Introduction to Perl

1
Introduction to Perl
2
What is Perl

• Perl is an interpreted language. This means you
  run it through an interpreter, not a compiler.
• Similar to shell script but lot easier and more
  powerful.
• Perl is free to download and is also available
  for Windows and Macintosh.
• File name extension .pl
• The first line of a perl program should tell
  where to find the perl intepreter
• !/usr/bin/perl

3
Steps to Run a Perl Program

• Use your favorite editor to create a Perl
  program, say test.pl.
• Change the file permission and make it
  executable.
• chmod 700 test.pl
• Run Perl program
• test.pl

4
Comments

• The pound sign "" is the symbol for comment
  entry.
• Exception First line, !/usr/bin/perl, tells
  where to find the Perl compiler on your system

5
Variables

• Three types of variables.
• Scalar
• Array
• Hash

6
Scalars

• Scalar means single value
• In C/C, many different kinds of single values
• int, float, double, char, bool
• In Perl, Scalar variable can hold all these
  types, and more, such as string.
• In Perl, we do not need to declare the data type
  of variables

7
Scalars

• All Scalar variables begin with a
• Examples foo, a, zebra1, F87

8
Scalar Assignments

• Scalars hold any data type
• foo 3
• d 4.43
• temp Z could be double quote
• My_String Hello, Im Paul. could be single
  quote
• value TRUE

9
Arithmetic in Perl
a 1 2 Addition b 3 - 4
Subtraction c a b
Multiplication d a / b Division a
9 10 Exponentiation b 5 2
Modulo
10
Single and double quotes

• a 'apples' you can also use double quote
• b 'bananas' you can also use double quote
• print 'a and b'
• display a and b
• print "a and b"
• display apples and bananas
• Single quotation marks do not interpret, and
  double quotation marks do

11
Arrays

• Concept is the same as in C/C
• Groups of other values
• much more dynamic than C/C
• no declaration of size, type
• can hold any kinds of value, and multiple kinds
  of values
• All array variables start with the _at_ character
• _at_array, _at_foo, _at_My_Array, _at_temp34
• Array index stars from 0

12
Array Operations

• _at_a1 (3, 2, 1, 4)
• _at_a2 (34, z, Hi, 43.2)
• Assignment
• a22 X _at_a2 (34, z, X, 43.2)
• Copy array
• _at_a3 _at_a1 _at_a3 (3, 2, 1, 4)
• Merge array
• _at_a5 (_at_a1, _at_a2) _at_a5 (3, 2, 1, 4, 34, z,
  Hi, 43.2)
• How about this operation
• _at_a1 (_at_a1, _at_a2) _at_a1 (3, 2, 1, 4, 34, z,
  Hi, 43.2)

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Sort Array

• _at_a1 (3, 2, 1, 4)
• Sort array
• _at_a4 sort (_at_a1) _at_a4 (1, 2, 3, 4)
• How about this one?
• _at_a1 (3, 22, 11, 4)
• _at_a4 sort (_at_a1) _at_a4 (11, 22, 3, 4)
• This is same as
• _at_a4 sort a cmp b (_at_a1)
• Array is sorted alphabetically (elements are
  considered as string)

14
Sort Array (continued)

• Sorted alphabetically
• _at_a1 (3, 22, 11, 4)
• _at_a4 sort (_at_a1)
• or _at_a4 sort a cmp b (_at_a1)
• _at_a4 (11, 22, 3, 4)
• Sort numerically
• _at_a1 (3, 22, 111, 4)
• _at_a4 sort a ltgt b (_at_a1)
• _at_a4 (3, 4, 11, 22)
• Note
• Inside sort block, two variables must be a and
  b
• If a and b is exchanged, the sorting order is
  changed

15
More about arrays

• special variable for each array
• _at_foo (3, 25, 43, 31)
• foo (a variable last index of _at_foo, which is
  3).
• foo1 (size of array _at_foo, which is 4).

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Program Flow if statement

• if (x y)
• ...
• elsif (x (y1))
• ...
• else
• ...

17
Program Flow Comparing variables
Numbers ! lt gt
Strings eq ne lt gt
18
Program Flow Logical operators

• (logical and)
• (or)
• ! (negation)

19
Program Flow (Loops)

• for (t 0 t lt 100 t)
• ...
• while (x y)
• ...

