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Print cells with same rectangular sums in a matrix

Given a matrix of m x n matrix, we need to print all those cells at which sum of sub-matrix ended at this cell and sub-matrix starting at this cell is equal to remaining elements. For better understanding please see below diagram,

Examples :

Input : mat[][] = {1, 2, 3, 5,
                   4, 1, 0, 2,
                   0, 1, 2, 0,
                   7, 1, 1, 0};
Output : (1, 1), (2, 2)      

In above matrix, cell (1, 1) and cell (2, 2)
are our required cells because,
For cell (1, 1), sum of red and green areas is same
1+2+4+1+0+2+1+2+0+1+1+0 = 3+5+0+7        
Same is true for cell (2, 2)
1+2+3+4+1+0+0+1+2+0+1+0 = 5+2+7+1    

We need to print all blue boundary cells for 
which sum of red area is equal to green area.
redgreenmat



First we construct auxiliary sub-matrices similar to the linked article.

We construct two matrices sum[][] and sumr[][] such that sum[i][j] denotes sum of sub-matrix from mat[0][0] to mat[i][j]. And sumr for storing sum till last indices i.e. sumr[i][j] denotes sum of sub-matrix, mat[i][j] to mat[m – 1][n – 1].
Now we can use above matrices for solving this problem, red area shown in above diagram can be calculated by adding corresponding cells from sum and sumr matrices, as mat[i][j] is considered twice while calculating this sum we will subtract it once to get sum of red area. Getting sum of remaining element, the green part, is pretty easy, we will just subtract sum of red part from total sum of given matrix.
So to check whether a particular cell satisfies the given condition we will calculate the sum of red part as explained above and compare it with total sum of matrix, if this sum is half of total sum, current cell satisfies the condition and hence a candidate for result.

C/C++

// C++ program to print cells with same rectangular
// sum diagonally
#include <bits/stdc++.h>
using namespace std;
#define R 4
#define C 4
  
// Method prints cell index at which rectangular sum is
// same at prime diagonal and other diagonal
void printCellWithSameRectangularArea(int mat[R][C],
                                     int m, int n)
{
    /* sum[i][j] denotes sum of sub-matrix, mat[0][0]
       to mat[i][j]
       sumr[i][j] denotes sum of sub-matrix, mat[i][j]
       to mat[m - 1][n - 1]  */
    int sum[m][n], sumr[m][n];
  
    //  Initialize both sum matrices by mat
    int totalSum = 0;
    for (int i = 0; i < m; i++)
    {
        for (int j = 0; j < n; j++)
        {
            sumr[i][j] = sum[i][j] = mat[i][j];
            totalSum += mat[i][j];
        }
    }
  
    //  updating first and last row separately
    for (int i = 1; i < m; i++)
    {
        sum[i][0] += sum[i-1][0];
        sumr[m-i-1][n-1] += sumr[m-i][n-1];
    }
  
    //  updating first and last column separately
    for (int j = 1; j < n; j++)
    {
        sum[0][j] += sum[0][j-1];
        sumr[m-1][n-j-1] += sumr[m-1][n-j];
    }
  
    //  updating sum and sumr indices by nearby indices
    for (int i = 1; i < m; i++)
    {
        for (int j = 1; j < n; j++)
        {
            sum[i][j] += sum[i-1][j] + sum[i][j-1] -
                                      sum[i-1][j-1];
            sumr[m-i-1][n-j-1] += sumr[m-i][n-j-1] +
                                  sumr[m-i-1][n-j] -
                                  sumr[m-i][n-j];
        }
    }
  
    //  Uncomment below code to print sum and reverse sum
    // matrix
    /*
        for (int i = 0; i < m; i++)
        {
            for (int j = 0; j < n; j++)
            {
                cout << sum[i][j] << " ";
            }
            cout << endl;
        }
        cout << endl;
        for (int i = 0; i < m; i++)
        {
            for (int j = 0; j < n; j++)
            {
                cout << sumr[i][j] << " ";
            }
            cout << endl;
        }
        cout << endl;    */
  
    /* print all those indices at which sum of prime diagonal
       rectangles is half of the total sum of matrix  */
    for (int i = 0; i < m; i++)
    {
        for (int j = 0; j < n; j++)
        {
            int mainDiagRectangleSum = sum[i][j] + sumr[i][j] -
                                                   mat[i][j];
            if (totalSum == 2 * mainDiagRectangleSum)
                cout << "(" << i << ", " << j << ")" << endl;
        }
    }
}
  
