Difference between revisions of "cpp/algorithm/inner product"

Latest revision as of 14:31, 26 October 2024

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.

1) Initializes the accumulator 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.

2) Initializes the accumulator 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:

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].

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 &. 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 LegacyInputIterator.

[edit] Return value

acc after all modifications.

[edit] 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;
}

[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

Run this code

#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

transform_reduce (C++17)	applies an invocable, then reduces out of order (function template) [edit]
accumulate	sums up or folds a range of elements (function template) [edit]
partial_sum	computes the partial sum of a range of elements (function template) [edit]

@@ Line 1: / Line 1: @@
 {{cpp/title|inner_product}}
-{{cpp/algorithm/navbar}}
+{{cpp/algorithm/numeric/navbar}}
 {{dcl begin}}
-{{dcl header | numeric}}
+{{dcl header|numeric}}
-{{dcl | num=1 |
+{{dcla|num=1|constexpr=c++20|
 template< class InputIt1, class InputIt2, class T >
 T inner_product( InputIt1 first1, InputIt1 last1,
-                  InputIt2 first2, T value );
+                  InputIt2 first2, T init );
 }}
-{{dcl | num=2 |
+{{dcla|num=2|constexpr=c++20|
-template<
+template< class InputIt1, class InputIt2, class T,
-    class InputIt1,
+          class BinaryOp1, class BinaryOp2 >
-    class InputIt2,
+T inner_product( InputIt1 first1, InputIt1 last1,
-    class T,
+                 InputIt2 first2, T init,
-    class BinaryOperation1,
+                 BinaryOp1 op1, BinaryOp2 op2 );
-    class BinaryOperation2
-> T inner_product( InputIt1 first1, InputIt1 last1,
-                   InputIt2 first2, T value,
-                   BinaryOperation1 op1,
-                   BinaryOperation2 op2 );
 }}
 {{dcl end}}
-Computes inner product (i.e. sum of products) of the range {{tt|[first1, last1)}} and another range beginning at {{tt|first2}}. The first version uses {{tt|operator*}} to compute product of the element pairs and {{tt|operator+}} to sum up the products, the second version uses {{tt|op2}} and {{tt|op1}} for these tasks respectively.
+Computes inner product (i.e. sum of products) or performs ordered map/reduce operation on the range {{range|first1|last1}} and the range of {{c|std::distance(first1, last1)}} elements beginning at {{c|first2}}.
+@1@ Initializes the accumulator {{c|acc}} (of type {{tt|T}}) with the initial value {{c|init}} and then  modifies it with the expression {{rev inl|until=c++20|{{c|1=acc = acc + (*i1) * (*i2)}}}}{{rev inl|since=c++20|{{c|1=acc = std::move(acc) + (*i1) * (*i2)}}}} for every iterator {{c|i1}} in the range {{range|first1|last1}} in order and its corresponding iterator {{c|i2}} in the range beginning at {{c|first2}}. For built-in meaning of + and *, this computes inner product of the two ranges.
+@2@ Initializes the accumulator {{c|acc}} (of type {{tt|T}}) with the initial value {{c|init}} and then  modifies it with the expression {{rev inl|until=c++20|{{c|1=acc = op1(acc, op2(*i1, *i2))}}}}{{rev inl|since=c++20|{{c|1=acc = op1(std::move(acc), op2(*i1, *i2))}}}} for every iterator {{c|i1}} in the range {{range|first1|last1}} in order and its corresponding iterator {{c|i2}} in the range beginning at {{c|first2}}.
+Given {{c|last2}} as the {{c|std::distance(first1, last1)}}{{su|b=th}} next iterator of {{c|first2}}, if any of the following conditions is satisfied, the behavior is undefined:
+* {{tt|T}} is not {{named req|CopyConstructible}}.
+* {{tt|T}} is not {{named req|CopyAssignable}}.
+* {{c|op1}} or {{c|op2}} modifies any element of {{range|first1|last1}} or {{range|first2|last2}}.
+* {{c|op1}} or {{c|op2}} invalidates any iterator or subrange in {{closed range|first1|last1}} or {{closed range|first2|last2}}.
 ===Parameters===
 {{par begin}}
-{{par | first1, last1 | the first range of elements}}
+{{par|first1, last1|the first range of elements}}
-{{par | first2 | the beginning of the second range of elements}}
