Prefix Sum Array – Implementation and Applications in Competitive Programming

Given an array arr[] of size n, its prefix sum array is another array prefixSum[] of same size such that the value of prefixSum[i] is arr[0] + arr[1] + arr[2] … arr[i].

Examples :

Input  : arr[] = {10, 20, 10, 5, 15}
Output : prefixSum[] = {10, 30, 40, 45, 60}

Explanation : While traversing the array, update 
the element by adding it with its previous element.
prefixSum[0] = 10, 
prefixSum[1] = prefixSum[0] + arr[1] = 30, 
prefixSum[2] = prefixSum[1] + arr[2] = 40 and so on.

To fill prefix sum array, we run through index 1 to last and keep on adding present element with previous value in prefix sum array.



Below is the implementation :

C++

// C++ program for Implementing
// prefix sum array
#include <bits/stdc++.h>
using namespace std;
  
// Fills prefix sum array
void fillPrefixSum(int arr[], int n, int prefixSum[])
{
    prefixSum[0] = arr[0];
  
    // Adding present element 
    // with previous element
    for (int i = 1; i < n; i++)
        prefixSum[i] = prefixSum[i-1] + arr[i];
}
  
// Driver Code
int main()
{
    int arr[] = { 10, 4, 16, 20 };
    int n = sizeof(arr) / sizeof(arr[0]);
    int prefixSum[n];
  
    fillPrefixSum(arr, n, prefixSum);
    for (int i = 0; i < n; i++)
        cout << prefixSum[i] << " ";
}

Java

// Java Program for Implementing
// prefix sum arrayclass 
class Prefix
{
    // Fills prefix sum array
    static void fillPrefixSum(int arr[], int n,
                               int prefixSum[])
    {
        prefixSum[0] = arr[0];
          
        // Adding present element
        // with previous element
        for( int i = 1; i < n; ++i )
            prefixSum[i] = prefixSum[i-1] + arr[i];
    }
      
    // Driver code
    public static void main(String[] args)
    {
        int arr[] = { 10, 4, 16, 20 };
        int n = arr.length;
        int prefixSum[] = new int[n];
          
        fillPrefixSum(arr, n, prefixSum);
          
        for (int i = 0; i < n; i++)
            System.out.print(prefixSum[i] + " ");
        System.out.println("");
    }
  
}
  
// This Code is Contributed by Saket Kumar

Python3

# Python Program for Implementing 
# prefix sum array
  
# Fills prefix sum array
def fillPrefixSum(arr, n, prefixSum):
  
    prefixSum[0] = arr[0]
  
    # Adding present element
    # with previous element
    for i in range(1, n):
        prefixSum[i] = prefixSum[i - 1] + arr[i]
  
# Driver code
arr =[10, 4, 16, 20 ]
n = len(arr)
  
prefixSum = [0 for i in range(n + 1)]
  
fillPrefixSum(arr, n, prefixSum)
  
for i in range(n):
    print(prefixSum[i] , " ", end="")
  
# This code is contributed
# by Anant Agarwal.

C#

// C# Program for Implementing
// prefix sum arrayclass
using System;
  
class GFG
{
    // Fills prefix sum array
    static void fillPrefixSum(int []arr, int n, 
                               int []prefixSum)
    {
        prefixSum[0] = arr[0];
          
        // Adding present element 
        // with previous element
        for( int i = 1; i < n; ++i )
            prefixSum[i] = prefixSum[i - 1] + arr[i];
    }
      
    // Driver code
    public static void Main()
    {
        int []arr = { 10, 4, 16, 20 };
        int n = arr.Length;
        int []prefixSum = new int[n];
          
        fillPrefixSum(arr, n, prefixSum);
          
        for (int i = 0; i < n; i++)
            Console.Write(prefixSum[i] + " ");
        Console.Write("");
    }
  
}
  
// This Code is Contributed by nitin mittal

PHP

<?php
// PHP program for 
// Implementing prefix 
// sum array
  
// Fills prefix sum array
function fillPrefixSum($arr
                       $n)
{
    $prefixSum = array();
    $prefixSum[0] = $arr[0];
  
    // Adding present element 
    // with previous element
    for ($i = 1; $i < $n; $i++)
        $prefixSum[$i] = $prefixSum[$i - 1] + 
                                    $arr[$i];
          
    for ($i = 0; $i < $n; $i++)
        echo $prefixSum[$i] . " ";
}
  
// Driver Code
$arr = array(10, 4, 16, 20);
$n = count($arr);
  
fillPrefixSum($arr, $n);
  
// This code is contributed
// by Sam007
?>


Output:

10 14 30 50

Applications :

An Example Problem :

Consider an array of size n with all initial values as 0. Perform given ‘m’ add operations from index ‘a’ to ‘b’ and evaluate highest element in array. An add operation adds 100 to all elements from a to b (both inclusive).

Example :

Input : n = 5 // We consider array {0, 0, 0, 0, 0}
        m = 3.
        a = 2, b = 4.
        a = 1, b = 3.
        a = 1, b = 2.
Output : 300

Explanation : 

After I operation -
A : 0 100 100 100 0

After II operation -
A : 100 200 200 100 0

After III operation -
A : 200 300 200 100 0

Highest element : 300

A simple approach is running a loop ‘m’ times. Inputting a and b and running a loop from a to b, adding all elements by 100.

Efficient approach using Prefix Sum Array :

1 : Run a loop for 'm' times, inputting 'a' and 'b'.
2 : Add 100 at index 'a' and subtract 100 from index 'b+1'.
3 : After completion of 'm' operations, compute the prefix sum array.
4 : Scan the largest element and we're done.

What we did was adding 100 at ‘a’ because this will add 100 to all elements while taking prefix sum array. Subtracting 100 from ‘b+1’ will reverse the changes made by adding 100 to elements from ‘b’ onward.

For better understanding :


After I operation -
A : 0 100 0 0 -100 
Prefix Sum Array : 0 100 100 100 0

After II operation -
A : 100 100 0 -100 -100
Prefix Sum Array : 100 200 200 100 0

After III operation -
A : 200 100 -100 -100 -100
Prefix Sum Array : 200 300 200 100 0

Final Prefix Sum Array : 200 300 200 100 0 

The required highest element : 300


Recent Articles on Prefix Sum Technique

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Improved By : nitin mittal, Sam007