Harnessing Elements - Perl

To modify a particular element of an array, use the index/key notation to accomplish your task, like this:


#!/usr/bin/perl 
# define array 
@friends = ("Rachel", "Monica", "Phoebe", "Chandler", "Joey", "Ross"); 
# now change one of its elements 
$friends[3] = "Janice"; 
# array now looks like this 
@friends = ("Rachel", "Monica", "Phoebe", "Janice", "Joey", "Ross"); 



This works with associative arrays too:


#!/usr/bin/perl 
# define hash 
%dinner = ("starter" => "fried squid rings", "main" => "roast chicken", "dessert" => "chocolate cake"); 
 
# change element 
$dinner{"dessert"} = "tiramisu"; 
 
# hash now looks like this 
%dinner = ("starter" => "fried squid rings", "main" => "roast chicken", "dessert" => "tiramisu"); 



You can print the contents of an array simply by using the array in a print() function call, as below:


#!/usr/bin/perl 
# define array 
@friends = ("Rachel", "Monica", "Phoebe", "Chandler", "Joey", "Ross"); 
# print contents 
print "@friends "; 



The fact that array values can be accessed and manipulated using a numeric index makes them particularly well-suited for use in a loop. Consider the following example, which asks the user for a set of values (the user can define the size of the set), stores these values in an array, and then prints them back out.


#!/usr/bin/perl 
# get array size 
print ("How many items? "); 
$size = ; 
chomp ($size); 
 
# get array values 
for ($x=0; $x<$size; $x++) 
{ 
        print ("Enter item ", ($x+1), ": "); 
        $val = ; 
        chomp ($val); 
        $data[$x] = $val; 
} 
 
# iterate over array 
# print array values 
print ("Here is what you entered: n"); 
for ($x=0; $x<$size; $x++) 
{ 
        print ("Element $x: $data[$x]n"); 
} 



Here's what the output looks like:

How many items? 3
Enter item 1: red
Enter item 2: orange
Enter item 3: green
Here is what you entered:
Element 0: red
Element 1: orange
Element 2: green

The first thing I've done here is ask the user for the number of items to be entered - this will tell me the size of the array. Once that information has been obtained, it's pretty easy to set up a "for" loop to obtain that number of values through user input at the command prompt. Every time the user enters one of these values, a counter variable is incremented, this counter variable corresponds to the index in the @data array that is being constructed as the loop executes. Once all the values have been entered, another "for" loop is used to iterate over the newly-minted @data array, and print the values stored in it.

The second loop runs as many times as there are elements in the array. In this specific example, I know the size of the array because I had the user enter it at the beginning of the script, but you can also obtain the array size programmatically, as you'll see on the next page.