16.6: Array Traversal

Many computations can be implemented by looping through the elements of an array and performing an operation on each element. For example, the following loop squares the elements of a double array:

for (int i = 0; i < a.length; i++) {
    a[i] = Math.pow(a[i], 2.0);
}

Looping through the elements of an array is called a traversal. Another common pattern is a search, which involves traversing an array looking for a particular element. For example, the following method takes an int array and an integer value, and it returns the index where the value appears:

public static int search(double[] a, double target) {
    for (int i = 0; i < a.length; i++) {
        if (a[i] == target) {
            return i;
        }
    }
    return -1;
}

If we find the target value in the array, we return its index immediately. If the loop exits without finding the target, it returns -1, a special value chosen to indicate a failed search.

Another common traversal is a reduce operation, which “reduces” an array of values down to a single value. Examples include the sum or product of the elements, the minimum, and the maximum. The following method takes a double array and returns the sum of the elements:

public static double sum(double[] a) {
    double total = 0.0;
    for (int i = 0; i < a.length; i++) {
        total += a[i];
    }
    return total;
}

Before the loop, we initialize total to zero. Each time through the loop, we update total by adding one element from the array. At the end of the loop, total contains the sum of the elements. A variable used this way is sometimes called an accumulator.