std::replace_copy, std::replace_copy_if

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)
template< class ExecutionPolicy, class InputIt, class OutputIt, class T > OutputIt replace_copy( ExecutionPolicy&& policy, InputIt first, InputIt last, OutputIt d_first, const T& old_value, const T& new_value );	(2)	(since C++17)
template< class InputIt, class OutputIt, class UnaryPredicate, class T > OutputIt replace_copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPredicate p, const T& new_value );	(3)
template< class ExecutionPolicy, class InputIt, class OutputIt, class UnaryPredicate, class T > OutputIt replace_copy_if( ExecutionPolicy&& policy, InputIt first, InputIt last, OutputIt d_first, UnaryPredicate p, const T& new_value );	(4)	(since C++17)

Copies the all elements from the range [first, last) to another range beginning at d_first replacing all elements satisfying specific criteria with new_value. The source and destination ranges cannot overlap.

1) Replaces all elements that are equal to old_value.

3) Replaces all elements for which predicate p returns true.

2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

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 signature of the predicate function should be equivalent to the following: bool pred(const Type &a); The signature does not need to have const &, but the function must not modify the objects passed to it. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type.
new_value	-	the value to use as replacement
Type requirements
- `InputIt` must meet the requirements of `InputIterator`.
- `OutputIt` must meet the requirements of `OutputIterator`.

[edit] Return value

Iterator to the element past the last element copied.

[edit] Complexity

Exactly last - first applications of the predicate.

[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,

if policy is std::parallel_vector_execution_policy, std::terminate is called
if policy is std::sequential_execution_policy or std::parallel_execution_policy, the algorithm exits with an std::exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in std::exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered.
if policy is some other type, the behavior is implementation-defined

If the algorithm fails to allocate memory (either for itself or to construct an std::exception_list when handling a user exception), std::bad_alloc is thrown.

[edit] Possible implementation

First version

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;
}

Second version

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

[edit] Example

The following copy prints a vector, replacing all values over 5 with 99 on the fly.

Run this code

#include <algorithm>
#include <vector>
#include <iostream>
#include <iterator>
#include <functional>
 
int main()
{
    std::vector<int> v{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};
    std::replace_copy_if(v.begin(), v.end(),
                         std::ostream_iterator<int>(std::cout, " "),
                         [](int n){return n > 5;}, 99);
    std::cout << '\n';
}

Output:

5 99 4 2 99 99 1 99 0 3

[edit] See also

removeremove_if	removes elements satisfying specific criteria (function template)
std::experimental::parallel::replace_copy (parallelism TS)	parallelized version of `std::replace_copy` (function template)
std::experimental::parallel::replace_copy_if (parallelism TS)	parallelized version of `std::replace_copy_if` (function template)

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