Category: algorithms | Component type: function |
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 BinaryFunction1, class BinaryFunction2> T inner_product(InputIterator1 first1, InputIterator1 last1, InputIterator2 first2, T init, BinaryFunction1 binary_op1, BinaryFunction2 binary_op2);
The first version of inner_product returns init plus the inner product of the two ranges [1]. That is, it first initializes the result to init and then, for each iterator i in [first1, last1), in order from the beginning to the end of the range, updates the result by result = result + (*i) * *(first2 + (i - first1)).
The second version of inner_product is identical to the first, except that it uses two user-supplied function objects instead of operator+ and operator*. That is, it first initializes the result to init and then, for each iterator i in [first1, last1), in order from the beginning to the end of the range, updates the result by result = binary_op1(result, binary_op2(*i, *(first2 + (i - first1))). [2]
int main() { int A1[] = {1, 2, 3}; int A2[] = {4, 1, -2}; const int N1 = sizeof(A1) / sizeof(int); cout << "The inner product of A1 and A2 is " << inner_product(A1, A1 + N1, A2, 0) << endl; }
[1] There are several reasons why it is important that inner_product starts with the value init. One of the most basic is that this allows inner_product to have a well-defined result even if [first1, last1) is an empty range: if it is empty, the return value is init. The ordinary inner product corresponds to setting init to 0.
[2] Neither binary operation is required to be either associative or commutative: the order of all operations is specified.
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