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 ...