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std::accumulate

From cppreference.com
< cpp‎ | algorithm
 
 
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
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
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
accumulate
(C++17)
Operations on uninitialized storage
(C++17)
(C++17)
(C++17)
C library
 
Defined in header <numeric>
(1)
template< class InputIt, class T >
T accumulate( InputIt first, InputIt last, T init );
(until C++20)
template< class InputIt, class T >
constexpr T accumulate( InputIt first, InputIt last, T init );
(since C++20)
(2)
template< class InputIt, class T, class BinaryOperation >

T accumulate( InputIt first, InputIt last, T init,

              BinaryOperation op );
(until C++20)
template< class InputIt, class T, class BinaryOperation >

constexpr T accumulate( InputIt first, InputIt last, T init,

                        BinaryOperation op );
(since C++20)

Computes the sum of the given value init and the elements in the range [first, last). The first version uses operator+ to sum up the elements, the second version uses the given binary function op, both applying std::move to their operands on the left hand side (since C++20).

op must not invalidate any iterators, including the end iterators, nor modify any elements of the range involved, nor *last.

Contents

[edit] Parameters

first, last - the range of elements to sum
init - initial value of the sum
op - binary operation function object that will be applied. The binary operator takes the current accumulation value a (initialized to init) and the value of the current element b.

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 type Type1 must be such that an object of type T can be implicitly converted to Type1 . The type Type2 must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type2 . The type Ret must be such that an object of type T can be assigned a value of type Ret. ​

Type requirements
-
InputIt must meet the requirements of LegacyInputIterator.
-
T must meet the requirements of CopyAssignable and CopyConstructible.

[edit] Return value

1) The sum of the given value and elements in the given range.
2) The result of left fold of the given range over op.

[edit] Notes

std::accumulate performs a left fold. In order to perform a right fold, one must reverse the order of the arguments to the binary operator, and use reverse iterators.

[edit] Common mistakes

If left to type inference, op operates on values of the same type as init which can result in unwanted casting of the iterator elements. For example, std::accumulate(v.begin(), v.end(), 0) likely does not give the result one wishes for when v is std::vector<double>.

[edit] Possible implementation

accumulate (1)
template<class InputIt, class T>
constexpr // since C++20
T accumulate(InputIt first, InputIt last, T init)
{
    for (; first != last; ++first)
        init = std::move(init) + *first; // std::move since C++20
 
    return init;
}
accumulate (2)
template<class InputIt, class T, class BinaryOperation>
constexpr // since C++20
T accumulate(InputIt first, InputIt last, T init, BinaryOperation op)
{
    for (; first != last; ++first)
        init = op(std::move(init), *first); // std::move since C++20
 
    return init;
}

[edit] Example

#include <iostream>
#include <vector>
#include <numeric>
#include <string>
#include <functional>
 
int main()
{
    std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
 
    int sum = std::accumulate(v.begin(), v.end(), 0);
 
    int product = std::accumulate(v.begin(), v.end(), 1, std::multiplies<int>());
 
    auto dash_fold = [](std::string a, int b)
    {
        return std::move(a) + '-' + std::to_string(b);
    };
 
    std::string s = std::accumulate(std::next(v.begin()), v.end(),
                                    std::to_string(v[0]), // start with first element
                                    dash_fold);
 
    // Right fold using reverse iterators
    std::string rs = std::accumulate(std::next(v.rbegin()), v.rend(),
                                     std::to_string(v.back()), // start with last element
                                     dash_fold);
 
    std::cout << "sum: " << sum << '\n'
              << "product: " << product << '\n'
              << "dash-separated string: " << s << '\n'
              << "dash-separated string (right-folded): " << rs << '\n';
}

Output:

sum: 55
product: 3628800
dash-separated string: 1-2-3-4-5-6-7-8-9-10
dash-separated string (right-folded): 10-9-8-7-6-5-4-3-2-1

[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 op cannot have side effects it cannot modify the ranges involved

[edit] See also

computes the differences between adjacent elements in a range
(function template) [edit]
computes the inner product of two ranges of elements
(function template) [edit]
computes the partial sum of a range of elements
(function template) [edit]
(C++17)
similar to std::accumulate, except out of order
(function template) [edit]
left-folds a range of elements
(niebloid) [edit]