std::inner_product

Defined in header `<numeric>`
template< class InputIt1, class InputIt2, class T > T inner_product( InputIt1 first1, InputIt1 last1, InputIt2 first2, T value );	(1)
template< class ExecutionPolicy, class InputIt1, class InputIt2, class T > T inner_product( ExecutionPolicy&& policy, InputIt1 first1, InputIt1 last1, InputIt2 first2, T value );	(2)	(since C++17)
template<class InputIt1, class InputIt2, class T, class BinaryOperation1, class BinaryOperation2> T inner_product( InputIt1 first1, InputIt1 last1, InputIt2 first2, T value, BinaryOperation1 op1, BinaryOperation2 op2 );	(3)
template< class ExecutionPolicy,class InputIt1, class InputIt2, class T, class BinaryOperation1, class BinaryOperation2> T inner_product( ExecutionPolicy&& policy, InputIt1 first1, InputIt1 last1, InputIt2 first2, T value, BinaryOperation1 op1, BinaryOperation2 op2 );	(4)	(since C++17)

Computes inner product (i.e. sum of products) of the range [first1, last1) and another range beginning at first2.

1) Products are calculated using operator* and sums are calculated using operator+.

3) Products are calculated using op2 and sums are calculated using op1.

2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

`op1` and `op2` must not have side effects.	(until C++11)
`op1` and `op2` must not invalidate any iterators, including the end iterators, or modify any elements of the ranges involved.	(since C++11)

first1, last1	-	the first range of elements
first2	-	the beginning of the second range of elements
value	-	initial value of the sum of the products
policy	-	the execution policy to use. See execution policy for details.
op1	-	binary operation function object that will be applied. This "sum" function takes a value returned by op2 and the current value of the accumulator and produces a new value to be stored in the accumulator. The signature of the function should be equivalent to the following: Ret fun(const Type1 &a, const Type2 &b); The signature does not need to have const &. The types Type1 and Type2 must be such that objects of types T and Type3 can be implicitly converted to Type1 and Type2 respectively. The type Ret must be such that an object of type T can be assigned a value of type Ret.
op2	-	binary operation function object that will be applied. This "product" function takes one value from each range and produces a new value. The signature of the function should be equivalent to the following: Ret fun(const Type1 &a, const Type2 &b); The signature does not need to have const &. The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. The type Ret must be such that an object of type Type3 can be assigned a value of type Ret.
Type requirements
- `InputIt1, InputIt2` must meet the requirements of `InputIterator`.
- `T` must meet the requirements of `CopyAssignable` and `CopyConstructible`.

[edit] Return value

The inner product of two ranges.

[edit] Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

If execution of a function invoked as part of the algorithm throws an exception,

if policy is std::parallel_vector_execution_policy, std::terminate is called
if policy is std::sequential_execution_policy or std::parallel_execution_policy, the algorithm exits with an std::exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in std::exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered.
if policy is some other type, the behavior is implementation-defined

If the algorithm fails to allocate memory (either for itself or to construct an std::exception_list when handling a user exception), std::bad_alloc is thrown.

[edit] Possible implementation

First version

template<class InputIt1, class InputIt2, class T>
T inner_product(InputIt1 first1, InputIt1 last1,
                InputIt2 first2, T value)
{
    while (first1 != last1) {
         value = value + *first1 * *first2;
         ++first1;
         ++first2;
    }
    return value;
}

Second version

template<class InputIt1, class InputIt2,
         class T,
         class BinaryOperation1, class BinaryOperation2>
T inner_product(InputIt1 first1, InputIt1 last1,
                InputIt2 first2, T value,
                BinaryOperation1 op1
                BinaryOperation2 op2)
{
    while (first1 != last1) {
         value = op1(value, op2(*first1, *first2));
         ++first1;
         ++first2;
    }
    return value;
}

[edit] Example

Run this code

#include <numeric>
#include <iostream>
#include <vector>
#include <functional>
int main()
{
    std::vector<int> a{0, 1, 2, 3, 4};
    std::vector<int> b{5, 4, 2, 3, 1};
 
    int r1 = std::inner_product(a.begin(), a.end(), b.begin(), 0);
    std::cout << "Inner product of a and b: " << r1 << '\n';
 
    int r2 = std::inner_product(a.begin(), a.end(), b.begin(), 0,
                                std::plus<int>(), std::equal_to<int>());
    std::cout << "Number of pairwise matches between a and b: " <<  r2 << '\n';
}

Output:

Inner product of a and b: 21
Number of pairwise matches between a and b: 2

[edit] See also

accumulate	sums up a range of elements (function template)
partial_sum	computes the partial sum of a range of elements (function template)
std::experimental::parallel::inner_product (parallelism TS)	parallelized version of `std::inner_product` (function template)

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