//  Driver code to test above methods
int main()
{
    int mat[R][C] =
    {
        1, 2, 3, 5,
        4, 1, 0, 2,
        0, 1, 2, 0,
        7, 1, 1, 0
    };
  
    printCellWithSameRectangularArea(mat, R, C);
  
    return 0;
}

Java

// Java program to print cells with 
// same rectangular sum diagonally
  
class GFG {
      
static final int R = 4;
static final int C = 4;
  
// Method prints cell index at which 
// rectangular sum is same at 
// prime diagonal and other diagonal
static void printCellWithSameRectangularArea(int mat[][], 
                                             int m, int n) 
{
    /* sum[i][j] denotes sum of sub-matrix, mat[0][0]
    to mat[i][j]
    sumr[i][j] denotes sum of sub-matrix, mat[i][j]
    to mat[m - 1][n - 1] */
    int sum[][] = new int[m][n];
    int sumr[][] = new int[m][n];
  
    // Initialize both sum matrices by mat
    int totalSum = 0;
    for (int i = 0; i < m; i++) {
    for (int j = 0; j < n; j++) {
        sumr[i][j] = sum[i][j] = mat[i][j];
        totalSum += mat[i][j];
    }
    }
  
    // updating first and last row separately
    for (int i = 1; i < m; i++) {
    sum[i][0] += sum[i - 1][0];
    sumr[m - i - 1][n - 1] += sumr[m - i][n - 1];
    }
  
    // updating first and last column separately
    for (int j = 1; j < n; j++) {
    sum[0][j] += sum[0][j - 1];
    sumr[m - 1][n - j - 1] += sumr[m - 1][n - j];
    }
  
    // updating sum and sumr indices by nearby indices
    for (int i = 1; i < m; i++) {
    for (int j = 1; j < n; j++) {
        sum[i][j] += sum[i - 1][j] + sum[i][j - 1] - 
                                     sum[i - 1][j - 1];
        sumr[m - i - 1][n - j - 1] += sumr[m - i][n - j - 1] +
                                      sumr[m - i - 1][n - j] -
                                      sumr[m - i][n - j];
    }
    }
  
    // Uncomment below code to print sum and reverse sum
    // matrix
    /*
        for (int i = 0; i < m; i++)
        {
            for (int j = 0; j < n; j++)
            {
                System.out.print( sum[i][j] + " ");
            }
            System.out.println();
        }
        System.out.println();
        for (int i = 0; i < m; i++)
        {
            for (int j = 0; j < n; j++)
            {
                System.out.print(sumr[i][j] + " ");
            }
            System.out.println();
        }
        System.out.println(); */
  
    /* print all those indices at which sum of prime diagonal
    rectangles is half of the total sum of matrix */
    for (int i = 0; i < m; i++) {
    for (int j = 0; j < n; j++) {
        int mainDiagRectangleSum = sum[i][j] + 
                       sumr[i][j] - mat[i][j];
        if (totalSum == 2 * mainDiagRectangleSum)
        System.out.println("(" + i + ", " + j + ")");
    }
    }
}
  
// Driver code
public static void main(String[] args)
{
    int mat[][] = {{1, 2, 3, 5},
                   {4, 1, 0, 2}, 
                   {0, 1, 2, 0}, 
                   {7, 1, 1, 0}};
  
    printCellWithSameRectangularArea(mat, R, C);
}
}
  
// This code is contributed by Anant Agarwal.

Python

# Python program to print cells with same rectangular
# sum diagonally
# R 4
# C 4
   
# Method prints cell index at which rectangular sum is
# same at prime diagonal and other diagonal
def printCellWithSameRectangularArea(mat, m, n):
    # sum[i][j] denotes sum of sub-matrix, mat[0][0]
    #  to mat[i][j]
    # sumr[i][j] denotes sum of sub-matrix, mat[i][j]
    # to mat[m - 1][n - 1]  
    sum = [[0 for i in range(m)]for j in range(n)]
    sumr = [[0 for i in range(m)]for j in range(n)]
   
    #  Initialize both sum matrices by mat
    totalSum = 0
    for i in range(m):
        for j in range(n):
            sumr[i][j] = sum[i][j] = mat[i][j];
            totalSum += mat[i][j]
   
    #  updating first and last row separately
    for i in range(1,m):
        sum[i][0] += sum[i-1][0]
        sumr[m-i-1][n-1] += sumr[m-i][n-1]
   
