std::transform

Defined in header `<algorithm>`
template< class InputIt, class OutputIt, class UnaryOperation > OutputIt transform( InputIt first1, InputIt last1, OutputIt d_first, UnaryOperation unary_op );	(1)
template< class ExecutionPolicy, class InputIt, class OutputIt, class UnaryOperation > OutputIt transform( ExecutionPolicy&& policy, InputIt first1, InputIt last1, OutputIt d_first, UnaryOperation unary_op );	(2)	(since C++17)
template< class InputIt1, class InputIt2, class OutputIt, class BinaryOperation > OutputIt transform( InputIt1 first1, InputIt1 last1, InputIt2 first2, OutputIt d_first, BinaryOperation binary_op );	(3)
template< class ExecutionPolicy, class InputIt1, class InputIt2, class OutputIt, class BinaryOperation > OutputIt transform( ExecutionPolicy&& policy, InputIt1 first1, InputIt1 last1, InputIt2 first2, OutputIt d_first, BinaryOperation binary_op );	(4)	(since C++17)

std::transform applies the given function to a range and stores the result in another range, 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>> 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 the end iterators, or modify any elements of the ranges involved.	(since C++11)

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_op	-	unary operation function object that will be applied. The signature of the function should be equivalent to the following: Ret fun(const Type &a); 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_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 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` must meet the requirements of `InputIterator`.
- `InputIt1` must meet the requirements of `InputIterator`.
- `InputIt2` must meet the requirements of `InputIterator`.
- `OutputIt` must meet the requirements of `OutputIterator`.

[edit] Return value

Output iterator to the element past the last element transformed.

[edit] Complexity

1-2) Exactly std::distance(first1, last1) applications of unary_op

3-4) Exactly std::distance(first1, last1) applications of binary_op

[edit] 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,

if policy is std::parallel_vector_execution_policy, std::terminate is called
if policy is std::sequential_execution_policy or std::parallel_execution_policy, the algorithm exits with an std::exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in std::exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered.
if policy is some other type, the behavior is implementation-defined

If the algorithm fails to allocate memory (either for itself or to construct an std::exception_list when handling a user exception), std::bad_alloc is thrown.

[edit] 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;
}

[edit] 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

[edit] Example

The following code uses transform to convert a string to uppercase using the toupper function:

Run this code

#include <string>
#include <cctype>
#include <algorithm>
#include <iostream>
 
int main()
{
    std::string s("hello");
    std::transform(s.begin(), s.end(), s.begin(),
                   [](unsigned char c) { return std::toupper(c); });
    std::cout << s;
}

Output:

HELLO

[edit] See also

for_each	applies a function to a range of elements (function template)
std::experimental::parallel::transform (parallelism TS)	parallelized version of `std::transform` (function template)

Language
Standard library headers
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Technical Specifications