lec9_prefix_sum_2darray.java

package shreya.java;

import java.util.Scanner;

public class lec9_prefix_sum_2darray {
    static int findsum (int [][] matrix,int l1,int r1 ,int l2 , int r2){
        int sum =0;
        for(int i =l1;i<=l2;i++){
            for(int j=r1;j<=r2;j++){
                sum+= matrix[i][j];
            }
        }
        return sum;
    }
    //calculate row wise and col wise sum
    static void findprefixsummatrix(int[][]matrix){
        int r = matrix.length;
        int c = matrix[0].length;

        //traverse horizontally to calculate row wise prefix sum
        //matrix[i][j] = sumrectangle ((0,0) ,(i,j))
        for(int i=0;i<r;i++){
            for(int j=1;j<c;j++){
                matrix[i][j]+=matrix[i][j-1];
            }
        }
        //traverse vertically to calculate col wise sum
        for(int j=0;j<c;j++){
            for(int i=1;i<r;i++){
                matrix[i][j]+=matrix[i-1][j];  // with is we are adding i-1 element in i row
            }
        }
    }
    static int findsum2 (int [][] matrix,int l1,int r1 ,int l2 , int r2){
        int sum =0;
        findprefixsummatrix(matrix);
        for(int i=l1;i<=l2;i++){
            //r1 to r2 sum for row i
            if (r1 >= 1) {
                sum+= matrix[i][r2]-matrix[i][r1-1];
            }
           else {sum+=matrix[i][r2];
        }}
        return sum;

    }
    static int findsum3(int [][] matrix,int l1,int r1 ,int l2 , int r2){
        int ans =0;
        int sum =0 , up =0 , left=0 ,leftup =0;
        findprefixsummatrix(matrix);
        sum = matrix[l2][r2];
        if(r1>=1){
        left = matrix[l2][r1-1];}
        if(l1>=1)
        up =matrix[l1-1][r2];
        if(l1>=1&&r1>=1)
        leftup =matrix[l1-1][r1-1];


        ans =sum-up-left+leftup;



        return ans;}



    public static void main(String[] args) {
        Scanner sc =new Scanner(System.in);
        System.out.println("enter the no.of rows and cols");
        int r = sc.nextInt();
        int c = sc.nextInt();
        int [][] matrix = new int [r][c];
        int totalelements =r*c;
        System.out.println("enter "+ totalelements+" elements");
        for(int i=0;i<r;i++){
            for (int j=0;j<c;j++){
                matrix[i][j] =sc.nextInt();

            }
        }
        System.out.println("enter rectangle boundaries l1, r1, l2, r2 ");
        int l1 = sc.nextInt();
        int r1 = sc.nextInt();
        int l2 = sc.nextInt();
        int r2 = sc.nextInt();
        System.out.println("rectangle sum"+findsum(matrix,l1,r1,l2,r2));
        System.out.println("rectangle sum"+findsum3(matrix,l1,r1,l2,r2));

    }
}