How to find the sum of elements of a Vector using STL in C++?

Given a vector, find the sum of the elements of this vector using STL in C++.

Example:

Input: vec = {1, 45, 54, 71, 76, 12}
Output: 259

Input: vec = {1, 7, 5, 4, 6, 12}
Output: 35

Approach:

Sum can be found with the help of accumulate() function provided in STL.

Syntax:

accumulate(first_index, last_index, initial value of sum);

CPP

// C++ program to find the sum
// of Array using accumulate() in STL

#include <bits/stdc++.h>
using namespace std;

int main()
{
    // Get the vector
    vector<int> a = { 1, 45, 54, 71, 76, 12 };

    // Print the vector
    cout << "Vector: ";
    for (int i = 0; i < a.size(); i++)
        cout << a[i] << " ";
    cout << endl;

    // Find the sum of the vector
    cout << "\nSum = " << accumulate(a.begin(), a.end(), 0);
    return 0;
}

Output

Vector: 1 45 54 71 76 12 

Sum = 259

Time Complexity: It is linear in the distance between first_index and last_index i.e if your  vector contains n number of elements between two given indices , the time complexity will be O(n).
Auxiliary Space: O(1)

Another Approach: (Using the for_each() function)

The for_each() function is an STL algorithm that applies a given function to each element in a range defined by a pair of iterators.
To find the sum of all elements in a vector using the for_each() function, we can define a lambda function or a function object that adds each element to a running total.

Syntax:

for_each(InputIt first, InputIt last, UnaryFunction f);

C++

// C++ program to find the sum
// of Array using for_each() function in STL

#include <bits/stdc++.h>
using namespace std;

int main()
{
    // Get the vector
    vector<int> a = { 1, 45, 54, 71, 76, 12 };

    int sum = 0;

    // Print the vector and calculate sum
    cout << "Vector: ";
    for_each(a.begin(), a.end(), [&](int i) {
        cout << i << " ";
        sum += i;
    });

    // Print the sum of the vector
    cout << "\nSum = " << sum << endl;
    return 0;
}

// This code is contributed by Susobhan Akhuli

Output

Vector: 1 45 54 71 76 12 
Sum = 259

Time Complexity: O(N), , where N is the number of elements in the range.
Auxiliary Space: O(1)