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std::replace_copy, std::replace_copy_if

Defined in header `<algorithm>`
	(1)
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 );		(until C++20)
template< class InputIt, class OutputIt, class T > constexpr OutputIt replace_copy( InputIt first, InputIt last, OutputIt d_first, const T& old_value, const T& new_value );		(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 UnaryPredicate, class T > OutputIt replace_copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPredicate p, const T& new_value );		(until C++20)
template< class InputIt, class OutputIt, class UnaryPredicate, class T > constexpr OutputIt replace_copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPredicate p, const T& new_value );		(since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryPredicate, class T > ForwardIt2 replace_copy_if( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, UnaryPredicate p, const T& new_value );	(4)	(since C++17)

Copies the 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. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

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.

Return value

Iterator to the element past the last element copied.

Complexity

Exactly last - first applications of the predicate.

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.

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; }

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;
}

Example

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

#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