std::accumulate

std::accumulate

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Prerequisites

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1. What is std::accumulate?

std::accumulate is a standard algorithm that computes a single accumulated value from a range.

It processes elements sequentially from left to right, combining them using a given operation.

#include <numeric>
#include <vector>

std::vector<int> v = {1, 2, 3, 4};

int sum = std::accumulate(v.begin(), v.end(), 0);

10

(((init + v[0]) + v[1]) + v[2]) + ...

Always evaluated left-to-right

2. Function Signature

template<class InputIt, class T>
T accumulate(InputIt first, InputIt last, T init);

With Custom Operation

template<class InputIt, class T, class BinaryOp>
T accumulate(InputIt first, InputIt last,
             T init, BinaryOp op);

Example: Multiplication

int result = std::accumulate(v.begin(), v.end(), 1,
    [](int a, int b)
    {
        return a * b;
    });

24

Example: String Concatenation

#include <string>
#include <vector>

std::vector<std::string> words = {"Hello", " ", "World"};

std::string result = std::accumulate(words.begin(), words.end(), std::string(""));

"Hello World"

3. Characteristics

3-1. Sequential Execution

• always runs in order
• no parallelism
• deterministic result

3-2. Order Matters

std::accumulate(v.begin(), v.end(), 0,
    [](int a, int b)
    {
        return a - b;
    });

Result depends on order:

```text id=”yl7rxt” (((0 - 1) - 2) - 3) - 4

#### <b>3-3. Initial Value (`init`)</b>

* defines starting value
* affects result type and behavior

```cpp
std::accumulate(v.begin(), v.end(), 100);

Result:

110

4. Common Use Cases

✔️ Sum

```cpp id=”x8drgk” int sum = std::accumulate(v.begin(), v.end(), 0);

##### ✔️ Product

```cpp
int product = std::accumulate(v.begin(), v.end(), 1, std::multiplies<>());

✔️ Count with condition

int count = std::accumulate(v.begin(), v.end(), 0,
    [](int acc, int x)
    {
        return acc + (x % 2 == 0);
    });