introduction to perl day2

8/11/2019 Introduction to Perl Day2

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Perl Language

Day 2


2/23

Collections Of Variables: Arrays


3/23

3

Arrays

Array variable is denoted by the @ symbol @array = ( Larry, Curly, Moe );

To access the whole array, use the wholearray print @array; # prints : Larry Curly Moe

Notice that you do not need to loop throughthe whole array to print itPerl does this foryou


4/23

4

Arrays cont

Array Indexes start at 0 !!!!!

We can assign an array directly to another array.

E.g. @array_1 = @array_2;

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] ?

Answer1 : Value is set to 0 in Perl

Answer2: Anything in C!!!!!


5/23

5

Arrays cont ...

To find the index of the last element in thearrayprint $#array; # prints 2 in the previous

# example

Note another way to find the number ofelements in the array:$array_size = @array;

$array_size now has 3 in the above examplebecause there are 3 elements in the array


6/23

6

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 sortingsubroutines very easy


7/23

7

Numerical Sorting Example

#!/usr/local/bin/perl -w

@unsortedArray = (3, 10, 76, 23, 1, 54);

@sortedArray = sort numeric @unsortedArray;

print @unsortedArray\n; # prints 3 10 76 23 1 54print @sortedArray\n; # prints 1 3 10 23 54 76

sub numeric

{

return $a $b;

}

# Numbers: $a $b : -1 if $a$b

# Strings: $a cmp $b : -1 if $a$b


8/23

8

#!/usr/local/bin/perl -w

@unsortedArray = (Larry, Curly, moe);

@sortedArray = sort { lc($a) cmp lc($b)} @unsortedArray;

print @unsortedArray\n; # prints Larry Curly moeprint @sortedArray\n; # prints Curly Larry moe

String Sorting Example


9/23

9

Foreach

Foreach allows you to iterate over an array

Example:foreach $element (@array)

{

print $element\n;

}

This is similar to :for ($i = 0; $i


10/23

10

Sorting with Foreach

The sort function sorts the array and returns the list insorted order.

Example :

@array( Larry, Curly, Moe);foreach $element (sort @array)

{

print $element ;

}

Prints the elements in sorted order:

Curly Larry Moe


11/23

11

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]; }

}

O/P :: V -- 10100W -- 20670

X -- 15 -- 280

Y -- 20800

Z -- 10 -- 400


12/23

Sample Programs

1. Write a program to print the odd numbers from 1 to 500?

O/P :: 1,3,5,7,9,11, .

my $i = 0;

for ($i = 1; $i


13/23

Manipulating Arrays


14/23

14

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 sawpreviously

Split into characters

@stooge = split( //, curly );

# array @stooge has 5 elements: c, u, r, l, y


15/23

15

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


16/23

16

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


17/23

17

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


18/23

18

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)


19/23

19

Arrays as Stacks and Lists

To prepend to the beginning of an array@array = ( Larry, Curly, Moe );

unshift @array, Shemp;

print $array[3]; # prints Moeprint $array[0]; # prints Shemp

To remove the first element of the array$element = shift@array;print $element; # prints Shemp

The array now contains only :

Larry, Curly, Moe


20/23

20

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 couplesThe enquiry 2 was administered to six couples

The enquiry 3 was administered to eigh couples

Intofive couples were administered the enquiry 1.


21/23

21

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_s\n;

}


22/23

Multidimentional Arrays


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23

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 rowof tab

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