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Difference between revisions of "cpp/algorithm/ranges/fold left"

From cppreference.com
< cpp‎ | algorithm‎ | ranges
m (rm internal html comments)
m (→‎Notes: +'l')
 
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{{dcl header|algorithm}}
 
{{dcl header|algorithm}}
 
{{dcl h|Call signature}}
 
{{dcl h|Call signature}}
−
{{dcl|num=1|since=c++23|1=
+
{{dcl rev begin|num=1}}
−
template<
+
{{dcla|anchor=1|since=c++23|until=c++26|1=
−
  std::input_iterator I, std::sentinel_for<I> S, class T,
+
template< std::input_iterator I, std::sentinel_for<I> S, class T,
−
  __indirectly_binary_left_foldable<T, I> F
+
          /* indirectly-binary-left-foldable */<T, I> F >
−
>
+
 
constexpr auto fold_left( I first, S last, T init, F f );
 
constexpr auto fold_left( I first, S last, T init, F f );
 
}}
 
}}
−
{{dcl|num=2|since=c++23|1=
+
{{dcl|since=c++26|1=
−
template<
+
template< std::input_iterator I, std::sentinel_for<I> S,
−
  ranges::input_range R, class T,
+
          class T = std::iter_value_t<I>,
−
  __indirectly_binary_left_foldable<T, ranges::iterator_t<R>> F
+
          /* indirectly-binary-left-foldable */<T, I> F >
−
>
+
constexpr auto fold_left( I first, S last, T init, F f );
 +
}}
 +
{{dcl rev end}}
 +
{{dcl rev begin|num=2}}
 +
{{dcl|since=c++23|until=c++26|1=
 +
template< ranges::input_range R, class T,
 +
          /* indirectly-binary-left-foldable */
 +
              <T, ranges::iterator_t<R>> F >
 +
constexpr auto fold_left( R&& r, T init, F f );
 +
}}
 +
{{dcl|since=c++26|1=
 +
template< ranges::input_range R, class T = ranges::range_value_t<R>,
 +
          /* indirectly-binary-left-foldable */
 +
              <T, ranges::iterator_t<R>> F >
 
constexpr auto fold_left( R&& r, T init, F f );
 
constexpr auto fold_left( R&& r, T init, F f );
 
}}
 
}}
 +
{{dcl rev end}}
 
{{dcl h|Helper concepts}}
 
{{dcl h|Helper concepts}}
−
{{dcl|num=3|since=c++23|1=
+
{{dcl|num=3|notes={{mark expos}}|1=
 
template< class F, class T, class I >
 
template< class F, class T, class I >
−
  concept __indirectly_binary_left_foldable =    // exposition only
+
concept /* indirectly-binary-left-foldable */ = /* see description */;
−
    /* see description */;
+
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
−
Left-[[enwiki:Fold (higher-order function)|folds]] the elements of given range, that is, returns the result of evaluation of the chain expression:<br>{{tt|f(f(f(f(init, x{{sub|1}}), x{{sub|2}}), ...), x{{sub|n}})}}, where {{tt|x{{sub|1}}}}, {{tt|x{{sub|2}}}}, ..., {{tt|x{{sub|n}}}} are elements of the range.
+
Left-{{enwiki|Fold (higher-order function)|folds}} the elements of given range, that is, returns the result of evaluation of the chain expression:<br>{{tt|f(f(f(f(init, x{{sub|1}}), x{{sub|2}}), ...), x{{sub|n}})}}, where {{tt|x{{sub|1}}}}, {{tt|x{{sub|2}}}}, ..., {{tt|x{{sub|n}}}} are elements of the range.
  
 
Informally, {{tt|ranges::fold_left}} behaves like {{lc|std::accumulate}}'s overload that accepts a binary predicate.
 
Informally, {{tt|ranges::fold_left}} behaves like {{lc|std::accumulate}}'s overload that accepts a binary predicate.
  
