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std::ranges::transform, std::ranges::unary_transform_result, std::ranges::binary_transform_result

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
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
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
 
Defined in header <algorithm>
Call signature
template< std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O,

          std::copy_constructible F, class Proj = std::identity >
requires std::indirectly_writable<O,
                                  std::indirect_result_t<F&, std::projected<I, Proj>>>
constexpr ranges::unary_transform_result<I, O>

  transform( I first1, S last1, O result, F op, Proj proj = {} );
(1) (since C++20)
template< ranges::input_range R, std::weakly_incrementable O,

          std::copy_constructible F, class Proj = std::identity >
requires std::indirectly_writable<
             O,
             std::indirect_result_t<F&, std::projected<ranges::iterator_t<R>, Proj>>>
constexpr ranges::unary_transform_result<ranges::borrowed_iterator_t<R>, O>

  transform( R&& r, O result, F op, Proj proj = {} );
(2) (since C++20)
template< std::input_iterator I1, std::sentinel_for<I1> S1,

          std::input_iterator I2, std::sentinel_for<I2> S2,
          std::weakly_incrementable O,
          std::copy_constructible F,
          class Proj1 = std::identity, class Proj2 = std::identity >
requires std::indirectly_writable<
             O,
             std::indirect_result_t<F&,
                                    std::projected<I1, Proj1>,
                                    std::projected<I2, Proj2>>>
constexpr ranges::binary_transform_result<I1, I2, O>
  transform( I1 first1, S1 last1, I2 first2, S2 last2, O result,

             F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {} );
(3) (since C++20)
template< std::input_range R1,

          std::input_range R2,
          std::weakly_incrementable O,
          std::copy_constructible F,
          class Proj1 = std::identity, class Proj2 = std::identity >
requires std::indirectly_writable<
             O,
             std::indirect_result_t<
                 F&,
                 std::projected<ranges::iterator_t<R1>, Proj>,
                 std::projected<ranges::iterator_t<R2>, Proj>>>
constexpr ranges::binary_transform_result<ranges::borrowed_iterator_t<R1>,
                                          ranges::borrowed_iterator_t<R2>, O>

  transform( R1&& r1, R2&& r2, O result, F binary_op, Proj proj = {} );
(4) (since C++20)
Helper types
template < class I, class O >
using unary_transform_result = ranges::in_out_result<I, O>;
(5) (since C++20)
template < class I1, class I2, class O >
using binary_transform_result = ranges::in_in_out_result<I1, I2, O>;
(6) (since C++20)

ranges::transform applies the given function to a range and stores the result in another range, beginning at result.

1) The unary operation op is applied to the range defined by [first1, last1) (after projecting with the projection proj).
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.
3) The binary operation binary_op is applied to pairs of elements from two ranges: one defined by [first1, last1) and the other defined by [first2, last2) (after respectively projecting with the projections proj1 and proj2).
4) Same as (3), but uses r1 as the first source range, as if using ranges::begin(r1) as first1 and ranges::end(r1) as last1, and similarly for r2.

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

first1, last1 - the first range of elements to transform
r, r1 - the first range of elements to transform
first2, last2 - the second range of elements to transform
r2 - the second range of elements to transform
result - the beginning of the destination range, may be equal to first1 or first2
op, binary_op - operation to apply to the projected element(s)
proj1 - projection to apply to the elements in the first range
proj2 - projection to apply to the elements in the second range

[edit] Return value

1-2) a unary_transform_result contains an input iterator equal to last and an output iterator to the element past the last element transformed.
3-4) a binary_transform_result contains input iterators to last transformed elements from ranges [first1, last1) and [first2, last2) as in1 and in2 respectively, and the output iterator to the element past the last element transformed as out.

[edit] Complexity

1-2) Exactly last1 - first1 applications of op and proj.
3-4) Exactly min(last1 - first1, last2 - first2) applications of binary_op and projections.

