(C++) std::accumulate

Function to summarize a range. Comparable to std::for_each (as mentioned by Meyers, 'Effective STL', item 37).

include <vector>

include <numeric>

include <cassert>


int main()
{
  std::vector<double> data(100,1.0); //Fill vector with 100 1.0's
  const double mySum = std::accumulate( data.begin(), data.end(), 0.0);
  assert(mySum == 100.0);
}

You can also use std::accumulate using a functor. The example shows how to calculate the product of all elements of a std::vector using the STL functor std::multiplies:

include <vector>

include <numeric>

include <cassert>


int main()
{
  std::vector<int> v;
  v.push_back(1);
  v.push_back(2);
  v.push_back(3);
  v.push_back(4);
  assert(std::accumulate(v.begin(),v.end(),1,std::multiplies<int>())==24);
}

A custom functor

This functor calculates the denominator of the Gibbs or Boltzmann distribution

            exp( q(a) / tau )
p(a) = ----------------------------
            SUM( exp( q(a) / tau) )
p(a): chance of selecting action a
exp: natural exponent
q(a): estimated value of action a 
tau: temperature

include <numeric>

include <vector>

include <cassert>

include <math>


template <class T>
struct GibbsFunctor : public std::binary_function<T, T, T>
{
  GibbsFunctor(const double& tau) : mTau(tau) {}
  T operator() (const T& sum, const T& x) const
  {
    return sum + std::exp( x / mTau );
  }
  const double mTau;
};
int main()
{
  std::vector<double> v;
  v.push_back(0.1);
  v.push_back(0.2);
  v.push_back(0.3);
  const double sumGibbs
    = std::accumulate(v.begin(),v.end(),0.0,GibbsFunctor<double>(1.0));
}

std::accumulate's definition

From the numeric header file supplied with C++ Builder 6:

// Copyright (c) 1994
// Hewlett-Packard Company
template <class InputIterator, class T>
T accumulate (InputIterator first, InputIterator last, T init)
{
  while (first != last) init = init + *first++; 
  return init;
}
template <class InputIterator, class T, class BinaryOperation>
T accumulate (InputIterator first, InputIterator last, T init,
              BinaryOperation binary_op)
{
  while (first != last) init = binary_op(init, *first++); 
  return init;
}

Code links

last edited (November 9, 2006) by bilderbikkel, Number of views: 3004, Current Rev: 7 (Diff)

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