Difference between revisions of "cpp/algorithm/accumulate"

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:

T is not CopyConstructible.
T is not CopyAssignable.
op modifies any element of [first, last).
op invalidates any iterator or subrange in [first, last].

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 &. 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.

[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

adjacent_difference	computes the differences between adjacent elements in a range (function template) [edit]
inner_product	computes the inner product of two ranges of elements (function template) [edit]
partial_sum	computes the partial sum of a range of elements (function template) [edit]
reduce (C++17)	similar to std::accumulate, except out of order (function template) [edit]
ranges::fold_left (C++23)	left-folds a range of elements (niebloid)[edit]

@@ Line 1: / Line 1: @@
 {{cpp/title|accumulate}}
-{{cpp/algorithm/navbar}}
+{{cpp/algorithm/numeric/navbar}}
 {{dcl begin}}
-{{dcl header | numeric}}
+{{dcl header|numeric}}
-{{dcl | num=1 |
+{{dcla|num=1|constexpr=c++20|
 template< class InputIt, class T >
 T accumulate( InputIt first, InputIt last, T init );
 }}
-{{dcl | num=2 |
+{{dcla|num=2|constexpr=c++20|
-template< class InputIt, class T, class BinaryOperation >
+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 );
-              BinaryOperation op );
 }}
 {{dcl end}}
-Computes the sum of the given value {{tt|init}} and the elements in the range {{tt|[first, last)}}. The first version uses {{tt|operator+}} to sum up the elements, the second version uses the given binary function {{tt|op}}.
+Computes the sum of the given value {{c|init}} and the elements in the range {{range|first|last}}.
-{{rev begin}}
+@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.
-{{rev | until=c++11 |
-{{tt|op}} must not have side effects.
+@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.
-}}
-{{rev | since=c++11 |
+If any of the following conditions is satisfied, the behavior is undefined:
-{{tt|op}} must not invalidate any iterators, including the end iterators, or modify any elements of the range involved.
+* {{tt|T}} is not {{named req|CopyConstructible}}.
-}}
+* {{tt|T}} is not {{named req|CopyAssignable}}.
-{{rev end}}
+* {{c|op}} modifies any element of {{range|first|last}}.
+* {{c|op}} invalidates any iterator or subrange in {{closed range|first|last}}.
 ===Parameters===
 {{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 req concept | InputIt | InputIterator}}
+{{par req named|InputIt|InputIterator}}
-{{par req concept | T | CopyConstructible | CopyAssignable}}
 {{par end}}
 ===Return value===
-@1@ The sum of the given value and elements in the given range.
+{{c|acc}} after all modifications.
-@2@ The result of [[enwiki:Fold (higher-order function)|left fold]] of the given range over {{tt|op}}
-===Notes===
-Although {{tt|std::accumulate}} performs left fold by default, right fold may be achieved by using reverse iterators, e.g. {{c|std::accumulate(v.rbegin(), v.rend(), init, binop)}}
 ===Possible implementation===
-{{eq fun
+{{eq impl
- | 1=
+|title1=accumulate (1)|ver1=1|1=
 template<class InputIt, class T>
+constexpr // since C++20
 T accumulate(InputIt first, InputIt last, T init)
 {
-     for (; first != last; ++first) {
+     for (; first != last; ++first)
-         init = init + *first;
+         init = std::move(init) + *first; // std::move since C++20
-     }
      return init;
 }
- | 2=
+|title2=accumulate (2)|ver2=2|2=
 template<class InputIt, class T, class BinaryOperation>
-T accumulate(InputIt first, InputIt last, T init,
+constexpr // since C++20
-             BinaryOperation op)
+T accumulate(InputIt first, InputIt last, T init, BinaryOperation op)
 {
-     for (; first != last; ++first) {
+     for (; first != last; ++first)
-         init = op(init, *first);
+         init = op(std::move(init), *first); // std::move since C++20
-     }
      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
- |
+|
- | code=
+|code=
+#include <functional>
 #include <iostream>
-#include <vector>
 #include <numeric>
 #include <string>
-#include <functional>
+#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>());
-     std::string s = std::accumulate(v.begin()+1, v.end(), std::to_string(v[0]),
+    auto dash_fold = [](std::string a, int b)
-                         [](const std::string& a, int b) {
+    {
-                               return a + '-' + std::to_string(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: " << s << '\n'
+              << "dash-separated string (right-folded): " << rs << '\n';
 }
- | output=
+|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
 }}
+===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===
 {{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/ranges/dsc fold_left}}
 {{dsc end}}
-[[de:cpp/algorithm/accumulate]]
+{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
-[[es:cpp/algorithm/accumulate]]
-[[fr:cpp/algorithm/accumulate]]
-[[it:cpp/algorithm/accumulate]]
-[[ja:cpp/algorithm/accumulate]]
-[[pt:cpp/algorithm/accumulate]]
-[[ru:cpp/algorithm/accumulate]]
-[[zh:cpp/algorithm/accumulate]]

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Difference between revisions of "cpp/algorithm/accumulate"

Latest revision as of 14:33, 15 September 2024

Contents

[edit] Parameters