Introduction to Perl Part

1
Introduction to PerlPart I

• By Cédric Notredame
• (Adapted from BT McInnes)

2
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

3
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

4
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

5
Web Sources for Perl

• Web
• www.perl.com
• www.perldoc.com
• www.perl.org
• www.perlmonks.org

6
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 ax hello.pl
• Run it as follows
• ./hello.pl

7
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

8

• Variables and Their Content

9
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

10
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 Wall

11
Types of Variables

• Types of variables
• Scalar variables a, b, c
• Array variables _at_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

12
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

13
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

14
Relational Operators for Strings

• Greater than
• Numeric gt String gt
• Greater than or equal to
• Numeric gt String ge
• Less than
• Numeric lt String lt
• Less than or equal to
• Numeric lt String le

15
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)

16
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

17
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'

18
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

19
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

20
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!

21

• Control Structures Loops and Conditions

22
If ... else ... statements

• if ( weather eq Rain )
• print Umbrella!\n
• elsif ( weather eq Sun )
• print Sunglasses!\n
• else
• print Anti Radiation Armor!\n

23
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!

24
While Loop

• Example
• i 0
• while ( i lt 1000 )
• print i\n
• i

25
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

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For Loops

• Syntax 1
• for ( i 0 i lt 1000 ii2 )
• print i\n
• Syntax 2
• for i(0..1000)
• print i\n

27
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 lt 10 i)
• if (i 1 i 3) next
• elsif(i 5) last
• else
• print i\n

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Answer

• 0
• 2
• 4

29
Exercise

• Use a loop structure and code a program that
  produces the following output
• AAAAAAAAAB
• AAABA
• AAABAA
• AAABAAA
• AAABAAAB
• ..
• TIP chain chain . A

30
Exercise

• ! /usr/bin/perlfor (i0, j0 ilt100 i)
• if ( j3)chain.Bj0
• else chain.A j
• print chain\n

31
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 99
• Patient 2 65
• Patient 3 89
• .
• Tip
• - use the srand to seed the random number
  generator
• -use rand 100 to generate values between 0 and
  100
• rand 100

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Exercise

• for (i0 ilt100 i)
• vrand 100
• print Patient i v\n
• printf Patient d .2f\n\n, i, vs
  chaines, stringsd integer
• f floating points

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• Collections Of Variables Arrays

34
Arrays

• Array variable is denoted by the _at_ symbol
• _at_array ( Larry, Curly, Moe )
• To access the whole array, use the whole array
• print _at_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

35
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 array0\n prints Larry
• Question
• What happens if we access array3 ?
• Answer1 Value is set to 0 in Perl
• Answer2 Anything in C!!!!!

36
Arrays cont ...

• To find the index of the last element in the
  array
• print array prints 2 in the previous
  example
• Note another way to find the number of elements
  in the array
• array_size _at_array
• array_size now has 3 in the above example
  because there are 3 elements in the array

37
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 ltgt and cmp operators make creating sorting
  subroutines very easy

38
Numerical Sorting Example

• !/usr/local/bin/perl -w
• _at_unsortedArray (3, 10, 76, 23, 1, 54)
• _at_sortedArray sort numeric _at_unsortedArray
• print _at_unsortedArray\n prints 3 10 76 23 1
  54
• print _at_sortedArray\n prints 1 3 10 23 54 76
• sub numeric
• return a ltgt b
• Numbers a ltgt b -1 if altb , 0 if a
  b, 1 if agtb
• Strings a cpm b -1 if altb , 0 if
  a b, 1 if agtb

39
String Sorting Example
!/usr/local/bin/perl -w _at_unsortedArray
(Larry, Curly, moe) _at_sortedArray sort
lc(a) cmp lc(b) _at_unsortedArray print
_at_unsortedArray\n prints Larry Curly
moe print _at_sortedArray\n prints Curly
Larry moe
40
Foreach

