# std::replace_copy, std::replace_copy_if

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< cpp‎ | algorithm

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 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)
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 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 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`. 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 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`.
- `ForwardIt1, ForwardIt2` must meet the requirements of `ForwardIterator`.

### 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 three 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;
    }

### 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

### See also

remove remove_if | removes elements satisfying specific criteria (function template)

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