−
The behavior is undefined if {{tt|[first, last)}} is not a valid range.
+
The behavior is undefined if {{range|first|last}} is not a valid range.
  
−
@1@ The range is {{tt|[first, last)}}. Equivalent to
+
@1@ The range is {{range|first|last}}. Equivalent to
 
{{c|return ranges::fold_left_with_iter(std::move(first), last, std::move(init), f).value}}.
 
{{c|return ranges::fold_left_with_iter(std::move(first), last, std::move(init), f).value}}.
  
−
@2@ Same as {{v|1}}, except that uses {{tt|r}} as the range, as if by using {{c|ranges::begin(r)}} as {{tt|first}} and {{c|ranges::end(r)}} as {{tt|last}}.
+
@2@ Same as {{v|1}}, except that uses {{c|r}} as the range, as if by using {{c|ranges::begin(r)}} as {{c|first}} and {{c|ranges::end(r)}} as {{c|last}}.
  
 
{{ranges_fold_algos_helper_concepts}}
 
{{ranges_fold_algos_helper_concepts}}
Line 49: Line 61:
  
 
===Return value===
 
===Return value===
−
An object of type {{c|U}} that contains the result of left-[[enwiki:Fold (higher-order function)|fold]] of the given range over {{c|f}}, where {{c|U}} is equivalent to {{c|std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>}}.
+
An object of type {{c|U}} that contains the result of left-{{enwiki|Fold (higher-order function)|fold}} of the given range over {{c|f}}, where {{c|U}} is equivalent to {{c|std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>}}.
  
 
If the range is empty, {{c|U(std::move(init))}} is returned.
 
If the range is empty, {{c|U(std::move(init))}} is returned.
  
 
===Possible implementations===
 
===Possible implementations===
−
{{eq fun
+
{{eq fun|1=
−
|1=
+
 
struct fold_left_fn
 
struct fold_left_fn
 
{
 
{
−
     template<
+
     template<std::input_iterator I, std::sentinel_for<I> S, class T = std::iter_value_t<I>,
−
        std::input_iterator I, std::sentinel_for<I> S, class T,
+
            /* indirectly-binary-left-foldable */<T, I> F>
−
        __indirectly_binary_left_foldable<T, I> F
+
     constexpr auto operator()(I first, S last, T init, F f) const
−
    >
+
−
     constexpr auto operator()( I first, S last, T init, F f ) const
+
 
     {
 
     {
 
         using U = std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>;
 
         using U = std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>;
Line 72: Line 81:
 
         return std::move(accum);
 
         return std::move(accum);
 
     }
 
     }
−
     template<
+
   
−
        ranges::input_range R, class T,
+
     template<ranges::input_range R, class T = ranges::range_value_t<R>,
−
        __indirectly_binary_left_foldable<T, ranges::iterator_t<R>> F
+
            /* indirectly-binary-left-foldable */<T, ranges::iterator_t<R>> F>
−
    >
+
     constexpr auto operator()(R&& r, T init, F f) const
−
     constexpr auto operator()( R&& r, T init, F f ) const
+
 
     {
 
     {
 
         return (*this)(ranges::begin(r), ranges::end(r), std::move(init), std::ref(f));
 
         return (*this)(ranges::begin(r), ranges::end(r), std::move(init), std::ref(f));
Line 91: Line 99:
 
{{ranges_fold_algos_table}}
 
{{ranges_fold_algos_table}}
  
−
 
+
{{ftm begin}}
−
{{feature test macro|__cpp_lib_ranges_fold|std=C++23|value=202207L|{{tt|std::ranges}} fold algorithms}}
+
{{ftm|__cpp_lib_ranges_fold|std=C++23|value=202207L|{{tt|std::ranges}} [[cpp/algorithm/ranges#Constrained fold operations|fold algorithms]]}}
 +
{{ftm|__cpp_lib_algorithm_default_value_type|value=202403L|std=C++26|[[cpp/language/list initialization|List-initialization]] for algorithms {{vl|1,2}}}}
 +
{{ftm end}}
  
