Computes inner product (i.e. sum of products) or performs ordered map/reduce operation on the range[first1, last1) and the range ofstd::distance(first1, last1) elements beginning atfirst2.

Givenlast2 as thestd::distance(first1, last1)th next iterator offirst2, if any of the following conditions is satisfied, the behavior is undefined:

• T is notCopyConstructible.
• T is notCopyAssignable.
• op1 orop2 modifies any element of[first1, last1) or[first2, last2).
• op1 orop2 invalidates any iterator or subrange in[first1, last1] or[first2, last2].

[edit]Parameters

[edit]Return value

acc after all modifications.

[edit]Possible implementation

template<class InputIt1,class InputIt2,class T>constexpr// since C++20T inner_product(InputIt1 first1, InputIt1 last1, InputIt2 first2, T init){while(first1!= last1){        init= std::move(init)+(*first1)*(*first2);// std::move since C++20++first1;++first2;} return init;}

template<class InputIt1,class InputIt2,class T,class BinaryOp1,class BinaryOp2>constexpr// since C++20T inner_product(InputIt1 first1, InputIt1 last1, InputIt2 first2, T init,                BinaryOp1 op1, BinaryOp2 op2){while(first1!= last1){        init= op1(std::move(init), op2(*first1,*first2));// std::move since C++20++first1;++first2;} return init;}

[edit]Notes

The parallelizable version of this algorithm,std::transform_reduce, requiresop1 andop2 to be commutative and associative, butstd::inner_product makes no such requirement, and always performs the operations in the order given.

[edit]Example

#include <functional>#include <iostream>#include <numeric>#include <vector> 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<>(),std::equal_to<>());std::cout<<"Number of pairwise matches between a and b: "<<  r2<<'\n';}

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

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

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