std::inner_product
| Defined in header <numeric>
|
||
template< class InputIt1, class InputIt2, class T > T inner_product( InputIt1 first1, InputIt1 last1, InputIt2 first2, T init ); |
(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 ); |
(2) | (constexpr since C++20) |
Computes inner product (i.e. sum of products) or performs ordered map/reduce operation on the range [first1, last1) and the range of std::distance(first1, last1) elements beginning at first2.
acc (of type T) with the initial value init and then modifies it with the expression acc = acc + (*i1) * (*i2)(until C++20)acc = std::move(acc) + (*i1) * (*i2)(since C++20) for every iterator i1 in the range [first1, last1) in order and its corresponding iterator i2 in the range beginning at first2. For built-in meaning of + and *, this computes inner product of the two ranges.acc (of type T) with the initial value init and then modifies it with the expression acc = op1(acc, op2(*i1, *i2))(until C++20)acc = op1(std::move(acc), op2(*i1, *i2))(since C++20) for every iterator i1 in the range [first1, last1) in order and its corresponding iterator i2 in the range beginning at first2.Given last2 as the std::distance(first1, last1)th next iterator of first2, if any of the following conditions is satisfied, the behavior is undefined:
Tis not CopyConstructible.Tis not CopyAssignable.op1orop2modifies any element of[first1,last1)or[first2,last2).op1orop2invalidates any iterator or subrange in[first1,last1]or[first2,last2].
Parameters
| first1, last1 | - | the pair of iterators defining the range of elements to |
| first2 | - | the beginning of the second range of elements |
| init | - | initial value of the sum of the products |
| 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.
| ||
Return value
acc after all modifications.
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;
}
|
| 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;
}
|
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.
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
|
they cannot modify the ranges involved |
See also
(C++17) |
applies an invocable, then reduces out of order (function template) |
| sums up or folds a range of elements (function template) | |
| computes the partial sum of a range of elements (function template) |