 
===Example===
 
===Example===
−
{{example|code=
+
{{example
 +
|code=
 
#include <algorithm>
 
#include <algorithm>
 +
#include <complex>
 
#include <functional>
 
#include <functional>
 
#include <iostream>
 
#include <iostream>
 
#include <ranges>
 
#include <ranges>
 
#include <string>
 
#include <string>
 +
#include <utility>
 
#include <vector>
 
#include <vector>
  
 
int main()
 
int main()
 
{
 
{
−
     std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8};
+
     namespace ranges = std::ranges;
  
−
     int sum = std::ranges::fold_left(v.begin(), v.end(), 0, std::plus<int>()); // (1)
+
     std::vector v{1, 2, 3, 4, 5, 6, 7, 8};
 +
   
 +
    int sum = ranges::fold_left(v.begin(), v.end(), 0, std::plus<int>()); // (1)
 
     std::cout << "sum: " << sum << '\n';
 
     std::cout << "sum: " << sum << '\n';
−
 
+
   
−
     int mul = std::ranges::fold_left(v, 1, std::multiplies<int>()); // (2)
+
     int mul = ranges::fold_left(v, 1, std::multiplies<int>()); // (2)
 
     std::cout << "mul: " << mul << '\n';
 
     std::cout << "mul: " << mul << '\n';
−
 
+
   
 
     // get the product of the std::pair::second of all pairs in the vector:
 
     // get the product of the std::pair::second of all pairs in the vector:
−
     std::vector<std::pair<char, float>> data = {{'A', 2.f}, {'B', 3.f}, {'C', 3.5f}};
+
     std::vector<std::pair<char, float>> data {<!---->{'A', 2.f}, {'B', 3.f}, {'C', 3.5f}<!---->};
−
     float sec = std::ranges::fold_left
+
     float sec = ranges::fold_left
 
     (
 
     (
−
         data {{!}} std::ranges::views::values, 2.0f, std::multiplies<>()
+
         data {{!}} ranges::views::values, 2.0f, std::multiplies<>()
 
     );
 
     );
 
     std::cout << "sec: " << sec << '\n';
 
     std::cout << "sec: " << sec << '\n';
−
 
+
   
 
     // use a program defined function object (lambda-expression):
 
     // use a program defined function object (lambda-expression):
−
     std::string str = std::ranges::fold_left
+
     std::string str = ranges::fold_left
 
     (
 
     (
 
         v, "A", [](std::string s, int x) { return s + ':' + std::to_string(x); }
 
         v, "A", [](std::string s, int x) { return s + ':' + std::to_string(x); }
 
     );
 
     );
 
     std::cout << "str: " << str << '\n';
 
     std::cout << "str: " << str << '\n';
 +
 +
    using CD = std::complex<double>;
 +
    std::vector<CD> nums{<!---->{1, 1}, {2, 0}, {3, 0}<!---->};
 +
    #ifdef __cpp_lib_algorithm_default_value_type
 +
        auto res = ranges::fold_left(nums, {7, 0}, std::multiplies{}); // (2)
 +
    #else
 +
        auto res = ranges::fold_left(nums, CD{7, 0}, std::multiplies{}); // (2)
 +
    #endif
 +
    std::cout << "res: " << res << '\n';
 
}
 
}
 
|output=
 
|output=
Line 133: Line 157:
 
sec: 42
 
sec: 42
 
str: A:1:2:3:4:5:6:7:8
 
str: A:1:2:3:4:5:6:7:8
 +
res: (42,42)
 
}}
 
}}
 +
 +
===References===
 +
{{ref std c++23}}
 +
{{ref std|title=Fold|id=alg.fold|section=27.6.18}}
 +
{{ref std end}}
  
