I am supposed to write a recursive(!) method to count the amount of occurrences of a given integer in an integer array and return true if even and false if odd. This is my code so far:

public static boolean evenNumberOf(int x, int[] arr) {

    if (arr == null || arr.length == 0)
        return false;
    int count = counting(x, arr, 0, 0);
    if (count % 2 == 0) {
        System.out.print("true");
        return true;
    } else {
        System.out.print("false");
        return false;
    }
}

public static int counting(int x, int[] arr, int index, int count) {

    if (arr[index] == x && index < arr.length) {
        return counting(x, arr, index  , count  );
    } else {
        return counting(x, arr, index  , count);
    }
}

It works for

evenNumberOf(2, new int[] { 1, 2, 3, 2 });

but it gives java.lang.StackOverflowError for

evenNumberOf(1, new int[] { 1, 2, 3, 2 });

I am not sure how to prevent this endless recursive loop, as I'm new to programming and it's my first time using recursion. Thanks in advance.

CodePudding user response：

Any recursion should have a stop condition and 1 or more recursive calls.

in this example, the stop condition is index >= arr.length so you start writing the function as folows:

public static int counting(int x, int[] arr, int index) {
    if (index >= arr.length) {
        return 0;//there are 0 x's between index and arr.length
    }

after you handled the stop condition, you need to write the rest:

public static int counting(int x, int[] arr, int index) {
    if (index >= arr.length) {
        return 0;//there are 0 x's between index and arr.length
    }
    int countAfterIndex = counting(x, arr, index 1);//the amount of x's starting from index 1
    if (x == arr[index]) {
        return countAfterIndex   1;//there is 1 more to add
    } else {
        return countAfterIndex; //the amount of x's after index is the same as the amount from index.
    }
}