Namespaces
Variants
Views
Actions

std::replace_copy, std::replace_copy_if

From cppreference.com
< cpp‎ | algorithm
 
C++
 
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)
(C++11)
Swap operations
Transformation operations
replace_copyreplace_copy_if

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
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Defined in header <algorithm>
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 );
 (1) (constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt1, class ForwardIt2, class T >
ForwardIt2 replace_copy
    ( ExecutionPolicy&& policy,
      ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first,

      const T& old_value, const T& new_value );
 (2) (since C++17)
(3)
template< class InputIt, class OutputIt, class UnaryPred, class T >

OutputIt replace_copy_if
    ( InputIt first, InputIt last, OutputIt d_first,

      UnaryPred p, const T& new_value );
 (constexpr since C++20)
(until C++26)
template< class InputIt, class OutputIt, class UnaryPred,

          class T = typename std::iterator_traits
                        <OutputIt>::value_type >
constexpr OutputIt replace_copy_if
    ( InputIt first, InputIt last, OutputIt d_first,

      UnaryPred p, const T& new_value );
 (since C++26)
(4)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2,

          class UnaryPred, class T >
ForwardIt2 replace_copy_if
    ( ExecutionPolicy&& policy,
      ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first,

      UnaryPred p, const T& new_value );
 (since C++17)
(until C++26)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2,

          class UnaryPred, class T = typename std::iterator_traits
                                         <ForwardIt2>::value_type >
ForwardIt2 replace_copy_if
    ( ExecutionPolicy&& policy,
      ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first,

      UnaryPred p, const T& new_value );
 (since C++26)

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.

1) Replaces all elements that are equal to old_value (using operator==).
3) Replaces all elements for which predicate p returns true.
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)

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.

Contents

[edit] Parameters

first, last - the range of elements to copy
d_first - the beginning of the destination range
old_value - the value of elements to replace
policy - the execution policy to use. See execution policy for details.
p - unary predicate which returns ​true if the element value should be replaced.

The expression p(v) must be convertible to bool for every argument v of type (possibly const) VT, where VT is the value type of InputIt, regardless of value category, and must not modify v. Thus, a parameter type of VT&is not allowed, nor is VT unless for VT a move is equivalent to a copy(since C++11). ​

new_value - the value to use as replacement
Type requirements
-
InputIt must meet the requirements of LegacyInputIterator.
-
OutputIt must meet the requirements of LegacyOutputIterator.
-
ForwardIt1, ForwardIt2 must meet the requirements of LegacyForwardIterator.

[edit] Return value

Iterator to the element past the last element copied.

[edit] Complexity

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

1,2) Exactly N comparisons using operator==.
3,4) Exactly N applications of the predicate p.

[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

replace_copy 
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;
}
replace_copy_if 
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

Feature-test macro Value Std Feature
__cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for algorithms (3,4)

[edit] Example

Run this code
#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);
}

Output: 

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.

DR Applied to Behavior as published Correct behavior
LWG 283 C++98 T was required to be CopyAssignable (and EqualityComparable for
replace_copy), but the value type of InputIt is not always T 		removed the requirement
LWG 337 C++98 replace_copy_if only required InputIt to
meet the requirements of LegacyIterator[1] 		corrected to
LegacyInputIterator
  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

 replaces all values satisfying specific criteria with another value
(function template) [edit]
removes elements satisfying specific criteria
(function template) [edit]
(C++20)(C++20)
 copies a range, replacing elements satisfying specific criteria with another value
(niebloid)[edit]
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/replace_copy&oldid=171916"