Product of two arrays using recursion Java

Question

I have two arrays a1 = {1, 2, 3, 4, 5} and a2 = {2, 4, 6, 8} with an integer n = 3. What I need to do is, using recursion, return a new array of the product of each element until reaching position marked by n. Final result should be a3 = {2, 8, 18}.

public class Product {

    public static int[] arrayProd(int [] v1, int [] v2, int n) {
        if(n >= 0){
            return a1[n] * a2[n];
        }
    }

    public static void main(String [] args) {
        int[] v1 = {1, 2, 3, 4, 5};
        int[] v2 = {2, 4, 6, 8};
        int n = 3;
    }
}

It is also suggested that we should use an auxiliary method like:

private static void auxArrayProd(int [] v1, int [] v2, int [] result, int n) {
    return null;
}

But I don't know how to return an array formed with integers

My specific problem is that when I try to return the product I get this error: error: incompatible types return a1[n] * a2[n]; ^ required: int[] found: int 1 error — Daniel, Mar 18 '16 at 00:31
Sorry for not being that specific I'm still trying to figure how this site works — Daniel, Mar 18 '16 at 00:32
Please post the entire relevant code - the comment you posted about your error is hard to interpret without more context. — M.S., Mar 18 '16 at 00:37
"don't know how to return an array" -- change `void` to `int[]` in `auxArrayProd`. You can't return null from a void method — OneCricketeer, Mar 18 '16 at 00:51
Thank you, so you also know what should I do with the auxiliary method? I'm very lost at this — Daniel, Mar 18 '16 at 00:53

score 2 · Answer 1 · answered Mar 18 '16 at 01:06

Here is a recursive approach :)

public static void arrayProd(int [] v1, int [] v2, int upperBound, int curIndex, int[] prod) {
    if(curIndex == upperBound) {
        return;
    }
    prod[curIndex] = v1[curIndex] * v2[curIndex];
    arrayProd(v1, v2, upperBound, curIndex+1, prod);
}

public static void main(String [] args) {
    int[] v1 = {1, 2, 3, 4, 5};
    int[] v2 = {2, 4, 6, 8};
    int n = 3;
    // array size cant be greater than 3 as per your example
    int[] prod = new int[n];
    arrayProd(v1, v2, n, 0, prod);
    System.out.println(Arrays.toString(prod));    // [2, 8, 18]
}

OneCricketeer · Accepted Answer · 2016-03-18T01:15:44.400

The auxiliary method is a good suggestion, but with a different method signature than that you listed in your code.

Reason being is that you need to keep track of the index less than n, then your base-case for the recursion is when that index is greater than n.

On the first call to the auxiliary method, you start the index at 0, then you recurse inside the auxiliary method much like a for-loop to increment the index.

This code will print your desired result of [2, 8, 18]

import java.util.Arrays;

public class Product {

    // auxiliary method
    private static void _arrayProd(int[] v1, int[] v2, int[] result, int n, int i) {
        if(i < n){
            result[i] = v1[i] * v2[i];
             _arrayProd(v1, v2, result, n, i+1);
        }
    }

    public static int[] arrayProd(int[] v1, int[] v2, int n) {
        int[] result = null;
        if(n >= 0){
            result = new int[n];
            _arrayProd(v1, v2, result, n, 0);
        }
        return result;
    }


    public static void main(String [] args) {
        int[] v1 = {1, 2, 3, 4, 5};
        int[] v2 = {2, 4, 6, 8};
        int n = 3;

        int[] v3 = arrayProd(v1, v2, n);

        System.out.println(Arrays.toString(v3));
    }
}

Thank you very much but I get an exception: Exception in thread "main" java.lang.Error: Unresolved compilation problem: result cannot be resolved to a variable — Daniel, Mar 18 '16 at 01:08
That's an easy fix. Are you using an IDE? Anyways, I've edited and fixed — OneCricketeer, Mar 18 '16 at 01:11
Thank you all I already figured what was wrong, yes I'm using Eclipse. Thank you for solving my problem! — Daniel, Mar 18 '16 at 01:13

Product of two arrays using recursion Java

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