How to check if two matrices have an identical row?

Question

I'm trying to develop an algorithm in Java, which, given two matrices (let's say a and b), returns true if at least one row is identical in a and b.

Here's my attempt of method:

public static boolean check_row(int a[][], int b[][]){
        boolean check = false;
        for(int i = 0; i < a.length; i++){
            for(int j = 0; j < b[0].length; j++){
                if(a[i][j] == b[i][j])
                    check = true;
            }
        }
        return check;
    }

And here's a simple main:

public static void main(String[] args){
        int a[][] = {{1,2}, {3,4}};
        int b[][] = {{1,2}, {7,8}};
        System.out.println(check_row(a, b));
    }

Here I get true because first row of both matrices is the same. But if I change the matrices initialization to this:

int a[][] = {{1,2}, {3,4}};
int b[][] = {{5,6}, {1,2}};

I get false, even though the first row of a and the second row of b are identical.

How should I modify the method in order to get true in both cases?

Answer 1

Your condition is too simple... High level idea is, that for each row from a and b pick a row and then identify whether it is the same, so you need 3 loops...

code:

public class SameRowFinder {

    public static void main(String[] args){
        int a[][] = {{1,2},{3,4}};
        int b[][] = {{1,2}, {7,8}};
        System.out.println(hasSameRow(a, b));

        int aa[][] = {{1,2},{3,4}};
        int bb[][] = {{5,6}, {1,2}};
        System.out.println(hasSameRow(aa, bb));
    }

    private static boolean hasSameRow(int[][] a, int[][] b) {
        for (int i = 0; i < a.length; i++) {
            for (int j = 0; j < b.length; j++) {
                if (isSameRow(a[i], b[j])) {
                    System.out.printf("Same rows are %d and %d (0-based).%n", i, j);
                    return true;
                }
            }
        }
        return false;
    }

    private static boolean isSameRow(int[] row1, int[] row2) {
        if (row1.length != row2.length) {
            throw new IllegalArgumentException("rows with different length");
        }
        for (int i = 0; i < row2.length; i++) {
            if (row1[i] != row2[i]) {
                return false;
            }
        }
        return true;
    }
}

Also you do not need to write your own function for array compare, but use Arrays.equal(int[], int[]), but it will just hide 3rd loop. Method above throws runtime exception in case of different length of arrays. It's definitelly worth look at Arrays.equal(int[], int[]) implementation for some tips (check for equality + null checks).