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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:
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purchases[]
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element
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value
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purchases[0]
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10.11
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purchases[1]
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5.0
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purchases[2]
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2.5
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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:
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index
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loop condition
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total
=
total + purchases[
index ];
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0
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0 <
3 = true
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0 + purchases[0]
= 0 + 10.11
= 10.11
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1
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1 <
3 = true
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10.11
+ purchases[1]
= 10.11 +
5.0 = 15.11
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2
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2 <
3 = true
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15.11
+ purchases[2]
= 15.11 +
2.5 = 15.61
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3
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3 <
3 = false
loop terminates
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and the value of
total
after the loop terminates is 15.61.
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