Chapter 9: Perl Programming

Practical Extraction and Report Language

Some materials are taken from Sams Teach Yourself Perl 5 in 21 Days, Second Edition

Learning to Use PerlObjectives

After studying this lesson, you should be able to:

• Learn the basics of the Perl language

• Learn three data types

– Scalars

– Arrays

– Associative Arrays

• Basic control flows

Introduction to Perl

• Perl (Practical Extraction and Report Language) was created in 1986 by Larry Wall as a simple report generator

• Perl provides the best of several worlds– Powerful and flexible, similar to a high-level

programming language such as C. – An interpreted language. no interpreter, or

compiler needed.– Provides all the features of the script

languages sed and awk, plus features not found in either of these two languages.

What can Perl do?

• anything involving text processing

• internet programming

• system administration

• bioinformatics

• quick prototyping

• database programming

What is bad of Perl?

• Bad at efficiency

• Not suitable for large scale computation

A simple program

• repeat.pl

#!/usr/bin/perl

$inputline = <STDIN>;

print( $inputline );

• chmod u+x repeat.pl

• Note: <STDIN> represents a line of input from the standard input file.

Identifying Data Types

• Data may be represented in a Perl program in a variety of ways

• You will learn about three basic types of data:ScalarsArraysAssociated Arrays

• There are others, but we’ll talk about them at a later time….

Data Types and Operations

ScalarsArraysAssociative Arrays

Scalars

• A scalar is a simple variable that holds a number or a string

• In C/C++, many different kinds of scalars, such as int, float, double, char, bool

• In Perl, scalar variable can hold all these types, without declaration

• Scalar variable names begin with a dollar sign ($)

Scalar Names

• Scalar variable names begin with a dollar sign ($)

• Next character is a letter• Remaining characters: letters, numbers, or _• Variable names can be between 1 and 251

characters in length• Legal samples: $f, $bar, $z1, $d_3• NOT legal: $, $_1, $47y, $x.y, $foo!• Perl variables are case-sensitive.

Assign Values to Scalars

Examples:

$var = <STDIN>; # get input from screen

$var = 5 + 6 * 4;

$var = 3.458;

$var = “HUSKER";

$var = ‘Hello World!’;

Data Types and Operations

ScalarsArraysAssociative Arrays

Arrays

• Arrays are variables that store an ordered list of scalar values that are accessed with numeric subscripts, starting at zero.

• An “at” sign (@) precedes the name of an array when assigning it values

Arrays

• A array can hold a list of any number or type of scalars:@array = ();@array = (23, “HUSKER”, 3.14159);@array = ($var1, $var2);

• Because Perl uses @ and $ to distinguish array variables from scalar variables, the same name can be used in an array variable and in a scalar variable. For example: $var = 1;@var = (11, 27.1, "a string");

Accessing an Element in an Array

Index of array begins with zero@array = (1, 2, 3, “HUSKER”); $array[3] = 5; # (1, 2, 3, 5)$scalar = $array[1]; # $scalar = 2;$index = 2; $scalar = $array[$index]; # use a scalar

variable as a subscript; $scalar = 3;

Notes:@array = (1, 2, 3, 4);$scalar = $array[4]; # scalar will be a null string (which is equivalent of zero)$array[7] = 7; # array will be (1, 2, 3, 4, “”, “”, “”, 7)$scalar = $array[-1]; # scalar will be a null string

Length of array

Retrieving the Length of a List:

$scalar = @array;

• Example:

@list = (“UNL", “HUSKER", “university");

$length = @list;

# $length is 3.

grep operation on array

• grep: extract the elements of a list that match a specified pattern

foundlist = grep (pattern, searchlist);

• Example:@list = (“UNL", “HUSKER", “university"); @foundlist = grep(/^[Uu]/, @list);

# foundlist = (“UNL”, “university”);

Push/Pop on Array

• Push: add an element to the end of a list@array = ("one", "two");$array[3] = "four"; # ("one", "two", "", "four ");push (@array, "five"); # ("one", "two", "", "four",

