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std::transform - cppreference.com

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

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.

The binary operation

is applied to pairs of elements from two ranges:

and another range of

elements starting from

.

If

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 all following conditions are satisfied:

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. ​

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. ​

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

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.

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.

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;
}

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;
}

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.

#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

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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