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Looping with the array index

A very common way to process all the elements of an array, is to use a loop variable for the array index.

The loop variable must start at 0, and go up to the <array>.length value. However, we must be careful not to attempt to retrieve an element with index of <array>.length , since array index values go from 0 .. (<array>.length - 1).

For example, to total all the values of a double array purchases, we could write the following loop, without having to know how the array was defined:

double total = 0;
int index ;

for( index = 0; index < purchases.length; index++)

total = total + purchases[ index ];
First we declare and initialise a double variable total to zero: int total = 0; then we declare our int index loop variable index: int index ; then we start the loop: for( index = 0; index < purchases.length; index++) Let us assume that the double array purchases contains the following: 
purchases[]
element
value
purchases[0]
10.11
purchases[1]
5.0
purchases[2]
2.5
Then purchases.length is 3, since there are 3 elements in this array.

Our loop, therefore, is evaluated at run time to:

for( index = 0; index < 3; index++) As the loop iterates the following happens: 
index
loop condition
total = 
total + purchases[ index ];
0
0 < 3 = true
0 + purchases[0] 
= 0 + 10.11 = 10.11
1
1 < 3 = true
10.11 + purchases[1] 
= 10.11 + 5.0 = 15.11
2
2 < 3 = true
15.11 + purchases[2] 
= 15.11 + 2.5 = 15.61
3
3 < 3 = false
loop terminates
 
and the value of total after the loop terminates is 15.61.

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