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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)

Parameters

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`.

Return value

The inner product of two ranges.

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, std::terminate is called.
If the algorithm fails to allocate memory, std::bad_alloc is thrown.

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; }

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;
}

Example

#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