std::transform

Defined in header `<algorithm>`
template< class InputIt, class OutputIt, class UnaryOp > OutputIt transform( InputIt first1, InputIt last1, OutputIt d_first, UnaryOp unary_op );	(1)	(constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryOp > ForwardIt2 transform( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 d_first, UnaryOp unary_op );	(2)	(since C++17)
template< class InputIt1, class InputIt2, class OutputIt, class BinaryOp > OutputIt transform( InputIt1 first1, InputIt1 last1, InputIt2 first2, OutputIt d_first, BinaryOp binary_op );	(3)	(constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class ForwardIt3, class BinaryOp > ForwardIt3 transform( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt3 d_first, BinaryOp binary_op );	(4)	(since C++17)

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)

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

[edit] Return value

Output iterator to the element that follows the last element transformed.

[edit] Complexity

Given $\scriptsize N$ $N$ as std::distance(first1, last1):

1,2) Exactly

N

applications of unary_op.

3,4) Exactly

N

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 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.

[edit] Possible implementation

transform (1)

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

transform (3)

template<class InputIt1, class InputIt2, 
         class OutputIt, class BinaryOp>
OutputIt transform(InputIt1 first1, InputIt1 last1, InputIt2 first2,
                   OutputIt d_first, BinaryOp 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 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>
 
void print_ordinals(const std::vector<std::size_t>& ordinals)
{
    std::cout << "ordinals: ";
    for (std::size_t ord : ordinals)
        std::cout << std::setw(3) << ord << ' ';
    std::cout << '\n';
}
 
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 = " << std::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 = " << std::quoted(g) << '\n';
 
    std::vector<std::size_t> ordinals;
    std::transform(s.cbegin(), s.cend(), std::back_inserter(ordinals),
                   [](unsigned char c) { return c; });
 
    print_ordinals(ordinals);
 
    std::transform(ordinals.cbegin(), ordinals.cend(), ordinals.cbegin(),
                   ordinals.begin(), std::plus<>{});
 
    print_ordinals(ordinals);
}

Output:

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

[edit] 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 effects	they cannot modify the ranges involved

[edit] See also

for_each	applies a function to a range of elements (function template) [edit]
ranges::transform (C++20)	applies a function to a range of elements (niebloid)[edit]

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[edit] Parameters