# Matrix Block Sum in C++

C++Server Side ProgrammingProgramming

Suppose we have one m * n matrix called mat and an integer K, we have to find another matrix answer where each answer[i][j] is the sum of all elements mat[r][c] for i - K <= r <= i + K, j - K <= c <= j + K, and (r, c) is a valid position in the matrix. So if the input is like −

 1 2 3 4 5 6 7 8 9

And k is 1, then the output will be −

 12 21 16 27 45 33 24 39 28

To solve this, we will follow these steps −

• n := number of rows, and m = number of columns
• define a matrix ans, whose order is n x m
• for i in range 0 to n – 1
• for j in range 0 to m – 1
• for r in range i – k to i + k
• for c in range j – k to j + k
• if r and c are inside the matrix indices, then
• ans[i, j] := ans[i, j] + mat[r, c]
• return ans

## Example

Let us see the following implementation to get better understanding −

class Solution {
public:
vector<vector<int>> matrixBlockSum(vector<vector<int>>& mat, int k) {
int n = mat.size();
int m = mat[0].size();
vector < vector <int> > ans(n , vector <int> (m));
for(int i = 0; i < n; i++){
for(int j = 0; j < m; j++){
for(int r = i - k;r <= i + k; r++){
for(int c = j - k; c <= j + k; c++){
if(r>= 0 && r < n && c >= 0 && c < m){
ans[i][j] += mat[r][c];
}
}
}
}
}
return ans;
}
};

## Input

[[1,2,3],[4,5,6],[7,8,9]]
1

## Output

[[12,21,16],[27,45,33],[24,39,28]]
Published on 18-Mar-2020 05:44:10