• Foreach allows you to iterate over an array
• Example
• foreach element (_at_array)
• print element\n
• This is similar to
• for (i 0 i lt array i)
• print arrayi\n

41
Sorting with Foreach

• The sort function sorts the array and returns the
  list in sorted order.
• Example
• _at_array( Larry, Curly, Moe)
• foreach element (sort _at_array)
• print element
• Prints the elements in sorted order
• Curly Larry Moe

42
Exercise Sorting According to Multiple Criterion

• Use the following initialization to sort
  individuals by age and then by income
• Syntax
• _at_sortedArray sort numeric _at_unsortedArray
• sub numeric
• return a ltgt b
• Data
• _at_index(0,1,2,3,4)_at_name(V,W,X,Y,Z)
  _at_age(10,20, 15, 20, 10)_at_income(100,670,
  280,800,400)
• Output
• Name X Age A Income I
• Tip
• -Sort the index, using information contained in
  the other arrays.

43
Exercise Sorting According to Multiple Criterion

• _at_index(0,1,2,3,4,5)_at_name(V,W,X,Y,Z)
  _at_age(10,20, 15, 20, 10)_at_income(100,670,
  280,800,400)foreach i ( sort my_numeric
  _at_index) print namei agei
  incomei sub my_numeric
• if (agea ageb)
• return incomealtgtincomeb
• else
• return agealtgtageb

44

• Manipulating Arrays

45
Strings to Arrays split

• Split a string into words and put into an array
• _at_array split( //, LarryCurlyMoe )
• _at_array (Larry, Curly, Moe)
• creates the same array as we saw
  previously
• Split into characters
• _at_stooge split( //, curly )
• array _at_stooge has 5 elements c, u, r, l, y

46
Split cont..

• Split on any character
• _at_array split( //, 10203040)
• array has 4 elements 10, 20, 30, 40
• Split on Multiple White Space
• _at_array split(/\s/, this is a test
• array has 4 elements this, is, a, test
• More on \s later

47
Arrays to Strings

• Array to space separated string
• _at_array (Larry, Curly, Moe)
• string join( , _at_array)
• string LarryCurlyMoe
• Array of characters to string
• _at_stooge (c, u, r, l, y)
• string join( , _at_stooge )
• string curly

48
Joining Arrays cont

• Join with any character you want
• _at_array ( 10, 20, 30, 40 )
• string join( , _at_array)
• string 10203040
• Join with multiple characters
• _at_array 10, 20, 30, 40)
• string join(-gt, _at_array)
• string 10-gt20-gt30-gt40

49
Arrays as Stacks and Lists

• To append to the end of an array
• _at_array ( Larry, Curly, Moe )
• push (_at_array, Shemp )
• print array3 prints Shemp
• To remove the last element of the array (LIFO)
• elment pop _at_array
• print element prints Shemp
• _at_array now has the original elements
• (Larry, Curly, Moe)

50
Arrays as Stacks and Lists

• To prepend to the beginning of an array
• _at_array ( Larry, Curly, Moe )
• unshift _at_array, Shemp
• print array3 prints Moe
• print array0 prints Shemp
• To remove the first element of the array
• element shift _at_array
• print element prints Shemp
• The array now contains only
• Larry, Curly, Moe

51
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
  couplesInto five couples were administered
  the enquiry 1.

52
Exercise Spliting

• Use split, shift and push to turn the following
  strings0 The enquiry 1 was administered
  to five coupless1 The enquiry 2 was
  administered to six coupless2 The enquiry
  3 was administered to eigh couples
• foreach s(_at_s)
• _at_s2split (/was administered to/, s)
• new_ss21 were admimistered s20
• print new_s\n

53

• Multidimentional Arrays

54
Multi Dimensional Arrays

• Better use Hash tables (cf later)
• If you need to
• _at_tab(Monday,Tuesday,
• Morning,Afternoon,Evening)
• atab00 a Monday
• tab2(midnight,  Twelve)
• tab2\_at_tab2 integrate tab2 as the last row
  of tab

55
Thank you ?