[edit] Possible implementation

struct transform_fn {
  template< std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O,
            std::copy_constructible F, class Proj = std::identity >
  requires std::indirectly_writable<O, std::indirect_result_t<F&, std::projected<I, Proj>>>
  constexpr ranges::unary_transform_result<I, O>
    operator()( I first1, S last1, O result, F op, Proj proj = {} ) const
  {
      for (; first1 != last1; ++first1, (void)++result) {
          *result = std::invoke(op, std::invoke(proj, *first1));
      }
 
      return {first1, result};
  }
 
  template< ranges::input_range R, std::weakly_incrementable O,
            std::copy_constructible F, class Proj = std::identity >
  requires std::indirectly_writable<
               O,
               std::indirect_result_t<F&, std::projected<ranges::iterator_t<R>, Proj>>>
  constexpr ranges::unary_transform_result<ranges::borrowed_iterator_t<R>, O>
    operator()( R&& r, O result, F op, Proj proj = {} ) const
  {
    return (*this)(ranges::begin(r), ranges::end(r), result, std::ref(proj));
  }
 
  template< std::input_iterator I1, std::sentinel_for<I1> S1,
            std::input_iterator I2, std::sentinel_for<I2> S2,
            std::weakly_incrementable O,
            std::copy_constructible F,
            class Proj1 = std::identity, class Proj2 = std::identity >
  requires std::indirectly_writable<
               O,
               std::indirect_result_t<F&,
                                      std::projected<I1, Proj1>,
                                      std::projected<I2, Proj2>>>
  constexpr ranges::binary_transform_result<I1, I2, O>
    operator()( I1 first1, S1 last1, I2 first2, S2 last2, O result,
                F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {} ) const
  {
    for (; first1 != last1 && first2 != last2; ++first1, (void)++first2, (void)++result) {
      *result = std::invoke(binary_op,
                            std::invoke(proj1, *first1),
                            std::invoke(proj2, *first2));
    }
 
    return {first1, first2, result};
  }
 
  template< ranges::input_range R1, ranges::input_range R2,
            std::weakly_incrementable O, std::copy_constructible F,
            class Proj1 = std::identity, class Proj2 = std::identity >
  requires std::indirectly_writable<
               O,
               std::projected<ranges::iterator_t<R1>, Proj1>,
               std::projected<ranges::iterator_t<R2>, Proj2>>>
  constexpr ranges::binary_transform_result<ranges::borrowed_iterator_t<R1>,
                                            ranges::borrowed_iterator_t<R2>, O>
    operator()( R1&& r1, R2&& r2, O result,
                F binary_op, Proj1 proj1 = {}, Proj2 proj2 = {} ) const
  {
    return (*this)(ranges::begin(r1), ranges::end(r1),
                   ranges::begin(r2), ranges::end(r2),
                   result, std::ref(binary_op),
                   std::ref(proj1), std::ref(proj2));
  }
};
 
inline constexpr transform_fn transform;

[edit] Notes

std::ranges::transform does not guarantee in-order application of op or binary_op. To apply a function to a sequence in-order or to apply a function that modifies the elements of a sequence, use std::ranges::for_each.

[edit] Example

The following code uses transform to convert a string in place to uppercase using the toupper function and then transforms each char to its ordinal value:

#include <algorithm>
#include <cctype>
#include <functional>
#include <iostream>
#include <string>
#include <vector>
 
int main()
{
    std::string s("hello");
 
    namespace ranges = std::ranges;
 
    ranges::transform(s.begin(), s.end(), s.begin(),
                   [](unsigned char c) -> unsigned char { return std::toupper(c); });
 
    std::vector<std::size_t> ordinals;
    ranges::transform(s, std::back_inserter(ordinals),
                      [](unsigned char c) -> std::size_t { return c; });
 
    std::cout << s << ':';
    for (auto ord : ordinals) {
       std::cout << ' ' << ord;
    }
 
    ranges::transform(ordinals, ordinals, ordinals.begin(), std::plus{});
 
    std::cout << '\n';
    for (auto ord : ordinals) {
       std::cout << ord << ' ';
    }
    std::cout << '\n';
}

Output:

HELLO: 72 69 76 76 79
144 138 152 152 158

[edit] See also

applies a function to a range of elements
(niebloid) [edit]
a view of a sequence that applies a transformation function to each element
(class template) (range adaptor object) [edit]
applies a function to a range of elements, storing results in a destination range
(function template) [edit]