1-D arrays: store, index, and traverse

Many values in one box

• An array holds many values of the same type in order.
• Until now one variable held one value. An array holds a whole list.
• For example, the test scores of a class can live in one int array.

Make an array with values

• If you know the values, list them in curly braces { }.
• The type is the element type plus [], like int[].
• Java counts how many values you gave and makes the array that size.

public class Main {
    public static void main(String[] args) {
        int[] scores = {90, 80, 70};
        System.out.println(scores[0]);   // 90
        System.out.println(scores[2]);   // 70
    }
}

Index positions

• Each value has an index (its position). Indexes start at 0.
• So the first value is scores[0], the second is scores[1], and so on.
• You can also change a value: scores[1] = 85;.

public class Main {
    public static void main(String[] args) {
        int[] scores = {90, 80, 70};
        scores[1] = 85;                  // change the middle value
        System.out.println(scores[1]);   // 85
    }
}

Make an empty array

• Use new int[n] to make an array of n values.
• Every int slot starts at 0 automatically.
• Then you fill the slots one by one.

public class Main {
    public static void main(String[] args) {
        int[] nums = new int[3];   // {0, 0, 0}
        nums[0] = 5;
        nums[1] = 6;
        System.out.println(nums[0] + " " + nums[1] + " " + nums[2]);   // 5 6 0
    }
}

.length

• array.length tells you how many values the array has.
• Note: no parentheses — it is scores.length, not scores.length().
• The last valid index is always length - 1.

public class Main {
    public static void main(String[] args) {
        int[] scores = {90, 80, 70};
        System.out.println(scores.length);            // 3
        System.out.println(scores[scores.length - 1]); // 70 (last value)
    }
}

Traverse with a for loop

• To visit every value, loop the index from 0 up to length - 1.
• Use i < array.length as the stop test.
• Below we add up every value.

public class Main {
    public static void main(String[] args) {
        int[] scores = {90, 80, 70};
        int total = 0;
        for (int i = 0; i < scores.length; i++) {
            total = total + scores[i];
        }
        System.out.println(total);   // 240
    }
}

Enhanced for loop

• The enhanced for (or "for-each") visits each value directly.
• Read it as: "for each int v in scores".
• Use it when you only need the values, not the index.

public class Main {
    public static void main(String[] args) {
        int[] scores = {90, 80, 70};
        int total = 0;
        for (int v : scores) {
            total = total + v;
        }
        System.out.println(total);   // 240
    }
}

Common mistakes

• a.length has no (); valid indexes run 0 to a.length - 1.
• Going out of range throws ArrayIndexOutOfBoundsException.

Now you try

• Some tasks print a result; others complete a method the Harness calls.
• Remember: indexes start at 0, and the last index is length - 1.
• Press Run to compile, then Check answer.