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C++ Program for Range sum queries without updates

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  • Last Updated : 12 Jun, 2022

Given an array arr of integers of size n. We need to compute sum of elements from index i to index j. The queries consisting of i and j index values will be executed multiple times. Examples:

Input : arr[] = {1, 2, 3, 4, 5}
        i = 1, j = 3
        i = 2, j = 4
Output :  9
         12         

Input : arr[] = {1, 2, 3, 4, 5}
        i = 0, j = 4 
        i = 1, j = 2 
Output : 15
          5

CPP




// CPP program to find sum between two indexes
// when there is no update.
#include <bits/stdc++.h>
using namespace std;
 
void preCompute(int arr[], int n, int pre[])
{
 pre[0] = arr[0];
 for (int i = 1; i < n; i++)
  pre[i] = arr[i] + pre[i - 1];
}
 
// Returns sum of elements in arr[i..j]
// It is assumed that i <= j
int rangeSum(int i, int j, int pre[])
{
 if (i == 0)
  return pre[j];
 
 return pre[j] - pre[i - 1];
}
 
// Driver code
int main()
{
 int arr[] = { 1, 2, 3, 4, 5 };
 int n = sizeof(arr) / sizeof(arr[0]);
 
 int pre[n];
 preCompute(arr, n, pre);
 cout << rangeSum(1, 3, pre) << endl;
 cout << rangeSum(2, 4, pre) << endl;
 
 return 0;
}

Output:

9
12

Time Complexity: O(n), where n represents the size of the given array.
Auxiliary Space: O(n), where n represents the size of the given array.

Please refer complete article on Range sum queries without updates for more details!

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