4.4 Array Traversals

1. What a traversal is

Traversing an array means using repetition statements (loops) to access all of the elements in an array, or an ordered sequence of them 4.4.A.1. “Ordered sequence” matters: a traversal can visit every element front to back, but also backward, every other element, or just part of the array.

2. Indexed for loops and while loops

A traversal with an indexed for loop or a while loop accesses the elements through their indexes 4.4.A.2. The loop variable is the index, and arr[i] is the element.

int[] temps = {12, 15, 9, 20};

for (int i = 0; i < temps.length; i++)
{
    System.out.println("Day " + i + ": " + temps[i]);
}

Output:

Day 0: 12
Day 1: 15
Day 2: 9
Day 3: 20

The same traversal written as a while loop: the three parts of the for header move to three places 4.4.A.2.

int[] temps = {12, 15, 9, 20};

int i = 0;                      // start
while (i < temps.length)        // condition
{
    System.out.println("Day " + i + ": " + temps[i]);
    i++;                        // update
}

Output:

Day 0: 12
Day 1: 15
Day 2: 9
Day 3: 20

Because you control the index, you can visit an ordered sequence instead of everything:

int[] temps = {12, 15, 9, 20, 17};

for (int i = temps.length - 1; i >= 0; i--)     // backward
{
    System.out.print(temps[i] + " ");
}
System.out.println();

for (int i = 0; i < temps.length; i += 2)       // every other element
{
    System.out.print(temps[i] + " ");
}
System.out.println();

Output:

17 20 9 15 12 
12 9 17 

An indexed loop can also change elements, because temps[i] on the left of = writes into the array:

int[] temps = {12, 15, 9, 20};
for (int i = 0; i < temps.length; i++)
{
    temps[i] = temps[i] * 2;
}
System.out.println(temps[0] + " " + temps[1] + " " + temps[2] + " " + temps[3]);

Output:

24 30 18 40

Watch out: the most common traversal bug is <= instead of <.

int[] temps = {12, 15, 9, 20};
for (int i = 0; i <= temps.length; i++)
{
    System.out.println(temps[i]);
}

Output:

12
15
9
20
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 4 out of bounds for length 4

3. The enhanced for loop

An enhanced for loop header declares a variable, called the enhanced for loop variable. On each iteration, that variable is assigned a copy of the next element, without using its index 4.4.A.3. Read the colon as “in”: “for each score in scores”.

int[] scores = {8, 6, 10};
int total = 0;

for (int score : scores)
{
    total += score;
}
System.out.println("Total: " + total);

Output:

Total: 24

The type of the loop variable must match the element type (int for an int[], String for a String[]).

4. Assigning to the loop variable changes nothing

Because the loop variable holds a copy, assigning a new value to it does not change the value stored in the array 4.4.A.4.

int[] scores = {8, 6, 10};

for (int score : scores)
{
    score = 0;                  // changes only the copy
}
System.out.println(scores[0] + " " + scores[1] + " " + scores[2]);

Output:

8 6 10

index	0	1	2
scores before	8	6	10
scores after	8	6	10

The code compiles and runs with no error, which is why this is a classic exam trap. To reset the array, use an indexed loop: scores[i] = 0;.

5. Enhanced for loops with object references

When an array stores object references, the loop variable gets a copy of each reference. The copy still points to the same object, so calling a method on the loop variable can modify that object’s attributes 4.4.A.5. What it does not change is the references stored in the array 4.4.A.5.

public class Team
{
    public static void main(String[] args)
    {
        Player[] squad = {new Player("Mia", 3), new Player("Sam", 5)};

        for (Player p : squad)
        {
            p.addPoints(10);            // changes the object p refers to
        }
        System.out.println(squad[0].getPoints() + " " + squad[1].getPoints());

        for (Player p : squad)
        {
            p = new Player("New", 0);   // changes only the copy of the reference
        }
        System.out.println(squad[0].getName() + " " + squad[1].getName());
    }
}

class Player
{
    private String name;
    private int points;

    public Player(String n, int pts)
    {
        name = n;
        points = pts;
    }

    public void addPoints(int amount)
    {
        points += amount;
    }

    public int getPoints()
    {
        return points;
    }

    public String getName()
    {
        return name;
    }
}

Output:

13 15
Mia Sam

The first loop changed the players’ points (an attribute of each object). The second loop tried to put new players in the array, but the array still refers to Mia and Sam.

6. Rewriting an enhanced for loop

Code that traverses an array with an enhanced for loop can always be rewritten with an indexed for loop or a while loop 4.4.A.6. The pattern: the loop variable x becomes arr[i].

String[] words = {"red", "blue", "green"};

for (String w : words)                        // enhanced for
{
    System.out.print(w.length() + " ");
}
System.out.println();

for (int i = 0; i < words.length; i++)        // indexed for
{
    System.out.print(words[i].length() + " ");
}
System.out.println();

int k = 0;                                    // while
while (k < words.length)
{
    System.out.print(words[k].length() + " ");
    k++;
}
System.out.println();

Output:

3 4 5 
3 4 5 
3 4 5 

The opposite is not always possible. Choose an indexed loop when you need any of these:

You need to…	Enhanced for?	Indexed for or while?
read every element, in order	yes	yes
know the position (index) of an element	no	yes
compare an element with its neighbor	no	yes
change the values stored in the array	no	yes
go backward or skip elements	no	yes