Is this implementation to the video correct? and what about garbage value if i=0 and j=0,also show correct output for 2 1 1 1 1

#1 
#include<iostream>
using namespace std;
int main()
{  
  int n;
  cin>>n;
  int a[100][100]={0};
  int presum[100][100];
  for(int i=0;i<n;i++)
  {
  	for(int j=0;j<n;j++)
  	{
  		cin>>a[i][j];
  		if(j==0)
  		{presum[i][j]=a[i][j];}
  		else{
  		presum[i][j]=presum[i][j-1]+a[i][j];}
  	}
  }
  for(int i=1;i<n;i++)
  {
  	for(int j=0;j<n;j++)
  	{
  		presum[j][i]+=presum[j-1][i];
  		}
  }
  
  int sum=0;
  for(int i=0;i<n;i++)
{ for (int j=0;j<n;j++)
{
	for(int k=i;k<n;k++)
	{
		for (int l=j;l<n;l++)
		{
			sum+=presum[k][l]-presum[i-1][l]-presum[k][j-1]+presum[i-1][j-1];
			
		}
		}
	
}}
cout<<sum;
 return 0;
}
#2

Hello @sagar_aggarwal,

The junk values are getting added and subtracted from the sum for the cases:

When li=0
when lj=0
You have to check for these cases before computing the sum.
Modified Code:

Hope, this would help.
Give a like if you are satisfied.

#3

#IS THIS CORRECT NOW?

#include<iostream>
using namespace std;
int main()
{  
  int n;
  cin>>n;
  int a[100][100]={0};
  int presum[100][100];
  for(int i=0;i<n;i++)
  {
  	for(int j=0;j<n;j++)
  	{
  		cin>>a[i][j];
  		if(j==0)
  		{presum[i][j]=a[i][j];}
  		else{
  		presum[i][j]=presum[i][j-1]+a[i][j];}
  	}
  }
  for(int i=0;i<n;i++)
  {
  	for(int j=0;j<n;j++)
  	{   if(j!=0){
  		presum[j][i]+=presum[j-1][i];}
  		}
  }
  for(int i=0;i<n;i++)
  {
  	for(int j=0;j<n;j++)
  	{
  		cout<<presum[i][j]<<" ";
  		}
  		cout<<endl;
  }
  
  int sum=0;
  for(int i=0;i<n;i++)
{ for (int j=0;j<n;j++)
{
	for(int k=i;k<n;k++)
	{
		for (int l=j;l<n;l++)
		{
			sum+=presum[k][l];
			if(i-1>=0){
			sum-=presum[i-1][l];}
			if(j-1>=0){
			sum-=presum[k][j-1];}
			if(i-1>=0&&j-1>=0)
			{
			sum+=presum[i-1][j-1];}
			
		}
		}
	
}}
cout<<sum;
 return 0;
}
#4

also brother we have used a nested loop for implementing presum matrix is time complexity still n^4 or n^2+ n^4??

#5

Hello @sagar_aggarwal,

  1. Yes, it seems correct.

  2. The complexity is O(n^4) we will consider the higher power of n.

#6

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