Exception in thread java.lang.ArrayIndexOutOfBoundsException: 5

Question

I'm a newbie who is trying to complete the below tutorial

    // Create a method called countEvens
    // Return the number of even ints in the given array. 
    // Note: the % "mod" operator computes the remainder, e.g. 5 % 2 is 1. 
/*
 * SAMPLE OUTPUT:
 *  
 * 3
 * 0
 * 2
 *  
 */

Below is my code

public static void main(String[] args) {

        int a[] = {2, 1, 2, 3, 4};
         countEvens(a); // -> 3
         int b[] = {2, 2, 0};
         countEvens(b); // -> 3
         int c[] = { 1, 3, 5};
         countEvens(c); // -> 0

    }


    public static void countEvens(int[] x){
        int i = 1;
        int count = 0;
        while ( i <= x.length){
            if (x[i] % 2 == 0){
                count ++;
            }
            i ++;
        }
        System.out.println(count);
    }

The code can be run, but I get the below error message

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5
    at apollo.exercises.ch05_conditionals.Ex5_CountEvens.countEvens(Ex5_CountEvens.java:23)
    at apollo.exercises.ch05_conditionals.Ex5_CountEvens.main(Ex5_CountEvens.java:10)

May I know what I'm doing wrong here?

Answer 1

The line

while ( i <= x.length)

should be

while ( i < x.length)

If the length of xis 5, for example, the indices are 0, 1, 2, 3 and 4. The index goes from 0 up to one less than the length of the array.

However the tidiest way to do this is to use a for each loop rather than a while loop:

public static void countEvens(int[] x) {
    int count = 0;
    for (int number : x)
        if (number % 2 == 0)
            count++;
    System.out.println(count);
}

Answer 2

'i' should go from 0 to length()-1, because array indices start at 0, and the index of the last element is length()-1.

Therefore, a correct version of your code would be:

public static void countEvens(int[] x){
    int i = 0;
    int count = 0;
    while ( i < x.length){
        if (x[i] % 2 == 0){
            count ++;
        }
        i ++;
    }
    System.out.println(count);
}

For your specific purpose, a for loop would be simpler.

for(int i = 0; i< x.length(); i++){
  if(x[i]%2==0){
  count++;
  }
}