Here is one way to sort the arrays by age and then by income: @sorted = sort { $age[$a] <=> $age[$b] || $income[$a] <=> $income[$b] } @index; This uses the spaceship operator (<=>) to first compare the ages, and if they are equal, then compares the incomes to break the tie.

1. Introduction to Perl
Part I
By: Amit Kr. Sinha
Nex-G Exuberant Solutions Pvt. Ltd.

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
November 22, 2012

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
November 22, 2012

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
November 22, 2012

5. Web Sources for Perl
 Web
 www.perl.com
 www.perldoc.com
 www.perl.org
 www.perlmonks.org
November 22, 2012

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 a+x hello.pl
 Run it as follows:
 ./hello.pl
November 22, 2012

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
November 22, 2012

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
November 22, 2012

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
November 22, 2012

11. 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
November 22, 2012

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;
November 22, 2012

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
November 22, 2012

14. 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
November 22, 2012

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);
November 22, 2012

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
November 22, 2012

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'
November 22, 2012

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
November 22, 2012

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 “Hellon”; # prints Hello and then a return
November 22, 2012

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!
November 22, 2012

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”;
}
November 22, 2012

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!
November 22, 2012

24. While Loop
 Example :
$i = 0;
while ( $i <= 1000 )
{
print “$in”;
$i++;
}
November 22, 2012

25. Until Loop
 The until function evaluates an expression
repeatedly until a specific condition is met.
 Example:
$i = 0;
until ($i == 1000) {
print “$in”;
$i++;
}
November 22, 2012

26. For Loops
 Syntax 1:
 for ( $i = 0; $i <= 1000; $i=$i+2 )
{
print “$in”;
}
 Syntax 2:
 for $i(0..1000)
{
print “$in”;
}
November 22, 2012

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 < 10; $i++)
{
if ($i == 1 || $i == 3) { next; }
elsif($i == 5) { last; }
else
{print “$in”;}
}
November 22, 2012

28. Answer
0
2
4

29. Exercise
 Use a loop structure and code a program that
produces the following output:
A
AA
AAA
AAAB
AAABA
AAABAA
AAABAAA
AAABAAAB
…..
November 22, 2012
TIP: $chain = $chain . “A”;

30. Exercise
#! /usr/bin/perl
for ($i=0, $j=0; $i<100; $i++)
{
if ( $j==3){$chain.=“B”;$j=0;}
else {$chain.=“A”; $j++;}
print “$chainn”;
}
November 22, 2012

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 :
November 22, 2012 rand 100

32. Exercise
for ($i=0; $i<100; $i++)
{
$v=rand 100;
#print “Patient $i $vn”;
printf “Patient %d %.2fnn”, $i, $v;
#%s : chaines, strings
#%d : integer
#%f : floating points
}
November 22, 2012

33. Collections Of Variables: Arrays

34. 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
November 22, 2012

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 “$array[0]n”; # prints : Larry
 Question:
 What happens if we access $array[3] ?

November 22, 2012
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 = @array;
 $array_size now has 3 in the above example
because there are 3 elements in the array
November 22, 2012

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 <=> and cmp operators make creating sorting
subroutines very easy
November 22, 2012

38. Numerical Sorting Example
#!/usr/local/bin/perl -w
@unsortedArray = (3, 10, 76, 23, 1, 54);
@sortedArray = sort numeric @unsortedArray;
print “@unsortedArrayn”; # prints 3 10 76 23 1 54
print “@sortedArrayn”; # 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 $a>$b
November 22, 2012

39. String Sorting Example
#!/usr/local/bin/perl -w
@unsortedArray = (“Larry”, “Curly”, “moe”);
@sortedArray = sort { lc($a) cmp lc($b)} @unsortedArray;
print “@unsortedArrayn”; # prints Larry Curly moe
print “@sortedArrayn”; # prints Curly Larry moe
November 22, 2012

40. Foreach
 Foreach allows you to iterate over an array
 Example:
foreach $element (@array)
{
print “$elementn”;
}
 This is similar to :
for ($i = 0; $i <= $#array; $i++)
{
print “$array[$i]n”;
}
November 22, 2012

41. 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 Moe
November 22, 2012

42. 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:
November 22, 2012
-Sort the index, using information contained in the other arrays.

43. 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]; }
}
November 22, 2012

44. Manipulating Arrays

45. 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
November 22, 2012

46. 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
November 22, 2012

47. 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”
November 22, 2012

48. 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”
November 22, 2012

49. 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”)
November 22, 2012

50. 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 :
November 22, 2012
 “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 couples”
Into
“five couples were administered the enquiry 1”
….
November 22, 2012

52. 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_sn”;
}
November 22, 2012

53. Multidimentional Arrays

54. 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
November 22, 2012

55. Thank you 