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std::transform

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
Swap operations
Transformation operations
transform
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
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 [first1last1).
If unary_op invalidates an iterator or modifies an element in any of the following ranges, the behavior is undefined:
  • [first1last1].
  • 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: [first1last1) 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:
  • [first1last1].
  • 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)

Contents

[edit] 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_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 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

#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

[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

applies a function to a range of elements
(function template) [edit]
applies a function to a range of elements
(niebloid)[edit]