 
===See also===
 
===See also===

Latest revision as of 02:32, 16 September 2024

 
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
Operations on uninitialized memory
 
Constrained algorithms
All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations (on sorted ranges)
       
       
    
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
       
       
Permutation operations
Fold operations
fold_left
(C++23)
(C++23)  
(C++23)
(C++23)  
Numeric operations
(C++23)            
Operations on uninitialized storage
Return types
 
Defined in header <algorithm>
Call signature
(1)
template< std::input_iterator I, std::sentinel_for<I> S, class T,

          /* indirectly-binary-left-foldable */<T, I> F >

constexpr auto fold_left( I first, S last, T init, F f );
 (since C++23)
(until C++26)
template< std::input_iterator I, std::sentinel_for<I> S,

          class T = std::iter_value_t<I>,
          /* indirectly-binary-left-foldable */<T, I> F >

constexpr auto fold_left( I first, S last, T init, F f );
 (since C++26)
(2)
template< ranges::input_range R, class T,

          /* indirectly-binary-left-foldable */
              <T, ranges::iterator_t<R>> F >

constexpr auto fold_left( R&& r, T init, F f );
 (since C++23)
(until C++26)
template< ranges::input_range R, class T = ranges::range_value_t<R>,

          /* indirectly-binary-left-foldable */
              <T, ranges::iterator_t<R>> F >

constexpr auto fold_left( R&& r, T init, F f );
 (since C++26)
Helper concepts
template< class F, class T, class I >
concept /* indirectly-binary-left-foldable */ = /* see description */;
 (3) (exposition only*)

Left-folds the elements of given range, that is, returns the result of evaluation of the chain expression:
f(f(f(f(init, x1), x2), ...), xn), where x1, x2, ..., xn are elements of the range.

Informally, ranges::fold_left behaves like std::accumulate's overload that accepts a binary predicate.

The behavior is undefined if [first, last) is not a valid range.

1) The range is [first, last). Equivalent to return ranges::fold_left_with_iter(std::move(first), last, std::move(init), f).value.
2) Same as (1), except that uses r as the range, as if by using ranges::begin(r) as first and ranges::end(r) as last.
3) Equivalent to:
Helper concepts
template< class F, class T, class I, class U >

concept /*indirectly-binary-left-foldable-impl*/ =
    std::movable<T> &&
    std::movable<U> &&
    std::convertible_to<T, U> &&
    std::invocable<F&, U, std::iter_reference_t<I>> &&
    std::assignable_from<U&,

        std::invoke_result_t<F&, U, std::iter_reference_t<I>>>;
 (3A) (exposition only*)
template< class F, class T, class I >

concept /*indirectly-binary-left-foldable*/ =
    std::copy_constructible<F> &&
    std::indirectly_readable<I> &&
    std::invocable<F&, T, std::iter_reference_t<I>> &&
    std::convertible_to<std::invoke_result_t<F&, T, std::iter_reference_t<I>>,
        std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>> &&
    /*indirectly-binary-left-foldable-impl*/<F, T, I,

        std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>>;
 (3B) (exposition only*)

The function-like entities described on this page are niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] Parameters

first, last - the range of elements to fold
r - the range of elements to fold
init - the initial value of the fold
f - the binary function object

[edit] Return value

An object of type U that contains the result of left-fold of the given range over f, where U is equivalent to std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>.

If the range is empty, U(std::move(init)) is returned.

[edit] Possible implementations

struct fold_left_fn
{
    template<std::input_iterator I, std::sentinel_for<I> S, class T = std::iter_value_t<I>,
             /* indirectly-binary-left-foldable */<T, I> F>
    constexpr auto operator()(I first, S last, T init, F f) const
    {
        using U = std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>;
        if (first == last)
            return U(std::move(init));
        U accum = std::invoke(f, std::move(init), *first);
        for (++first; first != last; ++first)
            accum = std::invoke(f, std::move(accum), *first);
        return std::move(accum);
    }
 
    template<ranges::input_range R, class T = ranges::range_value_t<R>,
             /* indirectly-binary-left-foldable */<T, ranges::iterator_t<R>> F>
    constexpr auto operator()(R&& r, T init, F f) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(init), std::ref(f));
    }
};
 
inline constexpr fold_left_fn fold_left;

[edit] Complexity

Exactly ranges::distance(first, last) applications of the function object f.