"five");

• Pop: remove the last element from the end of a list

$popped = pop (@array); # popped is "five"; array is ("one", "two", "", "four ");

shift and unshift operations

• shift: remove an element from the front of a list

@array = (1, 2, 3); $firstval = shift(@array); # mylist = 1, $array = ("2", "3")

• unshift: undo the effect of a shift function@array = (1, 2, 3); $num = unshift (@array, "newitem"); # returns the number of elements in the resulting list.

array = ("newitem", 1, 2, 3)

split operation

split: split a string into a list of elementslist = split (pattern, string);

Example 1:$line = "This:is:a:string"; @array = split (/:/, $line); #array is ("This", "is", "a", "string").

Example 2: @array = split(/[\s]+/, $line2); # split according to

white space.

sort operation

sort: sorts the elements of an array in alphabetical order and returns the sorted list list2 = sort (list1);

Example:@array = ("this", "is", "a", "test"); @array2 = sort (@array); #array2 is ("a", "is", "test", "this").

Note: sort treats its items as strings, not integers; items are sorted in alphabetical, not numeric, order.

@array = (70, 100, 8); @array = sort (@array); # array is (100, 70, 8), not (8, 70, 100)

reverse operation

reverse: reverses the order of the elements of a list or array variable, and returns the reversed list. list2 = reverse (list1);

Example:@array = ("backwards", "is", "array", "this");@array2 = reverse(@array); #array2 is ("this", "array", "is", "backwards")

Data Types and Operations

ScalarsArraysAssociative Array

Associative Array

• An associative array is a variable that represents a set of key/value pairs

• Instead of indexing by numbers as we did in arrays, we can look up the values by name.

• Why need it? A easy data structure keeps mapping.– Host name, ip address– ip address, hostname– Student name, score– Driver’s license number, name

Associative Array

• Associative arrays are preceded by a percent sign (%) when they are assigned values.

For example:

%fruit = ("apples“ => 6, "cherries“ => 8, "oranges“ => 11);

$d = 2*$fruit{“apples”} +3* $fruit{“cherries”};

# $d now is 28.

Or: %fruit = ("apples", 6, "cherries", 8, "oranges", 11);

Or: @fruit = ("apples", 6, "cherries", 8, "oranges", 11);

%fruit = @fruit;

Associative Array

• List array indexes

%fruit = ("apples", 9, "bananas", 23, "cherries", 11);

@fruitsubs = keys(%fruits);

# @fruitsubs is ("apples", "bananas", "cherries");

• List array values

@fruitvalues = values(%fruits);

# @ fruitvalues is (9, 23, 11)

Basic Control Flow: if

• if statement

if (expr_1) {

statement_block_1

} elsif (expr_2) {

statement_block_2

} else {

default_statement_block

}

• Example:

if ($number) { print ("The number is not zero.\n"); }

Basic Control Flow: while loop

• while statement

while (expr) {

statement_block

}

• Example:

while ($done == 0) {print ("The value of count is", $count, "\n");if ($count == 3) { $done = 1; }$count = $count + 1;

}

Basic Control Flow: for loop

• for statement

for (expr1; expr2; expr3) {

statement_block

}

• Example:

for ($count=0; $count < 5; $count++) {

# statements inside the loop go here

}

Basic Control Flow: foreach

• foreach statementforeach localvar (listexpr) { statement_block;}

• Example:

@words = ("Here", "is", "a", "list.");

foreach $word (@words) {

print ("$word\n");

}

Summary

• Perl is being extensively used as a powerful text-manipulation tool

• Perl has three basic data types:

– Scalars (begin with $)

– Arrays (begin with @)

– Associative Arrays (begin with %)

• A list is an ordered group of simple variables or literals, separated by commas

Chapter Summary

• Basic logic control– if

– while

– for

– foreach