Difference between revisions of "cpp/algorithm/accumulate"
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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 | + | template< class InputIt, class T, class BinaryOp > |
− | T accumulate( InputIt first, InputIt last, T init, | + | T accumulate( InputIt first, InputIt last, T init, BinaryOp op ); |
− | + | ||
}} | }} | ||
{{dcl end}} | {{dcl end}} | ||
− | Computes the sum of the given value {{ | + | 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++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. |
− | {{tt|op}} | + | |
− | | | + | @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. |
− | {{tt| | + | |
− | }} | + | 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=== | ||
{{par begin}} | {{par begin}} | ||
− | {{par | first, last | the range of elements to sum}} | + | {{par|first, last|the range of elements to sum}} |
− | {{par | init | initial value of the sum}} | + | {{par|init|initial value of the sum}} |
− | {{par op2 | op | t1=T | p2=InputIt | rt=T | + | {{par op2|op|t1=T|p2=InputIt|rt=T}} |
{{par hreq}} | {{par hreq}} | ||
− | {{par req | + | {{par req named|InputIt|InputIterator}} |
− | + | ||
{{par end}} | {{par end}} | ||
===Return value=== | ===Return value=== | ||
− | + | {{c|acc}} after all modifications. | |
− | + | ||
− | + | ||
− | + | ||
− | {{ | + | |
===Possible implementation=== | ===Possible implementation=== | ||
− | {{eq | + | {{eq impl |
− | + | |title1=accumulate (1)|ver1=1|1= | |
template<class InputIt, class T> | template<class InputIt, class T> | ||
+ | constexpr // since C++20 | ||
T accumulate(InputIt first, InputIt last, T init) | T accumulate(InputIt first, InputIt last, T init) | ||
{ | { | ||
− | for (; first != last; ++first) | + | for (; first != last; ++first) |
init = std::move(init) + *first; // std::move since C++20 | init = std::move(init) + *first; // std::move since C++20 | ||
− | + | ||
return init; | return init; | ||
} | } | ||
− | + | |title2=accumulate (2)|ver2=2|2= | |
template<class InputIt, class T, class BinaryOperation> | template<class InputIt, class T, class BinaryOperation> | ||
− | T accumulate(InputIt first, InputIt last, T init, | + | constexpr // since C++20 |
− | + | T accumulate(InputIt first, InputIt last, T init, BinaryOperation op) | |
{ | { | ||
− | for (; first != last; ++first) | + | for (; first != last; ++first) |
init = op(std::move(init), *first); // std::move since C++20 | init = op(std::move(init), *first); // std::move since C++20 | ||
− | + | ||
return init; | return init; | ||
} | } | ||
}} | }} | ||
+ | |||
+ | ===Notes=== | ||
+ | {{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>}}. | ||
===Example=== | ===Example=== | ||
{{example | {{example | ||
− | + | | | |
− | + | |code= | |
+ | #include <functional> | ||
#include <iostream> | #include <iostream> | ||
− | |||
#include <numeric> | #include <numeric> | ||
#include <string> | #include <string> | ||
− | #include < | + | #include <vector> |
− | + | ||
int main() | int main() | ||
{ | { | ||
std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; | std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; | ||
− | + | ||
int sum = std::accumulate(v.begin(), v.end(), 0); | int sum = std::accumulate(v.begin(), v.end(), 0); | ||
− | |||
int product = std::accumulate(v.begin(), v.end(), 1, std::multiplies<int>()); | 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::string s = std::accumulate(std::next(v.begin()), v.end(), | ||
std::to_string(v[0]), // start with first element | 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' | std::cout << "sum: " << sum << '\n' | ||
<< "product: " << product << '\n' | << "product: " << product << '\n' | ||
− | << "dash-separated string: " << s << '\n'; | + | << "dash-separated string: " << s << '\n' |
+ | << "dash-separated string (right-folded): " << rs << '\n'; | ||
} | } | ||
− | + | |output= | |
sum: 55 | sum: 55 | ||
product: 3628800 | product: 3628800 | ||
dash-separated string: 1-2-3-4-5-6-7-8-9-10 | 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 | ||
}} | }} | ||
+ | |||
+ | ===Defect reports=== | ||
+ | {{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 end}} | ||
===See also=== | ===See also=== | ||
{{dsc begin}} | {{dsc begin}} | ||
− | {{dsc inc | cpp/algorithm/dsc adjacent_difference}} | + | {{dsc inc|cpp/algorithm/dsc adjacent_difference}} |
− | {{dsc inc | cpp/algorithm/dsc inner_product}} | + | {{dsc inc|cpp/algorithm/dsc inner_product}} |
− | {{dsc inc | cpp/algorithm/dsc partial_sum}} | + | {{dsc inc|cpp/algorithm/dsc partial_sum}} |
− | {{dsc inc | cpp/algorithm/dsc reduce}} | + | {{dsc inc|cpp/algorithm/dsc reduce}} |
+ | {{dsc inc|cpp/algorithm/ranges/dsc fold_left}} | ||
{{dsc end}} | {{dsc end}} | ||
{{langlinks|de|es|fr|it|ja|pt|ru|zh}} | {{langlinks|de|es|fr|it|ja|pt|ru|zh}} |
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)
.
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.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:
-
T
is not CopyConstructible. -
T
is 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
#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) |