std::accumulate
- C++ course
- Intermediate
- Lesson 74/116
- 5 min
Learn aggregation with std::accumulate.
std::accumulate
std::accumulate combines all elements in a range. By default it sums them: int sum = std::accumulate(v.begin(), v.end(), 0);. The third parameter is the initial value; the accumulation starts from this value.
std::accumulate is defined in <numeric> (not in algorithm). The initial value sets the result type: 0 sums as int, 0.0 as double. You can pass a custom operation as the fourth parameter; for example std::multiplies<int>() for multiplication. This way accumulate is not just summation but a general 'fold' operation.
Initial value and type
The initial value sets not only where the sum starts but also the type of the result. Writing 0 sums as int, writing 0.0 sums as double. For a double vector, writing 0 loses the fractional part.
Example: for std::vector<double> prices = {9.99, 14.50, 3.75}; you write std::accumulate(prices.begin(), prices.end(), 0.0). This gives 28.24. If you write 0 the result is 28 (no fraction). That is why the initial value must match the vector type.
Practice Print the sum
For v = {1, 2, 3, 4, 5}, find the sum with std::accumulate and print it with std::cout. Use 0 as the initial value. Remember #include <numeric> is required.
Starter code
#include <iostream>
#include <vector>
#include <numeric>
int main() {
std::vector<int> v = {1, 2, 3, 4, 5};
// Print the sum with std::accumulate
return 0;
}Sample solution
#include <iostream>
#include <vector>
#include <numeric>
int main() {
std::vector<int> v = {1, 2, 3, 4, 5};
int sum = std::accumulate(v.begin(), v.end(), 0);
std::cout << sum;
return 0;
}Expected output
15
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match each piece to its role
| Term | Meaning |
|---|---|
std::accumulate |
Sum/fold |
v.begin() |
Range start |
v.end() |
Range end |
0 (third param) |
Initial value |
2 What is the third parameter of std::accumulate?
std::accumulate(v.begin(), v.end(), ???);- The last element
- A function object
- An index
- The initial value Correct answer
Explanation: The third parameter is the initial value.
3 Complete the accumulate call
int sum = std::____(v.begin(), v.end(), 0);int sum = std::accumulate(v.begin(), v.end(), 0);Explanation: std::accumulate sums the elements.