Introduction to Perl

Part I

By: Amit Kr. Sinha

Nex-G Exuberant Solutions Pvt. Ltd.

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

April 14, 2023

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

April 14, 2023

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

April 14, 2023

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 pico hello.pl 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

April 14, 2023

“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

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Variables and Their Content

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

April 14, 2023

String Literals

String Literals “There is more than one way to do it!” 'Just don't create a file called -rf.' “Beauty?\nWhat's that?\n” “” “Real programmers can write assembly in any

language.”

Quotes from Larry WallApril 14, 2023

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

April 14, 2023

Operators on Scalar Variables

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

… Not typical: ** for exponentiation

String Operators Concatenation: “.” - similar to strcat

$first_name = “Larry”;

$last_name = “Wall”;

$full_name = $first_name . “ “ . $last_name;

April 14, 2023

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

April 14, 2023

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

April 14, 2023

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);

April 14, 2023

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

April 14, 2023

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'

April 14, 2023

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

April 14, 2023

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

April 14, 2023

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!

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Control Structures: Loops and Conditions

If ... else ... statements

if ( $weather eq “Rain” )

{

print “Umbrella!\n”;

}

elsif ( $weather eq “Sun” ) {

print “Sunglasses!\n”;

}

else {

print “Anti Radiation Armor!\n”;

}

April 14, 2023

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!April 14, 2023

While Loop

Example :$i = 0;

while ( $i <= 1000 ) {

print “$i\n”;

$i++;

}

April 14, 2023

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++;

}April 14, 2023

For Loops

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

{ print “$i\n”; }

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

{ print “$i\n”; }

April 14, 2023

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”;}

}April 14, 2023

Answer

0

2

4

Exercise

Use a loop structure and code a program that produces the following output:

AAAAAAAAABAAABAAAABAAAAABAAAAAABAAAB

…..

TIP: $chain = $chain . “A”;April 14, 2023

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”;

}

April 14, 2023

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 the srand to seed the random number generator

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

rand 100April 14, 2023

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

}

April 14, 2023

Collections Of Variables: Arrays

Arrays

Array variable is denoted by the @ symbol @array = ( “Larry”, “Curly”, “Moe” );

To access the whole array, use the whole 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

April 14, 2023

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] ?

Answer1 : Value is set to 0 in Perl Answer2: Anything in C!!!!!

April 14, 2023

Arrays cont ...

To find the index of the last element in the arrayprint $#array; # prints 2 in the previous

# example

Note another way to find the number of elements in the array:$array_size = @array; $array_size now has 3 in the above example

because there are 3 elements in the arrayApril 14, 2023

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

April 14, 2023

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;

}

# Numbers: $a <=> $b : -1 if $a<$b , 0 if $a== $b, 1 if $a>$b

# Strings: $a cpm $b : -1 if $a<$b , 0 if $a== $b, 1 if $a>$bApril 14, 2023

#!/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

April 14, 2023

Foreach

Foreach allows you to iterate over an array Example:

foreach $element (@array) { print “$element\n”;}

This is similar to : for ($i = 0; $i <= $#array; $i++) { print “$array[$i]\n”;}

April 14, 2023

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 ”;

}

Prints the elements in sorted order:

Curly Larry MoeApril 14, 2023

Exercise: Sorting According to Multiple Criterion

Use the following initialization to sort individuals by age and then by income:

Syntax

@sortedArray = sort numeric @unsortedArray;sub numeric

{ return $a <=> $b;

}Data

@index=(0,1,2,3,4);@name=(“V”,“W”,”X”,”Y”,”Z”);@age=(10,20, 15, 20, 10);@income=(100,670, 280,800,400);

Output: Name X Age A Income I

…

Tip:-Sort the index, using information contained in the other arrays.April 14, 2023

Exercise: Sorting According to Multiple Criterion

@index=(0,1,2,3,4,5);@name=(“V”,“W”,”X”,”Y”,”Z”);@age=(10,20, 15, 20, 10);@income=(100,670, 280,800,400);

foreach $i ( sort my_numeric @index){

print “$name[$i] $age[$i] $income[$i];}

sub my_numeric {

if ($age[$a] == $age[$b]){return $income[$a]<=>$income[$b]; }

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

}April 14, 2023

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

April 14, 2023

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

April 14, 2023

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”

April 14, 2023

Joining Arrays cont…

Join with any character you want@array = ( “10”, “20”, “30”, “40” );

$string = join( “:”, @array);

# string = “10:20:30:40”

Join with multiple characters@array = “10”, “20”, “30”, “40”);

$string = join(“->”, @array);

# string = “10->20->30->40”

April 14, 2023

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

(“Larry”, “Curly”, “Moe”)

April 14, 2023

Arrays as Stacks and Lists

To prepend to the beginning of an array@array = ( “Larry”, “Curly”, “Moe” );unshift @array, “Shemp”;print $array[3]; # prints “Moe”print “$array[0]; # prints “Shemp”

To remove the first element of the array $element = shift @array;print $element; # prints “Shemp” The array now contains only :

“Larry”, “Curly”, “Moe”April 14, 2023

Exercise: Spliting Instructions

Remove shift: beginning, pop: end

Add 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“five couples were administered the enquiry 1”

….April 14, 2023

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”;

}

April 14, 2023

Multidimentional Arrays

Multi Dimensional Arrays

Better use Hash tables (cf later) If you need to:

@tab=([‘Monday’,’Tuesday’],

[‘Morning’,’Afternoon’,’Evening’]);

$a=$tab[0][0] # $a == ‘Monday’

$tab2=(‘midnight’,  ‘Twelve’);

$tab[2]=\@tab2 # integrate tab2 as the last row of tab

April 14, 2023

Thank you