Question

I’m learning how to use functors together with STL algorithms to calculate the dot product of two vectors. Here are my codes:

template<size_t DIM>
double Vector<DIM>::operator*(const Vector<DIM>& rhs) const {
    double dotPro = 0;
    std::for_each(vec, vec + DIM, std::bind2nd(dot_product<double>(rhs.get()), dotPro));
    return dotPro;
}
/*vec is a double array and the data member of Vector class. I want to get the 
dot product of rhs and *this by using std::for_each(). 
rhs.get()returns a const double* which is the start address of rhs's vec*/

/*The codes below define the functor.  dotPro is passed as a reference so as to 
it could be save the last result.*/  
template<typename T>
struct dot_product: public std::binary_function<T, T, void> {
    const T* arg;
    sum(const T* dbl) : arg(dbl){};
     void operator() (const T dbl, T& dotPro) {
        dotPro += *arg++ * dbl;
    }
};

Sorry I forget my quesion…
The question is my code doesn’t compile. Here is the compiling error:

error: no match for call to '(const dot_product<double>) (const double&, const double&)'|
note: candidates are: void dot_product<T>::operator()(T, T&) [with T = double]|

And here is a error occurs in binders.h

c:\program files\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.4.1\include\c++\backward\binders.h|147|error: return-statement with a value, in function returning 'void'|

Answer 1

Just use std::inner_product:

live demo

#include <algorithm>
#include <iostream>
#include <iterator>
#include <ostream>
#include <numeric>
#include <cstdlib>
#include <vector>

using namespace std;

int main()
{
    vector<int> v1,v2;
    generate_n( back_inserter(v1), 256, rand );
    generate_n( back_inserter(v2), v1.size(), rand );
    cout << inner_product( v1.begin(), v1.end(), v2.begin(), 0 ) << endl;
}

---

Yes, that’s a good solution. But I don’t want to use any methods provided in numeric.h.

Just define your own dot_product:

live demo

template<typename InputIterator1,typename InputIterator2,typename ValueType>
ValueType dot_product( InputIterator1 first, InputIterator1 last, InputIterator2 another, ValueType init)
{
    while(first!=last)
    {
        init += (*first) * (*another) ;
        ++first;
        ++another;
    }
    return init;
}