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, |
(1) | (constexpr since C++20) |
template< class InputIt1, class InputIt2, class T, class BinaryOp1, class BinaryOp2 > |
(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.
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.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:
-
T
is not CopyConstructible. -
T
is not CopyAssignable. - op1 or op2 modifies any element of
[
first1,
last1)
or[
first2,
last2)
. - op1 or op2 invalidates any iterator or subrange in
[
first1,
last1]
or[
first2,
last2]
.
Contents |
[edit] Parameters
first1, last1 | - | the first range of elements |
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: Ret fun(const Type1 &a, const Type2 &b); The signature does not need to have const &. |
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 &. |
Type requirements | ||
-InputIt1, InputIt2 must meet the requirements of LegacyInputIterator.
|
[edit] Return value
acc after all modifications.
[edit] Possible implementation
inner_product (1) |
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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; } |
[edit] 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.
[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'; }
Output:
Inner product of a and b: 21 Number 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.
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 |
[edit] 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) |