[edit] Notes

The following table compares all constrained folding algorithms:

Fold function template Starts from Initial value Return type
ranges::fold_left left init U
ranges::fold_left_first left first element std::optional<U>
ranges::fold_right right init U
ranges::fold_right_last right last element std::optional<U>
ranges::fold_left_with_iter left init

(1) ranges::in_value_result<I, U>

(2) ranges::in_value_result<BR, U>,

where BR is ranges::borrowed_iterator_t<R>

ranges::fold_left_first_with_iter left first element

(1) ranges::in_value_result<I, std::optional<U>>

(2) ranges::in_value_result<BR, std::optional<U>>

where BR is ranges::borrowed_iterator_t<R>

Feature-test macro Value Std Feature
__cpp_lib_ranges_fold 202207L (C++23) std::ranges fold algorithms
__cpp_lib_algorithm_default_value_type 202403L (C++26) List-initialization for algorithms (1,2)

[edit] Example

Run this code
#include <algorithm>
#include <complex>
#include <functional>
#include <iostream>
#include <ranges>
#include <string>
#include <utility>
#include <vector>
 
int main()
{
    namespace ranges = std::ranges;
 
    std::vector v{1, 2, 3, 4, 5, 6, 7, 8};
 
    int sum = ranges::fold_left(v.begin(), v.end(), 0, std::plus<int>()); // (1)
    std::cout << "sum: " << sum << '\n';
 
    int mul = ranges::fold_left(v, 1, std::multiplies<int>()); // (2)
    std::cout << "mul: " << mul << '\n';
 
    // get the product of the std::pair::second of all pairs in the vector:
    std::vector<std::pair<char, float>> data {{'A', 2.f}, {'B', 3.f}, {'C', 3.5f}};
    float sec = ranges::fold_left
    (
        data | ranges::views::values, 2.0f, std::multiplies<>()
    );
    std::cout << "sec: " << sec << '\n';
 
    // use a program defined function object (lambda-expression):
    std::string str = ranges::fold_left
    (
        v, "A", [](std::string s, int x) { return s + ':' + std::to_string(x); }
    );
    std::cout << "str: " << str << '\n';
 
    using CD = std::complex<double>;
    std::vector<CD> nums{{1, 1}, {2, 0}, {3, 0}};
    #ifdef __cpp_lib_algorithm_default_value_type
        auto res = ranges::fold_left(nums, {7, 0}, std::multiplies{}); // (2)
    #else
        auto res = ranges::fold_left(nums, CD{7, 0}, std::multiplies{}); // (2)
    #endif
    std::cout << "res: " << res << '\n';
}

Output: 

sum: 36
mul: 40320
sec: 42
str: A:1:2:3:4:5:6:7:8
res: (42,42)

[edit] References

  • C++23 standard (ISO/IEC 14882:2024):
  • 27.6.18 Fold [alg.fold]

[edit] See also

(C++23)
 left-folds a range of elements using the first element as an initial value
(niebloid)[edit]
(C++23)
 right-folds a range of elements
(niebloid)[edit]
(C++23)
 right-folds a range of elements using the last element as an initial value
(niebloid)[edit]
(C++23)
 left-folds a range of elements, and returns a pair (iterator, value)
(niebloid)[edit]
(C++23)
 left-folds a range of elements using the first element as an initial value, and returns a pair (iterator, optional)
(niebloid)[edit]
sums up or folds a range of elements
(function template) [edit]
(C++17)
 similar to std::accumulate, except out of order
(function template) [edit]
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