std::inner_product
| Defined in header <numeric>
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template< class InputIt1, class InputIt2, class T >
T inner_product( InputIt1 first1, InputIt1 last1,
InputIt2 first2, T init );
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(1) | (constexpr since C++20) |
template< class InputIt1, class InputIt2, class T,
class BinaryOp1, class BinaryOp2 >
T inner_product( InputIt1 first1, InputIt1 last1,
InputIt2 first2, T init,
BinaryOp1 op1, BinaryOp2 op2 );
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(2) | (constexpr since C++20) |
Computes inner product (i.e. sum of products) or performs ordered map/reduce operation on the two target ranges [first1, last1) and [first2, last2), where last2 is std::next(first2, std::distance(first1, last1)).
Initializes the accumulator acc (of type T) with the initial value init, and then for every iterator iter1 in the first target range and its corresponding iterator iter2 in the second target range in order, acc is modified with:
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(until C++20) |
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(since C++20) |
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(until C++20) |
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(since C++20) |
If any of the following conditions is satisfied, the behavior is undefined:
Tis not CopyConstructible.Tis not CopyAssignable.op1orop2modifies any element of the two target ranges.op1orop2invalidates any iterator or subrange in[first1,last1]or[first2,last2].
Parameters
| first1, last1 | - | the pair of iterators defining the first target range |
| first2 | - | the beginning of the second target range |
| init | - | the initial value of the accumulator |
| 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:
The signature does not need to have |
| 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:
The signature does not need to have |
| Type requirements | ||
-InputIt1, InputIt2 must meet the requirements of LegacyInputIterator.
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Return value
acc after all modifications.
Notes
The parallelizable version of this algorithm, std::transform_reduce, requires op1 and op2 to be commutative and associative, but std::inner_product makes no such requirement, and always performs the operations in the order given.
Possible implementation
| inner_product (1) |
|---|
template<class InputIt1, class InputIt2, class T>
constexpr // since C++20
T 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;
}
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| inner_product (2) |
template<class InputIt1, class InputIt2, class T,
class BinaryOp1, class BinaryOp2>
constexpr // since C++20
T 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;
}
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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';
}
Output:
Inner product of a and b: 21
Number of pairwise matches between a and b: 2
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 242 | C++98 | op1 and op2 could not have side effects
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they cannot modify the ranges involved |
See also
| sums up or folds a range of elements (function template) | |
(C++17) |
applies an invocable, then reduces out of order (function template) |
| computes the partial sum of a range of elements (function template) |