{

return $a <=> $b;

}

#!/usr/local/bin/perl -w

@unsortedArray = (“Larry”, “Curly”, “moe”);

@sortedArray = sort { lc($a) cmp lc($b)} @unsortedArray;

print “@unsortedArray\n”; # prints Larry Curly moe

print “@sortedArray\n”; # prints Curly Larry moe

String Sorting Example

Sorting with Foreach

 The sort function sorts the array and returns the list in sorted order.

 Example :

@array( “Larry”, “Curly”, “Moe”);

foreach $element (sort @array)

{ print “$element ”;

}

Exercise: Sorting According to

Exercise: Sorting According to

{

if ($age[$a] == $age[$b])

{return $income[$a]<=>$income[$b]; } else

Manipulating Arrays

Strings to Arrays : split

 Split a string into words and put into an array

@array = split( /;/, “Larry;Curly;Moe” );

@array= (“Larry”, “Curly”, “Moe”);

# creates the same array as we saw previously

 Split into characters

@stooge = split( //, “curly” );

# array @stooge has 5 elements: c, u, r, l, y

Split cont

 Split on any character

@array = split( /:/, “10:20:30:40”);

# array has 4 elements : 10, 20, 30, 40

 Split on Multiple White Space

@array = split(/\s+/, “this is a test”;

# array has 4 elements : this, is, a, test

 More on ‘\s+’ later

Arrays to Strings

 Array to space separated string

@array = (“Larry”, “Curly”, “Moe”);

$string = join( “;“, @array);

# string = “Larry;Curly;Moe”

 Array of characters to string

@stooge = (“c”, “u”, “r”, “l”, “y”);

$string = join( “”, @stooge );

# string = “curly”

Joining Arrays cont…

 Join with any character you want

Arrays as Stacks and Lists

 To append to the end of an array :

@array = ( “Larry”, “Curly”, “Moe” );

push (@array, “Shemp” );

print $array[3]; # prints “Shemp”

 To remove the last element of the array (LIFO)

$elment = pop @array;

print $element; # prints “Shemp”

 @array now has the original elements

Arrays as Stacks and Lists

@array = ( “Larry”, “Curly”, “Moe” );

unshift @array, “Shemp”;

print $array[3]; # prints “Moe”

print “$array[0]; # prints “Shemp”

$element = shift @array;

print $element; # prints “Shemp”

 The array now contains only :

 Unshift: beginning, push: end

 Use split, shift and push to turn the following string:

“The enquiry 1 was administered to five couples”

“The enquiry 2 was administered to six couples”

“The enquiry 3 was administered to eigh couples”

Into

Exercise: Spliting

 Use split, shift and push to turn the following string:

$s[0]= “The enquiry 1 was administered to five couples”;

$s[1]= “The enquiry 2 was administered to six couples”;

$s[2]= “The enquiry 3 was administered to eigh couples”;

foreach $s(@s)

{

@s2=split (/was administered to/, $s);

$new_s=“$s2[1] were admimistered $s2[0]”;

print “$new_s\n”;

}

Multidimentional Arrays

Multi Dimensional Arrays

 Better use Hash tables (cf later)

 If you need to:

Thank you 