Arrays - Building Java Programs: A Back to Basics Approach - page 429

[0] [1] [2] [3] [4]

list 31 33 35 37 39

Suppose that we want to report the first, middle, and last values in the list. From
an examination of the preceding diagram, we can see that these values occur at
indexes 0, 2, and 4, which means we could write the following code:

// works only for an array of length 5
System.out.println("first = " + list[0]);
System.out.println("middle = " + list[2]);
System.out.println("last = " + list[4]);

This technique works when the array is of length 5, but suppose that we change
the length of the array? If the array has a length of 10, for example, this code will
report the wrong values. We need to modify it to incorporate list.length , just as
we modified the standard traversing loop.
The first element of the array will always be at index 0, so the first line of code
doesn't need to change. You might at first think that we could fix the third line of
code by replacing the 4 with list.length :

// doesn't work
System.out.println("last = " + list[list.length]);

However, this code doesn't work. The culprit is zero-based indexing. In our example,
the last value is stored at index 4, not index 5, when list.length is 5. More generally,
the last value will be at index list.length - 1 . We can use this expression directly in
our println statement:

// this one works
System.out.println("last = " + list[list.length - 1]);

Notice that what appears inside the square brackets is an integer expression (the
result of subtracting 1 from list.length ).
A simple approach to finding the middle value is to divide the length of the list
in half:

// is this right?
System.out.println("middle = " + list[list.length / 2]);

When list.length is 5, this expression evaluates to 2 , which prints the correct
value. But what about when list.length is 10? In that case the expression evalu-
ates to 5 , and we would print list[5] . But when the list has an even length, there
are actually two values in the middle. For a list of length 10, the two values are at
list[4] and list[5] . In general, the preceding expression always returns the second
of the two values in the middle when the list is of even length.