20
foreach Statement

• This statement takes an array variable and
  assigns one item at a time to a scalar variable,
  executing a block of code.
• For example, _at_list an array
• foreach var (_at_list)

21
Basic IO

• Output to terminal
• The print statement.
• Example
• print My name is name\n
• Input from keyboard
• The ltgt operator
• Example
• input ltgt
• read one line from keyboard, and save in variable
  input

22
Task 1

• Write a Perl program to ask the user to enter a
  name, then it will display
• Hello name_user_rntered

23
Perl Program for Task 1

• !/usr/bin/perl
• print "Enter your name"
• name ltgt
• print "Hello name\n"

24
Chomp

• When reading in, carriage return (\n) is
  included.
• Usually dont want that.
• chomp will take off the last character of a
  string, if it is a \n.
• chomp (foo)

25
Perl Program for Task 1 (revised)

• !/usr/bin/perl
• print "Enter your name"
• name ltgt
• chomp(name)
• print "Hello name\n"

26
Read / Write to Files

• To read and write to files we should create
  something called handles which refer to the
  files.

27
Read from Files

• To create a file handle for reading
• Use the OPEN command
• Example
• open(filehandle1,"filename1")

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Read from Files (continued)

• Once the file handles have been obtained, we can
  read data (line by line) from the file.
• Example
• _at_lines ltfilehandle1gt
• This will result in each line being read from the
  file pointed by the file handle and all lines are
  stored in the array variable _at_lines, where index
  0 (lines0) contains first line of the file,
  index 1 (lines1) contains second line of the
  file, and so on.

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Read from Files (continued)

• After read file, we should close the file.
• close(filehandle1)

30
Task 2

• Write a Perl program that can read a file
  (test.cpp) and display each line with a line
  number

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Perl program for Task 2

• !/usr/bin/perl
• open(fh1, "test.cpp")
• _at_input_linesltfh1gt
• chomp(_at_input_lines)
• close(fh1)
• i1
• foreach line(_at_input_lines)
• print "i line\n"
• ii1

32
Write to Files

• To create a file handle for writing
• Use the OPEN command
• Example
• open(filehandle2, "gtfilename2")

33
Write to Files (continued)

• Once the file handles have been obtained, we can
  write data to the file.
• Example
• print filehandle2 "linevalue"
• This will result in the value of linevalue being
  written to the file pointed by the filehandle2.

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Write to Files (continued)

• After write to file, we should close the file.
• close(filehandle2)

35
Task 3

• Rewrite the Perl program for Task 2 so that the
  result will be write to a file (test_c.cpp)
  instead of displaying on the screen.

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Perl program for Task 3

• !/usr/bin/perl
• open(fh1, "test.cpp")
• _at_input_linesltfh1gt
• chomp(_at_input_lines)
• close(fh1)
• i1
• open(fh2, "gttest_c.cpp")
• foreach line(_at_input_lines)
• print fh2 "i line\n"
• ii1
• close(fh2)

37
Subroutines (functions)

• To define your own subroutine, use the keyword
  sub
• Can be defined anywhere in your program
• sub function_name
• commands

38
Function Calls

• Name getname() return a value
• getname() not returning a value

39
Parameters of Functions

• Parameters are passed a function as an array.
• The parameter is taken in as an array which is
  denoted by _at__ inside the function.
• So if you pass only one parameter the size of
  array _at__ will be one. If you pass two parameters
  then the _at__ size will be two and the two
  parameters can be accessed by _0,_1 , and
  so on.

40
Subroutines

• !/usr/bin/perl
• result max(11, 12)
• Print The largest number is result \n
• sub max
• if(_0 gt _1)
• return _0
• else
• return _1

Output The largest number is 12
41
More About Functions

• The variables declared in the main program are by
  default global so they will continue to have
  their values in the function also.
• Local variables are declared by putting my key
  word while declaring the variable.

42
Subroutines local variable example

• !/usr/bin/perl
• sub max
• my _at_num _at__
• if(num0 gt num1)
• return num0
• else
• return num1
• result max(11, 12)
• Print The largest number is result \n

43
A routine (user defined) to read web pages

• sub getweb
• my url _0
• require LWPUserAgent
• my ua LWPUserAgent-gtnew
• ua-gttimeout(10)
• ua-gtenv_proxy
• my response ua-gtget(url)
• return response-gtcontent

This routine takes one parameter (a web address)
and returns the contents of a web page as one
string
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Task 4

• Display the html code of www.google.com

45
Perl program for task 4

• !/usr/bin/perl
• google getweb("http//www.google.com")
• print google the entire page is saved as one
  string
• sub getweb
• my url _0
• require LWPUserAgent
• my ua LWPUserAgent-gtnew
• ua-gttimeout(10)
• ua-gtenv_proxy
• my response ua-gtget(url)
• return response-gtcontent