Question

I have declared an array that i would like to multiply the value of the first column by value of the second column of each row and create a grand sum of these products. I have tried the code listing below, what am i missing

public class Arrays {
    public static void(String[] args) {

        int array_x[][]={{9,8},{2,17},{49,4},{13,119},{2,19},{11,47},{3,73}};
        int sum = 0;

        for (int i = 0; i < array_x.length; i++) {
            for (int j = 0; j < array_x.length; j++) {
                array_x[i][j] = i * j;
                System.out.println("\n" + array_x[i][j])
            }
        }

    }
}

The output should be something like
9*8=72
2*17=34 etc then sum the whole results as 72+34+….

Answer 1

The code you wrote had several issues, including the fact that it would not compile because you had a different number of open and closed brackets, you didn’t specify the function name (which I assumed to be main) and there was a ; missing. However the biggest issue was a logical one: you only need a single for to do what you want to do. You know that the indices of the second dimension of the array are going to be 0 and 1, because as you said the array has only two columns. Also, you need to accumulate the products into sum, instead you initialized sum to 0 and never updated it. Finally, the instruction array_x[i][j] = i * j multiplies the indices instead of the values, so the result is not what you expect, and this result is put into array_x, which is the wrong place because you really don’t need to alter the input array.

class Arrays{

    public static void main(String[] args){

        int array_x[][]={{9,8},{2,17},{49,4},{13,119},{2,19},{11,47},{3,73}};

        int sum=0;

        for(int i=0;i<array_x.length;i++) {
            int prod = array_x[i][0] * array_x[i][1];
            System.out.println("\n"+prod);
            sum += prod;
        }

        System.out.println("Final: " + sum);
    }
}

The code you originally wrote is actually what you need to build a multiplication table, but in that case you need an array with an equal number of rows and columns.