+{{par|first2|the beginning of the second range of elements}}
-{{par | value | initial value of the sum of the products}}
+{{par|init|initial value of the sum of the products}}
-{{par op2 | op1 | 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. | t1=T | t2=Type3 | rt=T}}
+{{par op2|op1|This "sum" function takes a value returned by {{c|op2}} and the current value of the accumulator and produces a new value to be stored in the accumulator.|t1=T|t2=Type3|rt=T}}
-{{par op2 | op2 | This "product" function takes one value from each range and produces a new value. | p1=InputIt1 | p2=InputIt2 | rt=Type3}}
+{{par op2|op2|This "product" function takes one value from each range and produces a new value.|p1=InputIt1|p2=InputIt2|rt=Type3}}
 {{par hreq}}
-{{par req concept | InputIt1, InputIt2 | InputIterator}}
+{{par req named|InputIt1, InputIt2|InputIterator}}
-{{par req concept | T | CopyConstructible | CopyAssignable}}
 {{par end}}
 ===Return value===
-The inner product of two ranges.
+{{c|acc}} after all modifications.
 ===Possible implementation===
-{{eq fun
+{{eq impl
- | 1=
+|title1=inner_product (1)|ver1=1|1=
 template<class InputIt1, class InputIt2, class T>
-T inner_product(InputIt1 first1, InputIt1 last1,
+constexpr // since C++20
-                InputIt2 first2, T value)
+T inner_product(InputIt1 first1, InputIt1 last1, InputIt2 first2, T init)
 {
-     while (first1 != last1) {
+     while (first1 != last1)
-         value = value + *first1 * *first2;
+    {
-         ++first1;
+        init = std::move(init) + (*first1) * (*first2); // std::move since C++20
-         ++first2;
+        ++first1;
+        ++first2;
      }
-     return value;
+     return init;
 }
- | 2=
+|title2=inner_product (2)|ver2=2|2=
-template<class InputIt1, class InputIt2,
+template<class InputIt1, class InputIt2, class T,
-         class T,
+          class BinaryOp1, class BinaryOp2>
-          class BinaryOperation1, class BinaryOperation2>
+constexpr // since C++20
-T inner_product(InputIt1 first1, InputIt1 last1,
+T inner_product(InputIt1 first1, InputIt1 last1, InputIt2 first2, T init,
-                InputIt2 first2, T value,
+                 BinaryOp1 op1, BinaryOp2 op2)
-                 BinaryOperation1 op1
-                BinaryOperation2 op2)
 {
-     while (first1 != last1) {
+     while (first1 != last1)
-         value = op1(value, op2(*first1, *first2));
+    {
-         ++first1;
+        init = op1(std::move(init), op2(*first1, *first2)); // std::move since C++20
-         ++first2;
+        ++first1;
+        ++first2;
      }
-     return value;
+     return init;
 }
 }}
+===Notes===
+The parallelizable version of this algorithm, {{lc|std::transform_reduce}}, requires {{c|op1}} and {{c|op2}} to be commutative and associative, but {{tt|std::inner_product}} makes no such requirement, and always performs the operations in the order given.
 ===Example===
 {{example
- |
+|
- | code=
+|code=
-#include <numeric>
+#include <functional>
 #include <iostream>
+#include <numeric>
 #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::plus<>(), std::equal_to<>());
      std::cout << "Number of pairwise matches between a and b: " <<  r2 << '\n';
 }
- | output=
+|output=
 Inner product of a and b: 21
 Number of pairwise matches between a and b: 2
 }}
+===Defect reports===
+{{dr list begin}}
+{{dr list item|wg=lwg|dr=242|std=C++98|before={{c|op1}} and {{c|op2}} could not have side effects|after=they cannot modify the ranges involved}}
+{{dr list end}}
 ===See also===
 {{dsc begin}}
-{{dsc inc | cpp/algorithm/dsc accumulate}}
+{{dsc inc|cpp/algorithm/dsc transform_reduce}}
-{{dsc inc | cpp/algorithm/dsc partial_sum}}
+{{dsc inc|cpp/algorithm/dsc accumulate}}
+{{dsc inc|cpp/algorithm/dsc partial_sum}}
 {{dsc end}}
-[[de:cpp/algorithm/inner product]]
+{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
-[[es:cpp/algorithm/inner product]]
-[[fr:cpp/algorithm/inner product]]
-[[it:cpp/algorithm/inner product]]
-[[ja:cpp/algorithm/inner product]]
-[[pt:cpp/algorithm/inner product]]
-[[ru:cpp/algorithm/inner product]]
-[[zh:cpp/algorithm/inner product]]

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Difference between revisions of "cpp/algorithm/inner product"

Latest revision as of 14:31, 26 October 2024

Contents

[edit] Parameters