std::transform(3)

NAME
std::transform - std::transform

Synopsis
   Defined in header <algorithm>
   template< class InputIt, class OutputIt, class UnaryOp >
   OutputIt transform( InputIt first1, InputIt last1, (1) (constexpr since C++20)
                       OutputIt d_first, UnaryOp unary_op
   );
   template< class ExecutionPolicy,
             class ForwardIt1, class ForwardIt2, class
   UnaryOp >
   ForwardIt2 transform( ExecutionPolicy&& policy, (2) (since C++17)
                        ForwardIt1 first1, ForwardIt1
   last1,
                        ForwardIt2 d_first, UnaryOp
   unary_op );
   template< class InputIt1, class InputIt2,
             class OutputIt, class BinaryOp >
   OutputIt transform( InputIt1 first1, InputIt1 last1, (3) (constexpr since C++20)
   InputIt2 first2,
                       OutputIt d_first, BinaryOp binary_op
   );
   template< class ExecutionPolicy,
             class ForwardIt1, class ForwardIt2,
             class ForwardIt3, class BinaryOp >
   ForwardIt3 transform( ExecutionPolicy&& policy,
                        ForwardIt1 first1, ForwardIt1 (4) (since C++17)
   last1,
                        ForwardIt2 first2,
                        ForwardIt3 d_first, BinaryOp
   binary_op );
   std::transform applies the given function to the elements of the given input
   range(s), and stores the result in an output range starting from d_first.
   1) The unary operation unary_op is applied to the elements of [first1, last1).
   If unary_op invalidates an iterator or modifies an element in any of the following
   ranges, the behavior is undefined:
     * [first1, last1].
     * The range of std::distance(first1, last1) + 1 elements starting from d_first.
   3) The binary operation binary_op is applied to pairs of elements from two ranges:
   [first1, last1) and another range of std::distance(first1, last1) elements starting
   from first2.
   If binary_op invalidates an iterator or modifies an element in any of the following
   ranges, the behavior is undefined:
     * [first1, last1].
     * The range of std::distance(first1, last1) + 1 elements starting from first2.
     * The range of std::distance(first1, last1) + 1 elements starting from d_first.
   2,4) Same as (1,3), but executed according to policy.
   These overloads participate in overload resolution only if
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
                        C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
                        C++20)

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 that follows the last element transformed.

Complexity
   Given \(\scriptsize N\)N as std::distance(first1, last1):
   1,2) Exactly \(\scriptsize N\)N applications of unary_op.
   3,4) Exactly \(\scriptsize N\)N 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
                        transform (1)
   template<class InputIt, class OutputIt, class UnaryOp>
   constexpr //< since C++20
   OutputIt transform(InputIt first1, InputIt last1,
                      OutputIt d_first, UnaryOp unary_op)
   {
       for (; first1 != last1; ++d_first, ++first1)
           *d_first = unary_op(*first1);
       return d_first;
   }
                        transform (3)
   template<class InputIt1, class InputIt2,
            class OutputIt, class BinaryOp>
   constexpr //< since C++20
   OutputIt transform(InputIt1 first1, InputIt1 last1, InputIt2 first2,
                      OutputIt d_first, BinaryOp binary_op)
   {
       for (; first1 != last1; ++d_first, ++first1, ++first2)
           *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
// Run this code
 #include <algorithm>
 #include <cctype>
 #include <iomanip>
 #include <iostream>
 #include <string>
 #include <utility>
 #include <vector>
 void print_ordinals(const std::vector<unsigned>& ordinals)
 {
     std::cout << "ordinals: ";
     for (unsigned ord : ordinals)
         std::cout << std::setw(3) << ord << ' ';
     std::cout << '\n';
 }
 char to_uppercase(unsigned char c)
 {
     return std::toupper(c);
 }
 void to_uppercase_inplace(char& c)
 {
     c = to_uppercase(c);
 }
 void unary_transform_example(std::string& hello, std::string world)
 {
     // Transform string to uppercase in-place
     std::transform(hello.cbegin(), hello.cend(), hello.begin(), to_uppercase);
     std::cout << "hello = " << std::quoted(hello) << '\n';
     // for_each version (see Notes above)
     std::for_each(world.begin(), world.end(), to_uppercase_inplace);
     std::cout << "world = " << std::quoted(world) << '\n';
 }
 void binary_transform_example(std::vector<unsigned> ordinals)
 {
     // Transform numbers to doubled values
     print_ordinals(ordinals);
     std::transform(ordinals.cbegin(), ordinals.cend(), ordinals.cbegin(),
                    ordinals.begin(), std::plus<>{});
     print_ordinals(ordinals);
 }
 int main()
 {
     std::string hello("hello");
     unary_transform_example(hello, "world");
     std::vector<unsigned> ordinals;
     std::copy(hello.cbegin(), hello.cend(), std::back_inserter(ordinals));
     binary_transform_example(std::move(ordinals));
 }

Output:
 hello = "HELLO"
 world = "WORLD"
 ordinals: 72 69 76 76 79
 ordinals: 144 138 152 152 158
   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 unary_op and binary_op could not have side they cannot modify the
                      effects ranges involved

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)