inner_product

Generalized Numeric Operation

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Computes the inner product A X B of two ranges A and B.

None

#include <numeric>
template <class InputIterator1, class InputIterator2,
          class T>
T inner_product (InputIterator1 first1, 
                 InputIterator1 last1,
                 InputIterator2 first2, T init);
template <class InputIterator1, class InputIterator2,
          class T,
          class BinaryOperation1,
          class BinaryOperation2>
T inner_product (InputIterator1 first1, 
                 InputIterator1 last1,
                 InputIterator2 first2, T init,
                 BinaryOperation1 binary_op1,
                 BinaryOperation2 binary_op2);

There are two versions of inner_product. The first computes an inner product using the default multiplication and addition operators, while the second allows you to specify binary operations to use in place of the default operations.

The first version of the function computes its result by initializing the accumulator acc with the initial value init and then modifying it with:

acc = acc + ((*i1) * (*i2))

for every iterator i1 in the range [first1, last1) and iterator i2 in the range [first2, first2 + (last1 - first1)). The algorithm returns acc.

The second version of the function initializes acc with init, then computes:

acc = binary_op1(acc, binary_op2(*i1, *i2))

for every iterator i1 in the range [first1, last1) and iterator i2 in the range [first2, first2 + (last1 - first1)).

The inner_product algorithm computes exactly (last1 - first1) applications of either:

acc + (*i1) * (*i2)

or

binary_op1(acc, binary_op2(*i1, *i2)).

//
// inr_prod.cpp
//
 #include <numeric>       //For inner_product
 #include <list>          //For list
 #include <vector>        //For vectors
 #include <functional>    //For plus and minus
 #include <iostream>
 using namespace std;
 int main()
 {
   //Initialize a list and an int using arrays of ints
   int a1[3] = {6, -3, -2};
   int a2[3] = {-2, -3, -2};
   list<int>   l(a1, a1+3);
   vector<int> v(a2, a2+3);
   //Calculate the inner product of the two sets of values
   int inner_prod =
        inner_product(l.begin(), l.end(), v.begin(), 0);
   //Calculate a wacky inner product using the same values
   int wacky = 
         inner_product(l.begin(), l.end(), v.begin(), 0,
                       plus<int>(), minus<int>());
   //Print the output
   cout << "For the two sets of numbers: " << endl 
        << "     ";
   copy(v.begin(),v.end(),
        ostream_iterator<int,char>(cout," "));
   cout << endl << " and  ";
   copy(l.begin(),l.end(),
        ostream_iterator<int,char>(cout," "));
   cout << "," << endl << endl;
   cout << "The inner product is: " << inner_prod << endl;
   cout << "The wacky result is: " << wacky << endl;
   return 0;
 }

Program Output

For the two sets of numbers:
     -2 -3 -2
 and  6 -3 -2 ,
The inner product is: 1
The wacky result is: 8

If your compiler does not support default template parameters, then you always need to supply the Allocator template argument. For instance, you have to write:

list<int, allocator<int> > and vector<int, allocator<int> >

instead of

list<int> and vector<int>

If your compiler does not support namespaces, then you do not need the using declaration for std.