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Find the Submatrix which holds the given co-ordinate

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  • Difficulty Level : Medium
  • Last Updated : 19 Sep, 2022
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Given a matrix mat of N*N (N is a perfect square|) and two points x and y, the task is to return all the elements of the submatrix in which the element A[x][y] lies.

Note: The matrix is divided into N equal submatrix each of size K*K (where K is the square root of N)

Examples:

Input: N = 9, x = 4, y = 4
mat = {{1, 2, 3, 9, 8, 7, 1, 2, 1}, {4, 5, 6, 1, 2, 3, 7, 9, 8}, {7, 8, 9, 2, 2, 0, 1, 5, 7},  
{0, 2, 9, 1, 2, 3, 4, 5, 3}, {9, 8, 7, 4, 5, 6, 7, 4, 1}, {1, 4, 7, 7, 8, 9, 9, 8, 7},  
{5, 6, 1, 9, 8, 7, 5, 2, 3}, {4, 5, 1, 6, 5, 4, 9, 7, 4}, {8, 7, 9, 3, 2, 1, 9, 4, 9}}, 
Output: {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}
Explanation: Given x = 4, y = 4 that element lies in the middle grid.

Input Matrix

Input: N = 9, x = 6, y= 4
mat = {{1, 2, 3, 9, 8, 7, 1, 2, 1}, {4, 5, 6, 1, 2, 3, 7, 9, 8}, {7, 8, 9, 2, 2, 0, 1, 5, 7},  
{0, 2, 9, 1, 2, 3, 4, 5, 3}, {9, 8, 7, 4, 5, 6, 7, 4, 1}, {1, 4, 7, 7, 8, 9, 9, 8, 7}, 
{5, 6, 1, 9, 8, 7, 5, 2, 3}, {4, 5, 1, 6, 5, 4, 9, 7, 4}, {8, 7, 9, 3, 2, 1, 9, 4, 9}}
Output: {{9, 8, 7}, {6, 5, 4}, {3, 2, 1}}
Explanation: Given x =6, y = 4 that element lies in the grid shown below.

Input Matrix

Approach: The problem can be solved based on the following observation:

An element at index (x, y) in a square matrix of perfect square length, lies in submatrix[n*(x/n), (n*(y/n)], where each value shows the positioning with respect to other submatrices. So the idea is to just print that submatrix. 

Follow the steps mentioned below to implement the idea:

  • Find square root on N.
  • Store the submatrix where the coordinate (x, y) lies in a new matrix.
  • Return the new array.

Below is the implementation of the above approach.

C++




// C++ code to implement the approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to print submatrix
vector<vector<int> >
getSubGrid(int N, vector<vector<int> >& matrix, int x,
           int y)
{
    int n = sqrt(N);
    vector<vector<int> > subGrid(n, vector<int>(n));
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            subGrid[i][j]
                = matrix[n * (x / n) + (j / n) + i]
                        [n * (y / n) + j % n];
        }
    }
 
    // Return the submatrix
    return subGrid;
}
 
// Driver Code
int main()
{
    int N = 9;
    vector<vector<int> > matrix
        = { { 1, 2, 3, 9, 8, 7, 1, 2, 1 },
            { 4, 5, 6, 1, 2, 3, 7, 9, 8 },
            { 7, 8, 9, 2, 2, 0, 1, 5, 7 },
            { 0, 2, 9, 1, 2, 3, 4, 5, 3 },
            { 9, 8, 7, 4, 5, 6, 7, 4, 1 },
            { 1, 4, 7, 7, 8, 9, 9, 8, 7 },
            { 5, 6, 1, 9, 8, 7, 5, 2, 3 },
            { 4, 5, 1, 6, 5, 4, 9, 7, 4 },
            { 8, 7, 9, 3, 2, 1, 9, 4, 9 } };
    int x = 4;
    int y = 4;
 
    // Function call
    vector<vector<int> > subGrid
        = getSubGrid(N, matrix, x, y);
    for (int i = 0; i < subGrid.size(); i++) {
        cout << "[";
        int j = 0;
        for (; j < subGrid.size() - 1; j++) {
            cout << subGrid[i][j] << ", ";
        }
        cout << subGrid[i][j] << "] ";
    }
 
    return 0;
}
 
// This code is contributed by Rohit Pradhan

Java




// Java code to implement the approach
 
import java.io.*;
import java.util.*;
 
class GFG {
 
    // Function to print submatrix
    static int[][] getSubGrid(int N, int[][] matrix,
                              int x, int y)
    {
        int n = (int)Math.sqrt(N);
        int subGrid[][] = new int[n][n];
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                subGrid[i][j]
                    = matrix[n * (x / n) + (j / n) + i]
                            [n * (y / n) + j % n];
            }
        }
 
        // Return the submatrix
        return subGrid;
    }
 
    // Driver code
    public static void main(String[] args)
    {
        int N = 9;
        int matrix[][] = { { 1, 2, 3, 9, 8, 7, 1, 2, 1 },
                           { 4, 5, 6, 1, 2, 3, 7, 9, 8 },
                           { 7, 8, 9, 2, 2, 0, 1, 5, 7 },
                           { 0, 2, 9, 1, 2, 3, 4, 5, 3 },
                           { 9, 8, 7, 4, 5, 6, 7, 4, 1 },
                           { 1, 4, 7, 7, 8, 9, 9, 8, 7 },
                           { 5, 6, 1, 9, 8, 7, 5, 2, 3 },
                           { 4, 5, 1, 6, 5, 4, 9, 7, 4 },
                           { 8, 7, 9, 3, 2, 1, 9, 4, 9 } };
        int x = 4;
        int y = 4;
 