    #  updating first and last column separately
    for j in range(1,n):
        sum[0][j] += sum[0][j-1];
        sumr[m-1][n-j-1] += sumr[m-1][n-j]
   
    #  updating sum and sumr indices by nearby indices
    for i in range(1,m):
        for j in range(1,n):
            sum[i][j] += sum[i-1][j] + sum[i][j-1] - sum[i-1][j-1]
            sumr[m-i-1][n-j-1] += sumr[m-i][n-j-1] + sumr[m-i-1][n-j] - sumr[m-i][n-j]
   
    #  Uncomment below code to print sum and reverse sum
    # matrix
   
    # print all those indices at which sum of prime diagonal
    #   rectangles is half of the total sum of matrix  
    for i in range(m):
        for j in range(n):
            mainDiagRectangleSum = sum[i][j] + sumr[i][j] - mat[i][j]
            if (totalSum == 2 * mainDiagRectangleSum):
                print "(",
                print i,
                print ",",
                print j,
                print ")",
   
#  Driver code to test above methods
mat =[[1, 2, 3, 5,],
     [4, 1, 0, 2,],
     [0, 1, 2, 0],
     [7, 1, 1, 0]]
printCellWithSameRectangularArea(mat, 4, 4)
  
# Contributed by Afzal

C#

// C# program to print cells with 
// same rectangular sum diagonally
using System;
  
class GFG {
      
static int R = 4;
static int C = 4;
  
// Method prints cell index at which 
// rectangular sum is same at 
// prime diagonal and other diagonal
static void printCellWithSameRectangularArea(int [,]mat, 
                                             int m, int n) 
{
    /* sum[i][j] denotes sum of sub-
       matrix, mat[0][0] to mat[i][j]
       sumr[i][j] denotes sum of sub-matrix,
       mat[i][j] to mat[m - 1][n - 1] */
    int [,]sum = new int[m, n];
    int [,]sumr = new int[m, n];
  
    // Initialize both sum matrices by mat
    int totalSum = 0;
    for (int i = 0; i < m; i++) {
    for (int j = 0; j < n; j++) {
        sumr[i, j] = sum[i, j] = mat[i, j];
        totalSum += mat[i, j];
    }
    }
  
    // updating first and last row separately
    for (int i = 1; i < m; i++)
    {
    sum[i, 0] += sum[i - 1, 0];
    sumr[m - i - 1, n - 1] += sumr[m - i, n - 1];
    }
  
    // updating first and last column separately
    for (int j = 1; j < n; j++) 
    {
    sum[0,j] += sum[0,j - 1];
    sumr[m - 1,n - j - 1] += sumr[m - 1,n - j];
    }
  
    // updating sum and sumr indices by nearby indices
    for (int i = 1; i < m; i++) {
    for (int j = 1; j < n; j++) {
        sum[i,j] += sum[i - 1,j] + sum[i,j - 1] - 
                                   sum[i - 1,j - 1];
        sumr[m - i - 1,n - j - 1] += sumr[m - i,n - j - 1] +
                                     sumr[m - i - 1,n - j] -
                                     sumr[m - i,n - j];
    }
    }
  
    // Uncomment below code to print sum and reverse sum
    // matrix
    /*
        for (int i = 0; i < m; i++)
        {
            for (int j = 0; j < n; j++)
            {
                System.out.print( sum[i][j] + " ");
            }
            System.out.println();
        }
        System.out.println();
        for (int i = 0; i < m; i++)
        {
            for (int j = 0; j < n; j++)
            {
                System.out.print(sumr[i][j] + " ");
            }
            System.out.println();
        }
        System.out.println(); */
  
    /* print all those indices at which sum
       of prime diagonal rectangles is half 
       of the total sum of matrix */
    for (int i = 0; i < m; i++) {
    for (int j = 0; j < n; j++) {
        int mainDiagRectangleSum = sum[i,j] + 
                    sumr[i,j] - mat[i,j];
                      
        if (totalSum == 2 * mainDiagRectangleSum)
        Console.WriteLine("(" + i + ", " + j + ")");
    }
    }
}
  
// Driver code
public static void Main()
{
    int [,]mat = {{1, 2, 3, 5},
                  {4, 1, 0, 2}, 
                  {0, 1, 2, 0}, 
                  {7, 1, 1, 0}};
  
    printCellWithSameRectangularArea(mat, R, C);
}
}
  
// This code is contributed by vt_m.


Output:

(1, 1)
(2, 2)

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.



This article is attributed to GeeksforGeeks.org

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