4.2 Traversing Arrays

Traversing with a for loop

To visit every element in an array, use a `for` loop with an index variable that runs from `0` to `array.length - 1`. This gives access to each index and its value, and lets you modify elements in place, e.g. `for (int i = 0; i < arr.length; i++) { arr[i] = arr[i] * 2; }`.

Enhanced for loop (for-each)

The enhanced for loop simplifies traversal when you only need each *value*, not the index: `for (int x : arr) { System.out.println(x); }`. It's cleaner but you cannot modify the original array elements through it, since `x` is a copy of each value.

Common traversal errors

Using `<=` instead of `<` in the loop condition causes an ArrayIndexOutOfBoundsException, since valid indices only go up to `array.length - 1`. Off-by-one mistakes are the most frequent bug when traversing arrays, so always double check loop bounds.

Key terms

Standard for loop traversal

for (int i = 0; i < arr.length; i++) — gives access to both index and value, allows modification of elements.

Enhanced for loop

for (type x : arr) — iterates over values only; cannot modify original array elements.

arr.length

A field (not a method) that returns the number of elements in array arr.

Off-by-one error

A common bug from using <= instead of < in loop conditions, leading to out-of-bounds access.

ArrayIndexOutOfBoundsException

Runtime error thrown when accessing an index outside the valid range (0 to length - 1).

Try an AP-style question

int[] a = {3, 8, 2, 5};
int s = 0;
for (int i = 0; i < a.length; i++)
    s += a[i];

What is s?

1. 13
2. 18
3. 8
4. 5