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

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< cpp‎ | algorithm
(Add "sum" and "product" labels to the functor types to help make the text clearer.)
(restrictions on the functor: we only had them in std::transform, but they are for 5 functions (LWG 242 was the C++11 change))
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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) 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.
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 +
{{rev begin}}
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{{rev | until=c++11 |
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{{tt|op1}} and {{tt|op2}} must not have side effects.
 +
}}
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{{rev | since=c++11 |
 +
{{tt|op1}} and {{tt|op2}} must not invalidate any iterators, including the end iterators, or modify any elements of the ranges involved.
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}}
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{{rev end}}
  
 
===Parameters===
 
===Parameters===

Revision as of 06:38, 21 November 2013

 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
(C++11)
inner_product
  
Operations on uninitialized memory
 
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 InputIt1,
    class InputIt2,
    class T,
    class BinaryOperation1,
    class BinaryOperation2
> T inner_product( InputIt1 first1, InputIt1 last1,
                   InputIt2 first2, T value,
                   BinaryOperation1 op1,

                   BinaryOperation2 op2 );
 (2)

Computes inner product (i.e. sum of products) of the range [first1, last1) and another range beginning at first2. The first version uses operator* to compute product of the element pairs and operator+ to sum up the products, the second version uses op2 and op1 for these tasks respectively.

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

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
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

Template:par req concept Template:par req concept

Return value

The inner product of two ranges.

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;
}

Example

Run this code
#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

See also

 sums up or folds a range of elements
(function template) [edit]
computes the partial sum of a range of elements
(function template) [edit]
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