# 11.3. Computing the Sum and Mean of Elements in a Container

## Problem

You want to compute the sum and mean of elements in a container of numbers.

## Solution

You can use the `accumulate`

function from the
`<numeric>`

header to compute the sum, and then
divide by the size to get the mean. Example
11-5 demonstrates this using a vector.

Example 11-5. Computing the sum and mean of a container

#include <numeric> #include <iostream> #include <vector> using namespace std; int main() { vector<int> v; v.push_back(1); v.push_back(2); v.push_back(3); v.push_back(4); int sum = accumulate(v.begin(), v.end(), 0); double mean = double(sum) / v.size(); cout << "sum = " << sum << endl; cout << "count = " << v.size() << endl; cout << "mean = " << mean << endl; }

The program in Example 11-5 produces the following output:

sum = 10 count = 4 mean = 2.5

## Discussion

The `accumulate`

function generally provides the most
efficient and simplest method to find the sum of all the elements in a container.

Even though this recipe has a relatively simple solution, writing your own generic function to compute a mean is not so easy. Example 11-6 shows one way to write such a generic function:

Example 11-6. A generic function to compute the mean

template<class Iter_T> double computeMean(Iter_T first, Iter_T last) { return static_cast<double>(accumulate(first, last, 0.0)) / distance(first, last); }

The `computeMean`

function in Example 11-6 is sufficient for most purposes but it has one restriction: it doesn’t work with input iterators ...

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