        // Function call
        int subGrid[][] = getSubGrid(N, matrix, x, y);
 
        System.out.println(Arrays.deepToString(subGrid));
    }
}

Python3




# python3 code to implement the approach
import math
 
# Function to print submatrix
def getSubGrid(N, matrix, x, y):
 
    n = int(math.sqrt(N))
    subGrid = [[0 for _ in range(n)] for _ in range(n)]
    for i in range(0, n):
        for j in range(0, n):
            subGrid[i][j] = matrix[n *
                                   (x // n) + (j // n) + i][n * (y // n) + j % n]
 
    # Return the submatrix
    return subGrid
 
# Driver Code
if __name__ == "__main__":
 
    N = 9
    matrix = [[1, 2, 3, 9, 8, 7, 1, 2, 1],
              [4, 5, 6, 1, 2, 3, 7, 9, 8],
              [7, 8, 9, 2, 2, 0, 1, 5, 7],
              [0, 2, 9, 1, 2, 3, 4, 5, 3],
              [9, 8, 7, 4, 5, 6, 7, 4, 1],
              [1, 4, 7, 7, 8, 9, 9, 8, 7],
              [5, 6, 1, 9, 8, 7, 5, 2, 3],
              [4, 5, 1, 6, 5, 4, 9, 7, 4],
              [8, 7, 9, 3, 2, 1, 9, 4, 9]]
    x = 4
    y = 4
 
    # Function call
    subGrid = getSubGrid(N, matrix, x, y)
    for i in range(0, len(subGrid)):
        print("[", end="")
        j = 0
        while(j < len(subGrid) - 1):
            print(subGrid[i][j], end=", ")
            j += 1
 
        print(subGrid[i][j], end="] ")
 
    # This code is contributed by rakeshsahni

C#




// C# code to implement the approach
 
using System;
 
 
class GFG {
 
    // Function to print submatrix
    static int[,] getSubGrid(int N, int[,] matrix,
                              int x, int y)
    {
        int n = (int)Math.Sqrt(N);
        int [,]subGrid = new int[n,n];
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                subGrid[i,j] = matrix[n * (x / n) + (j / n) + i,n * (y / n) + j % n];
            }
        }
 
        // Return the submatrix
        return subGrid;
    }
 
    // Driver code
    public static void Main(string[] args)
    {
        int N = 9;
        int [,]matrix = { { 1, 2, 3, 9, 8, 7, 1, 2, 1 },
                           { 4, 5, 6, 1, 2, 3, 7, 9, 8 },
                           { 7, 8, 9, 2, 2, 0, 1, 5, 7 },
                           { 0, 2, 9, 1, 2, 3, 4, 5, 3 },
                           { 9, 8, 7, 4, 5, 6, 7, 4, 1 },
                           { 1, 4, 7, 7, 8, 9, 9, 8, 7 },
                           { 5, 6, 1, 9, 8, 7, 5, 2, 3 },
                           { 4, 5, 1, 6, 5, 4, 9, 7, 4 },
                           { 8, 7, 9, 3, 2, 1, 9, 4, 9 } };
        int x = 4;
        int y = 4;
 
        // Function call
        int [,]subGrid = getSubGrid(N, matrix, x, y);
        Console.Write("[") ;
        for (int i = 0; i< subGrid.GetLength(0); i++){
            Console.Write("[") ;
            for (int j = 0; j < subGrid.GetLength(1) ; j++)
                Console.Write(subGrid[i,j] + " ,");
            Console.Write("] ");
        }
        Console.Write("]") ;
 
    }
}
 
// This code is contributed by AnkThon

Javascript




<script>
// javascript code to implement the approach
   // Function to print submatrix
    function getSubGrid(N, matrix,
                              x , y)
    {
        var n = parseInt(Math.sqrt(N));
        var subGrid = Array(n).fill(0).map(x => Array(n).fill(0));
        for (var i = 0; i < n; i++) {
            for (var j = 0; j < n; j++) {
                subGrid[i][j]
                    = matrix[n * parseInt(x / n) + parseInt(j / n) + i]
                            [n * parseInt(y / n) + j % n];
            }
        }
 
        // Return the submatrix
        return subGrid;
    }
 
    // Driver code
     
     
        var N = 9;
        var matrix = [ [ 1, 2, 3, 9, 8, 7, 1, 2, 1 ],
                           [ 4, 5, 6, 1, 2, 3, 7, 9, 8 ],
                           [ 7, 8, 9, 2, 2, 0, 1, 5, 7 ],
                           [ 0, 2, 9, 1, 2, 3, 4, 5, 3 ],
                           [ 9, 8, 7, 4, 5, 6, 7, 4, 1 ],
                           [ 1, 4, 7, 7, 8, 9, 9, 8, 7 ],
                           [ 5, 6, 1, 9, 8, 7, 5, 2, 3 ],
                           [ 4, 5, 1, 6, 5, 4, 9, 7, 4 ],
                           [ 8, 7, 9, 3, 2, 1, 9, 4, 9 ] ];
        var x = 4;
        var y = 4;
 
        // Function call
        var subGrid = getSubGrid(N, matrix, x, y);
        document.write("[");
        for (let i = 0; i < subGrid.length; i++) {
            document.write("[");
            for (let j = 0; j < subGrid[i].length; j++) {
                document.write(subGrid[i][j]+", ");
            }
            document.write("]");
         }
         document.write("]");
         
 
// This code contributed by shikhasingrajput
</script>

Output

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

Time Complexity: O(N)
Auxiliary Space: O(1)


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