Chapter 9: Perl Programming

1
Chapter 9 Perl Programming

• Practical Extraction and Report Language

Some materials are taken from Sams Teach Yourself
Perl 5 in 21 Days, Second Edition
2
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

3
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.

4
What can Perl do?

• anything involving text processing
• internet programming
• system administration
• bioinformatics
• quick prototyping
• database programming

5
What is bad of Perl?

• Bad at efficiency
• Not suitable for large scale computation

6
A simple program

• repeat.pl
• !/usr/bin/perl
• inputline ltSTDINgt
• print( inputline )
• chmod ux repeat.pl
• Note ltSTDINgt represents a line of input from the
  standard input file.

7
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
• Scalars
• Arrays
• Associated Arrays
• There are others, but well talk about them at a
  later time.

8
Data Types and Operations

• Scalars
• Arrays
• Associative Arrays

9
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 ()

10
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.

11
Assign Values to Scalars

• Examples
• var ltSTDINgt get input from screen
• var 5 6 4
• var 3.458
• var HUSKER"
• var Hello World!

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Data Types and Operations

• Scalars
• Arrays
• Associative Arrays

13
Arrays

• Arrays are variables that store an ordered list
  of scalar values that are accessed with numeric
  subscripts, starting at zero.
• An at sign (_at_) precedes the name of an array
  when assigning it values

14
Arrays

• A array can hold a list of any number or type of
  scalars
• _at_array ()
• _at_array (23, HUSKER, 3.14159)
• _at_array (var1, var2)
• Because Perl uses _at_ 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
• _at_var (11, 27.1, "a string")

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Accessing an Element in an Array

• Index of array begins with zero
• _at_array (1, 2, 3, HUSKER)
• array3 5 (1, 2, 3, 5)
• scalar array1 scalar 2
• index 2 scalar arrayindex use a
  scalar variable as a subscript scalar 3
• Notes
• _at_array (1, 2, 3, 4)
• scalar array4 scalar will be a null
  string (which is equivalent of zero)
• array7 7 array will be (1, 2, 3, 4, ,
  , , 7)
• scalar array-1 scalar will be a null
  string

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Length of array

• Retrieving the Length of a List
• scalar _at_array
• Example
• _at_list (UNL", HUSKER", university")
• length _at_list
• length is 3.

17
grep operation on array

• grep extract the elements of a list that match a
  specified pattern
• foundlist grep (pattern, searchlist)
• Example
• _at_list (UNL", HUSKER", university")
• _at_foundlist grep(/Uu/, _at_list)
• foundlist (UNL, university)

18
Push/Pop on Array

• Push add an element to the end of a list
• _at_array ("one", "two")
• array3 "four" ("one", "two", "", "four
  ")
• push (_at_array, "five") ("one", "two", "",
  "four", "five")
• Pop remove the last element from the end of a
  list
• popped pop (_at_array)
• popped is "five" array is ("one", "two", "",
  "four ")

19
shift and unshift operations

• shift remove an element from the front of a list
• _at_array (1, 2, 3)
• firstval shift(_at_array)
• mylist 1, array ("2", "3")
• unshift undo the effect of a shift function
• _at_array (1, 2, 3)
• num unshift (_at_array, "newitem")
• returns the number of elements in the resulting
  list. array ("newitem", 1, 2, 3)

20
split operation

• split split a string into a list of elements
• list split (pattern, string)
• Example 1
• line "Thisisastring"
• _at_array split (//, line)
• array is ("This", "is", "a", "string").
• Example 2
• _at_array split(/\s/, line2) split
  according to white space.

21
sort operation

• sort sorts the elements of an array in
  alphabetical order and returns the sorted list
• list2 sort (list1)
• Example
• _at_array ("this", "is", "a", "test")
• _at_array2 sort (_at_array)
• array2 is ("a", "is", "test", "this").
• Note sort treats its items as strings, not
  integers items are sorted in alphabetical, not
  numeric, order.
• _at_array (70, 100, 8)
• _at_array sort (_at_array)
• array is (100, 70, 8), not (8, 70, 100)

22
reverse operation

• reverse reverses the order of the elements of a
  list or array variable, and returns the reversed
  list.
• list2 reverse (list1)
• Example
• _at_array ("backwards", "is", "array", "this")
• _at_array2 reverse(_at_array)
• array2 is ("this", "array", "is", "backwards")

23
Data Types and Operations

• Scalars
• Arrays
• Associative Array

24
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
• Drivers license number, name

25
Associative Array

• Associative arrays are preceded by a percent sign
  () when they are assigned values.
• For example
• fruit ("apples gt 6, "cherries gt 8,
  "oranges gt 11)
• d 2fruitapples 3 fruitcherries
• d now is 28.
• Or fruit ("apples", 6, "cherries", 8,
  "oranges", 11)
• Or _at_fruit ("apples", 6, "cherries", 8,
  "oranges", 11)
• fruit _at_fruit

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Associative Array

• List array indexes
• fruit ("apples", 9, "bananas", 23, "cherries",
  11)
• _at_fruitsubs keys(fruits)
• _at_fruitsubs is ("apples", "bananas",
  "cherries")
• List array values
• _at_fruitvalues values(fruits)
• _at_ fruitvalues is (9, 23, 11)

27
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")

28
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

29
Basic Control Flow for loop

• for statement
• for (expr1 expr2 expr3)
• statement_block
• Example
• for (count0 count lt 5 count)
• statements inside the loop go here

30
Basic Control Flow foreach

• foreach statement
• foreach localvar (listexpr)
• statement_block
• Example
• _at_words ("Here", "is", "a", "list.")
• foreach word (_at_words)
• print ("word\n")

31
Summary

• Perl is being extensively used as a powerful
  text-manipulation tool
• Perl has three basic data types
• Scalars (begin with )
• Arrays (begin with _at_)
• Associative Arrays (begin with )
• A list is an ordered group of simple variables or
  literals, separated by commas

32
Chapter Summary

• Basic logic control
• if
• while
• for
• foreach