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std::inner_product

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
(C++11)
inner_product
C library
 
Defined in header <numeric>
template< class InputIt1, class InputIt2, class T >

T inner_product( InputIt1 first1, InputIt1 last1,

                 InputIt2 first2, T value );
(1)
template< class ExecutionPolicy, class InputIt1, class InputIt2, class T >

T inner_product( ExecutionPolicy&& policy, InputIt1 first1, InputIt1 last1,

                 InputIt2 first2, T value );
(2) (since C++17)
template<class InputIt1, class InputIt2, class T,

         class BinaryOperation1, class BinaryOperation2>
T inner_product( InputIt1 first1, InputIt1 last1,
                 InputIt2 first2, T value,
                 BinaryOperation1 op1,

                 BinaryOperation2 op2 );
(3)
template< class ExecutionPolicy,class InputIt1, class InputIt2, class T,

         class BinaryOperation1, class BinaryOperation2>
T inner_product( ExecutionPolicy&& policy, InputIt1 first1, InputIt1 last1,
                 InputIt2 first2, T value,
                 BinaryOperation1 op1,

                 BinaryOperation2 op2 );
(4) (since C++17)

Computes inner product (i.e. sum of products) of the range [first1, last1) and another range beginning at first2.

1) Products are calculated using operator* and sums are calculated using operator+.
3) Products are calculated using op2 and sums are calculated using op1.
2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

op1 and op2 must not have side effects.

(until C++11)

op1 and op2 must not invalidate any iterators, including the end iterators, or modify any elements of the ranges involved.

(since C++11)

Contents

[edit] Parameters

first1, last1 - the first range of elements
first2 - the beginning of the second range of elements
value - initial value of the sum of the products
policy - the execution policy to use. See execution policy for details.
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 InputIterator.
-
T must meet the requirements of CopyAssignable and CopyConstructible.

[edit] Return value

The inner product of two ranges.

[edit] Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

  • If execution of a function invoked as part of the algorithm throws an exception, std::terminate is called.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

First version
template<class InputIt1, class InputIt2, class T>
T inner_product(InputIt1 first1, InputIt1 last1,
                InputIt2 first2, T value)
{
    while (first1 != last1) {
         value = value + *first1 * *first2;
         ++first1;
         ++first2;
    }
    return value;
}
Second version
template<class InputIt1, class InputIt2,
         class T,
         class BinaryOperation1, class BinaryOperation2>
T inner_product(InputIt1 first1, InputIt1 last1,
                InputIt2 first2, T value,
                BinaryOperation1 op1
                BinaryOperation2 op2)
{
    while (first1 != last1) {
         value = op1(value, op2(*first1, *first2));
         ++first1;
         ++first2;
    }
    return value;
}

[edit] Example

#include <numeric>
#include <iostream>
#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::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] See also

sums up a range of elements
(function template) [edit]
computes the partial sum of a range of elements
(function template) [edit]
parallelized version of std::inner_product
(function template) [edit]