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C++ Cookbook
book

C++ Cookbook

by D. Ryan Stephens, Christopher Diggins, Jonathan Turkanis, Jeff Cogswell
November 2005
Beginner to intermediate content levelBeginner to intermediate
594 pages
16h 23m
English
O'Reilly Media, Inc.
Content preview from C++ Cookbook

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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Publisher Resources

ISBN: 0596007612Supplemental ContentErrata Page