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//The diagonals of a matrix are such that i = j. 
int matrix[row][col];
for (int i = 0; i < row; i++){
   //For matrices such that max({row,col}) = row
   if(i > col){
      break;
   }else{
      printf("%d \n", &matrx[i][i]);
   }
}

[(matrix[i][i], matrix[i][len(matrix) -i-1])  for i in range(len(matrix))]

Printing diagonals of a matrix is quite easy, but you need to understand the relation between matrix and diagonals. If you want perfect diagonals from matrix then your matrix must be square, I mean your matrix should be NxN. For example 2×2, 4×4 or 9×9 etc. Because in rectangular matrix (2×3 or 5×7) it is not possible to find perfect diagonals.

1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16

Now if you see above matrix carefully there are two diagonals in this one is 1, 6, 11, 16 and other is 4, 7, 10, 13.

I found this implementation, which is already done, you can check algo on below website link

somanyword.com/2013/12/print-diagonals-of-a-given-matrix/

public static void main(String []args){
        System.out.println("Hello World");
        int [][] array = {{00,01,02,03,04},
                          {10,11,12,13,14},
                        {20,21,22,23,24},
                        {30,31,32,33,34},
                        {40,41,42,43,44}};
                        
        for(int i =0; i < array.length; i++){
            System.out.print(array[i][i]+" ");
        }
        System.out.println();

        for (int j = 0 ; j < array.length; j++ ){
            System.out.print(array[j][(array.length-1)-j]+" ");
            
        }
     }

This will work only for square matrix....

// test2.cpp : main project file.

#include "stdafx.h"
#include <iostream>

using namespace System;

#define M 3
#define N 3

bool testDiagonal(int n, int m, int input [][N])
{
    if (m != n) return false;

    for (int i=0; i<m; i++)
    {
        printf ("Diagonal: %d\n", input[i][i]);
    }
}



int main(array<System::String ^> ^args)
{
    int input[M][N] = {{1,2,3}, {4,5,6}, {7,8,9}};
    testDiagonal(M, N, input);

    return 0;
}

// test2.cpp : main project file.

#include "stdafx.h"
#include <iostream>

using namespace System;

#define M 3
#define N 3

bool testDiagonal(int n, int m, int input [][N])
{
    if (m != n) return false;

    for (int i=0; i<m; i++)
    {
        printf ("Diagonal: %d\n", input[i][i]);
    }
}



int main(array<System::String ^> ^args)
{
    int input[M][N] = {{1,2,3}, {4,5,6}, {7,8,9}};
    testDiagonal(M, N, input);

    return 0;
}

public class PrintMatrixDiagonally {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub

		int[][] mat = { {1,2,3,4,5},
						{6,7,8,9,10},
						{11,12,13,14,15},
						{16,17,18,19,20}
					};
		printMatrixDiagonally(mat);

	}

	private static void printMatrixDiagonally(int[][] mat) {
		
		//Down side
		int col = 0;
		for(int row = 0; row < mat.length; row++){
			diagonally(mat, row, col);
			System.out.print("\n");
		}
		//Right side
		int row = mat.length - 1;
		for(col = 1; col < mat[row].length; col++){
			diagonally(mat, row, col);
			System.out.print("\n");
		}
		
	}
	//using recursion printing matrix diagonally
	private static void diagonally(int[][] mat, int row, int col) {
		
		//Bundary check
		if(row < 0 || col < 0 || row >= mat.length || col >= mat[0].length){
			return;
		}
		System.out.print(mat[row][col]+" ");
		diagonally(mat, --row, ++col);
		
	}

}

Solution for main and incidental diagonals

public class Diagonals {

    public static void main(String[] args) {
        int[][] matrix = {{1,2,3}, {4,5,6}, {7,8,9}};
        new Diagonals().printDiagonals(matrix);
    }

    public void printDiagonals(int[][] matrix) {
        StringBuilder mainDiagonal = new StringBuilder();
        StringBuilder incidentalDiagonal = new StringBuilder();

        for (int i = 0; i < matrix.length; i++) {
            if (matrix[i].length != matrix.length) {
                System.out.print("Not square matrix");
                return;
            }

            mainDiagonal.append(matrix[i][i]);
            mainDiagonal.append(' ');

            incidentalDiagonal.append(matrix[i][matrix.length - i - 1]);
            incidentalDiagonal.append(' ');
        }

        System.out.println("main Diagonal: " + mainDiagonal);
        System.out.println("incidental Diagonal: " + incidentalDiagonal);
    }
}