Find duplicate elements in 2d array

Question

Suppose I have a 3*3 array(2D array):

1 2 4

1 5 7

2 4 8

So 1,2 and 4 appear more than once. The result I want to get is 3. How can I do that?

Answer 1

Here's a brute-force straight forward way of counting duplicates. Turn the 2d array into a 1d array (List<Integer>), then loop through the 1d array counting the duplicates as you find them and removing them so you don't count them more than once.

public static void main(String[] args) throws Exception {
    int[][] _2dArray = {
        { 1, 2, 4 },
        { 1, 5, 7 },
        { 2, 4, 8 }
    };

    // Convert to a flat 1d array
    List<Integer> flatArray = new ArrayList();
    for (int[] row : _2dArray) {
        for (int col : row) {
            flatArray.add(col);
        }
    }

    // Count and remove duplicates as you find them
    int dupCount = 0;
    for (int i = 0; i < flatArray.size(); i++) {
        boolean dupFound = false;
        for (int j = i + 1; j < flatArray.size(); j++) {
            if (flatArray.get(i) == flatArray.get(j)) {
                dupFound = true;
                // Remove duplicate found in inner loop
                flatArray.remove(j);
                j--;
            }
        }

        if (dupFound) {
            dupCount++;
            // Remove duplicate found in outer loop
            flatArray.remove(i);
            i--;
        }
    }
    System.out.println(dupCount);
}

Results:

3

Java 8 Streams

If you're interested in streams you can go this route also. It groups together all the numbers in the array into a map where the number is the key and the frequency of that number is the value. We then keep only the key/value pairs who have a value > 1 (The duplicates).

public static void main(String[] args) throws Exception {
    int[][] _2dArray = {
        { 1, 2, 4 },
        { 1, 5, 7 },
        { 2, 4, 8 }
    };

    // Convert to a flat 1d array
    int[] flatArray = Arrays.stream(_2dArray)
        .flatMapToInt(Arrays::stream)
        .toArray();

    //Count duplicates
    Object[] duplicates = Arrays.stream(flatArray).boxed()
            // Group all integers together
            .collect(Collectors.groupingBy(i -> i, Collectors.counting()))
            // Keep key/value pairs whose value > 1
            .entrySet()
            .stream()
            .filter(entry -> entry.getValue() > 1)
            .toArray();

    System.out.println(duplicates.length);       
}

Results:

3

Answer 2

Here is the solution to find count of duplicate elements in a 2D matrix without using collection public class DuplicateElementsCountIn2DMetrixWithoutUsingCollections{

public static void main(String args[]){
    String[][] matrix = {{"1","2","1","3",},{"7","6","null","2",},{"5","6","3","null",},{"3","10","9","5",}};
    System.out.println(findDuplicate(matrix));
}

public static int findDuplicate(String[][] matrix){
    String strArr[] = new String[(matrix.length)*(matrix[0].length)];
    int count = 0;
    for(int i = 0; i < matrix.length; i++){
        for(int j = 0; j < matrix[0].length; j++){
            for(int k = 0; k < matrix.length; k++){
                for(int l = 0; l < matrix[0].length; l++){
                    if((i!=k || j!=l)){
                        if(matrix[i][j] == matrix[k][l]){
                            int x = 0;
                            boolean flag = false;
                            while(strArr[x] != null){
                                if(null != matrix[i][j] && matrix[i][j].equals(strArr[x])){
                                    flag = true;                                    
                                }
                                x++;
                            }
                            if(flag==false){
                                strArr[count] = matrix[i][j];
                                count++;
                            }
                        }
                    }
                }
            }
        }
    }
    return count;
}

}

Output

6

Note : 1, 2, 3, 6, null are duplicate in matrix, hence count is 6