std::transform(3)

NAME
std::transform - std::transform

Synopsis
   Defined in header <algorithm>
   template< class InputIt,
   class OutputIt,
   class UnaryOperation >
   OutputIt transform( InputIt first1, (until C++20)
   InputIt last1,
   OutputIt d_first,
   UnaryOperation unary_op );
   template< class InputIt,
   class OutputIt,
   class UnaryOperation >
   constexpr OutputIt transform( InputIt first1, (since C++20)
   InputIt last1,
   OutputIt d_first,
   UnaryOperation unary_op );
   template< class ExecutionPolicy,
   class ForwardIt1,
   class ForwardIt2,
   class UnaryOperation >
   ForwardIt2 transform( ExecutionPolicy&& policy, (2) (since C++17)
   ForwardIt1 first1,
   ForwardIt1 last1,
   ForwardIt2 d_first,
   UnaryOperation unary_op );
   template< class InputIt1,
   class InputIt2, (1)
   class OutputIt,
   class BinaryOperation >
   OutputIt transform( InputIt1 first1, (until C++20)
   InputIt1 last1,
   InputIt2 first2,
   OutputIt d_first,
   BinaryOperation binary_op );
   template< class InputIt1,
   class InputIt2,
   class OutputIt,
   class BinaryOperation >
   constexpr OutputIt transform( InputIt1 first1, (since C++20)
   InputIt1 last1, (3)
   InputIt2 first2,
   OutputIt d_first,
   BinaryOperation binary_op );
   template< class ExecutionPolicy,
   class ForwardIt1,
   class ForwardIt2,
   class ForwardIt3,
   class BinaryOperation >
   ForwardIt3 transform( ExecutionPolicy&& policy, (4) (since C++17)
   ForwardIt1 first1,
   ForwardIt1 last1,
   ForwardIt2 first2,
   ForwardIt3 d_first,
   BinaryOperation binary_op );
   std::transform applies the given function to a range and stores the result in
   another range, keeping the original elements order and beginning at d_first.
   1) The unary operation unary_op is applied to the range defined by [first1, last1).
   3) The binary operation binary_op is applied to pairs of elements from two ranges:
   one defined by [first1, last1) and the other beginning at first2.
   2,4) Same as (1,3), but executed according to policy. These overloads do not
   participate in overload resolution unless
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.
   unary_op and binary_op must not have side effects. (until C++11)
   unary_op and binary_op must not invalidate any iterators, including (since C++11)
   the end iterators, or modify any elements of the ranges involved.

Parameters
   first1, last1 - the first range of elements to transform
   first2 - the beginning of the second range of elements to transform
   d_first - the beginning of the destination range, may be equal to first1 or
                   first2
   policy - the execution policy to use. See execution policy for details.
                   unary operation function object that will be applied.
                   The signature of the function should be equivalent to the following:
                   Ret fun(const Type &a);
   unary_op -
                   The signature does not need to have const &.
                   The type Type must be such that an object of type InputIt can be
                   dereferenced and then implicitly converted to Type. The type Ret
                   must be such that an object of type OutputIt can be dereferenced and
                   assigned a value of type Ret.
                   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);
   binary_op -
                   The signature does not need to have const &.
                   The types Type1 and Type2 must be such that objects of types
                   InputIt1 and InputIt2 can be dereferenced and then implicitly
                   converted to Type1 and Type2 respectively. The type Ret must be such
                   that an object of type OutputIt can be dereferenced and assigned a
                   value of type Ret.

Type requirements
   -
   InputIt, InputIt1, InputIt2 must meet the requirements of LegacyInputIterator.
   -
   OutputIt must meet the requirements of LegacyOutputIterator.
   -
   ForwardIt1, ForwardIt2, ForwardIt3 must meet the requirements of
   LegacyForwardIterator.

Return value
   Output iterator to the element past the last element transformed.

Complexity
   1-2) Exactly std::distance(first1, last1) applications of unary_op
   3-4) Exactly std::distance(first1, last1) applications of binary_op

Exceptions
   The overloads with a template parameter named ExecutionPolicy report errors as
   follows:
     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation

First version
   template< class InputIt,
             class OutputIt,
             class UnaryOperation >
   OutputIt transform( InputIt first1,
                       InputIt last1,
                       OutputIt d_first,
                       UnaryOperation unary_op )
   {
       while (first1 != last1) {
           *d_first++ = unary_op(*first1++);
       }
       return d_first;
   }

Second version
   template< class InputIt1,
             class InputIt2,
             class OutputIt,
             class BinaryOperation >
   OutputIt transform( InputIt1 first1,
                       InputIt1 last1,
                       InputIt2 first2,
                       OutputIt d_first,
                       BinaryOperation binary_op )
   {
       while (first1 != last1) {
           *d_first++ = binary_op(*first1++, *first2++);
       }
       return d_first;
   }

Notes
   std::transform does not guarantee in-order application of unary_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::for_each.

Example
   The following code uses transform to convert a string in place to uppercase using
   the std::toupper function and then transforms each char to its ordinal value: // Run this code
 #include <algorithm>
 #include <cctype>
 #include <iomanip>
 #include <iostream>
 #include <string>
 #include <vector>
 int main()
 {
     std::string s{"hello"};
     std::transform(s.cbegin(), s.cend(),
                    s.begin(), // write to the same location
                    [](unsigned char c) { return std::toupper(c); });
     std::cout << "s = " << quoted(s) << '\n';
     // achieving the same with std::for_each (see Notes above)
     std::string g{"hello"};
     std::for_each(g.begin(), g.end(), [](char& c) { // modify in-place
         c = std::toupper(static_cast<unsigned char>(c));
     });
     std::cout << "g = " << quoted(g) << '\n';
     std::vector<std::size_t> ordinals;
     std::transform(s.cbegin(), s.cend(), std::back_inserter(ordinals),
                    [](unsigned char c) { return c; });
     std::cout << "ordinals: ";
     for (auto ord : ordinals) {
        std::cout << ord << ' ';
     }
     std::transform(ordinals.cbegin(), ordinals.cend(), ordinals.cbegin(),
                    ordinals.begin(), std::plus<>{});
     std::cout << "\nordinals: ";
     for (auto ord : ordinals) {
        std::cout << ord << ' ';
     }
     std::cout << '\n';
 }

Output:
 s = "HELLO"
 g = "HELLO"
 ordinals: 72 69 76 76 79
 ordinals: 144 138 152 152 158

See also
   for_each applies a function to a range of elements
                     (function template)
   ranges::transform applies a function to a range of elements
   (C++20) (niebloid)