Difference between revisions of "cpp/algorithm/replace copy"

Latest revision as of 21:22, 20 May 2024

Copies the elements from the range [first, last) to another range beginning at d_first, while replacing all elements satisfying specific criteria with new_value.

If any of the results of the expressions *first and new_value is not writable to d_first, the program is ill-formed.

If the source and destination ranges overlap, the behavior is undefined.

[edit] Parameters

[edit] Return value

Iterator to the element past the last element copied.

[edit] Complexity

Given \(\scriptsize N\)N as std::distance(first, last):

[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

template<class InputIt, class OutputIt, class T>
OutputIt replace_copy(InputIt first, InputIt last, OutputIt d_first,
                      const T& old_value, const T& new_value)
{
    for (; first != last; ++first)
        *d_first++ = (*first == old_value) ? new_value : *first;
    return d_first;
}

template<class InputIt, class OutputIt, class UnaryPred,
         class T = typename std::iterator_traits<ForwardIt>::value_type>
OutputIt replace_copy_if(InputIt first, InputIt last, OutputIt d_first,
                         UnaryPred p, const T& new_value)
{
    for (; first != last; ++first)
        *d_first++ = p(*first) ? new_value : *first;
    return d_first;
}

[edit] Notes

[edit] Example

#include <algorithm>
#include <complex>
#include <iostream>
#include <vector>
 
void println(const auto& seq)
{
    for (const auto& e : seq)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::vector<short> src{3, 1, 4, 1, 5, 9, 2, 6, 5};
    println(src);
    std::vector<int> dst(src.size());
    std::replace_copy_if(src.cbegin(), src.cend(),
                         dst.begin(),
                         [](short n){ return n > 5; }, 0);
    println(dst);
 
    std::vector<std::complex<double>> src2{{1, 3}, {2, 4}, {3, 5}},
                                      dst2(src2.size());
    println(src2);
    #ifdef __cpp_lib_algorithm_default_value_type
        std::replace_copy_if(src2.cbegin(), src2.cend(), dst2.begin(),
            [](std::complex<double> z){ return std::abs(z) < 5; },
            {4, 2}); // Possible, since the T is deduced.
    #else
        std::replace_copy_if(src2.cbegin(), src2.cend(), dst2.begin(),
            [](std::complex<double> z){ return std::abs(z) < 5; },
            std::complex<double>{4, 2});
    #endif
    println(dst2);
}

3 1 4 1 5 9 2 6 5 
3 1 4 1 5 0 2 0 5 
(1,3) (2,4) (3,5) 
(4,2) (4,2) (3,5)

[edit] Defect reports

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

1. ↑ The actual defect in the C++ standard is that the template parameter InputIterator was misspecified as Iterator. This affects the type requirements because the C++ standard states that for the function templates in the algorithms library, the template type parameters whose name ends with Iterator imply the type requirements of the corresponding iterator categories.

[edit] See also