Difference between revisions of "cpp/algorithm/accumulate"
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(Added LWG issue #2055 DR (part 3/4).) |
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{{cpp/title|accumulate}} | {{cpp/title|accumulate}} | ||
| − | {{cpp/algorithm/navbar}} | + | {{cpp/algorithm/numeric/navbar}} |
{{dcl begin}} | {{dcl begin}} | ||
{{dcl header|numeric}} | {{dcl header|numeric}} | ||
| − | {{ | + | {{dcla|num=1|constexpr=c++20| |
| − | + | ||
template< class InputIt, class T > | template< class InputIt, class T > | ||
T accumulate( InputIt first, InputIt last, T init ); | T accumulate( InputIt first, InputIt last, T init ); | ||
| − | |||
| − | |||
| − | |||
}} | }} | ||
| − | {{ | + | {{dcla|num=2|constexpr=c++20| |
| − | + | template< class InputIt, class T, class BinaryOp > | |
| − | template< class InputIt, class T, class | + | T accumulate( InputIt first, InputIt last, T init, BinaryOp op ); |
| − | T accumulate( InputIt first, InputIt last, T init, | + | |
| − | + | ||
| − | + | ||
| − | + | ||
| − | + | ||
| − | + | ||
}} | }} | ||
{{dcl end}} | {{dcl end}} | ||
| Line 25: | Line 15: | ||
Computes the sum of the given value {{c|init}} and the elements in the range {{range|first|last}}. | Computes the sum of the given value {{c|init}} and the elements in the range {{range|first|last}}. | ||
| − | @1@ Initializes the accumulator {{c|acc}} (of type {{tt|T}}) with the initial value {{c|init}} and then modifies it with {{rev inl|until=c++ | + | @1@ Initializes the accumulator {{c|acc}} (of type {{tt|T}}) with the initial value {{c|init}} and then modifies it with {{rev inl|until=c++20|{{c|1=acc = acc + *i}}}}{{rev inl|since=c++20|{{c|1=acc = std::move(acc) + *i}}}} for every iterator {{c|i}} in the range {{range|first|last}} in order. |
| − | @2@ Initializes the accumulator {{c|acc}} (of type {{tt|T}}) with the initial value {{c|init}} and then modifies it with {{rev inl|until=c++ | + | @2@ Initializes the accumulator {{c|acc}} (of type {{tt|T}}) with the initial value {{c|init}} and then modifies it with {{rev inl|until=c++20|{{c|1=acc = op(acc, *i)}}}}{{rev inl|since=c++20|{{c|1=acc = op(std::move(acc), *i)}}}} for every iterator {{c|i}} in the range {{range|first|last}} in order. |
| − | + | 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|op}} modifies any element of {{range|first|last}}. | ||
| + | * {{c|op}} invalidates any iterator or subrange in {{closed range|first|last}}. | ||
===Parameters=== | ===Parameters=== | ||
| Line 38: | Line 32: | ||
{{par hreq}} | {{par hreq}} | ||
{{par req named|InputIt|InputIterator}} | {{par req named|InputIt|InputIterator}} | ||
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{{par end}} | {{par end}} | ||
| Line 52: | Line 45: | ||
{ | { | ||
for (; first != last; ++first) | for (; first != last; ++first) | ||
| − | init = std::move(init) + *first; // std::move since C++ | + | init = std::move(init) + *first; // std::move since C++20 |
return init; | return init; | ||
| Line 62: | Line 55: | ||
{ | { | ||
for (; first != last; ++first) | for (; first != last; ++first) | ||
| − | init = op(std::move(init), *first); // std::move since C++ | + | init = op(std::move(init), *first); // std::move since C++20 |
return init; | return init; | ||
| Line 71: | Line 64: | ||
{{tt|std::accumulate}} performs a left [[enwiki:Fold (higher-order function)|fold]]. In order to perform a right fold, one must reverse the order of the arguments to the binary operator, and use reverse iterators. | {{tt|std::accumulate}} performs a left [[enwiki:Fold (higher-order function)|fold]]. In order to perform a right fold, one must reverse the order of the arguments to the binary operator, and use reverse iterators. | ||
| − | + | If left to type inference, {{c|op}} operates on values of the same type as {{c|init}} which can result in unwanted casting of the iterator elements. For example, {{c|std::accumulate(v.begin(), v.end(), 0)}} likely does not give the result one wishes for when {{c|v}} is of type {{c/core|std::vector<double>}}. | |
| − | If left to type inference, {{c|op}} operates on values of the same type as {{c|init}} which can result in unwanted casting of the iterator elements. For example, {{c|std::accumulate(v.begin(), v.end(), 0)}} likely does not give the result one wishes for when {{c|v}} is {{c/core|std::vector<double>}}. | + | |
===Example=== | ===Example=== | ||
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| | | | ||
|code= | |code= | ||
| + | #include <functional> | ||
#include <iostream> | #include <iostream> | ||
| − | |||
#include <numeric> | #include <numeric> | ||
#include <string> | #include <string> | ||
| − | #include < | + | #include <vector> |
int main() | int main() | ||
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{{dr list begin}} | {{dr list begin}} | ||
{{dr list item|wg=lwg|dr=242|std=C++98|before={{c|op}} could not have side effects|after=it cannot modify the ranges involved}} | {{dr list item|wg=lwg|dr=242|std=C++98|before={{c|op}} could not have side effects|after=it cannot modify the ranges involved}} | ||
| − | |||
{{dr list end}} | {{dr list end}} | ||
Latest revision as of 14:33, 15 September 2024
| Defined in header <numeric>
|
||
template< class InputIt, class T > T accumulate( InputIt first, InputIt last, T init ); |
(1) | (constexpr since C++20) |
template< class InputIt, class T, class BinaryOp > T accumulate( InputIt first, InputIt last, T init, BinaryOp op ); |
(2) | (constexpr since C++20) |
Computes the sum of the given value init and the elements in the range [first, last).
1) Initializes the accumulator acc (of type
T) with the initial value init and then modifies it with acc = acc + *i(until C++20)acc = std::move(acc) + *i(since C++20) for every iterator i in the range [first, last) in order.2) Initializes the accumulator acc (of type
T) with the initial value init and then modifies it with acc = op(acc, *i)(until C++20)acc = op(std::move(acc), *i)(since C++20) for every iterator i in the range [first, last) in order.If any of the following conditions is satisfied, the behavior is undefined:
-
Tis not CopyConstructible. -
Tis not CopyAssignable. - op modifies any element of
[first,last). - op invalidates any iterator or subrange in
[first,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 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 | ||
-InputIt must meet the requirements of LegacyInputIterator.
| ||
[edit] Return value
acc after all modifications.
[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] 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.
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 of type std::vector<double>.
[edit] Example
Run this code
#include <functional> #include <iostream> #include <numeric> #include <string> #include <vector> 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 could not have side effects | it cannot modify the ranges involved |
[edit] See also
| computes the differences between adjacent elements in a range (function template) | |
| computes the inner product of two ranges of elements (function template) | |
| computes the partial sum of a range of elements (function template) | |
| (C++17) |
similar to std::accumulate, except out of order (function template) |
| (C++23) |
left-folds a range of elements (niebloid) |
