Use custom function and for_each to calculate mean value : for_each « STL Algorithms Non modifying sequence operations « C++ Tutorial

1. C++ Tutorial
2. STL Algorithms Non modifying sequence operations
3. for_each

/* The following code example is taken from the book
 * "The C++ Standard Library - A Tutorial and Reference"
 * by Nicolai M. Josuttis, Addison-Wesley, 1999
 *
 * (C) Copyright Nicolai M. Josuttis 1999.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.
 */

#include <iostream>
#include <vector>
#include <deque>
#include <list>
#include <set>
#include <map>
#include <string>
#include <algorithm>
#include <iterator>
#include <functional>
#include <numeric>

/* PRINT_ELEMENTS()
 * - prints optional C-string optcstr followed by
 * - all elements of the collection coll
 * - separated by spaces
 */
template <class T>
inline void PRINT_ELEMENTS (const T& coll, const char* optcstr="")
{
    typename T::const_iterator pos;

    std::cout << optcstr;
    for (pos=coll.begin(); pos!=coll.end(); ++pos) {
        std::cout << *pos << ' ';
    }
    std::cout << std::endl;
}

/* INSERT_ELEMENTS (collection, first, last)
 * - fill values from first to last into the collection
 * - NOTE: NO half-open range
 */
template <class T>
inline void INSERT_ELEMENTS (T& coll, int first, int last)
{
    for (int i=first; i<=last; ++i) {
        coll.insert(coll.end(),i);
    }
}

using namespace std;

// function object to process the mean value
class MeanValue {
  private:
    long num;    // number of elements
    long sum;    // sum of all element values
  public:
    // constructor
    MeanValue () : num(0), sum(0) {
    }

    // function call
    // - process one more element of the sequence
    void operator() (int elem) {
        num++;          // increment count
        sum += elem;    // add value
    }

    // return mean value (implicit type conversion)
    operator double() {
        return static_cast<double>(sum) / static_cast<double>(num);
    }
};

int main()
{
    vector<int> coll;

    INSERT_ELEMENTS(coll,1,8);

    // process and print mean value
    double mv = for_each (coll.begin(), coll.end(),  // range
                          MeanValue());              // operation
    cout << "mean value: " << mv << endl;
}

mean value: 4.5