# std::replace, std::replace_if

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class ForwardIt, class T > void replace( ForwardIt first, ForwardIt last, const T& old_value, const T& new_value ); |  | (until C++20)
template< class ForwardIt, class T > constexpr void replace( ForwardIt first, ForwardIt last, const T& old_value, const T& new_value ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt, class T > void replace( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& old_value, const T& new_value ); | (2) | (since C++17)
| (3) |
template< class ForwardIt, class UnaryPredicate, class T > void replace_if( ForwardIt first, ForwardIt last, UnaryPredicate p, const T& new_value ); |  | (until C++20)
template< class ForwardIt, class UnaryPredicate, class T > constexpr void replace_if( ForwardIt first, ForwardIt last, UnaryPredicate p, const T& new_value ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate, class T > void replace_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPredicate p, const T& new_value ); | (4) | (since C++17)

Replaces all elements satisfying specific criteria with `new_value` in the range `[first, last)`.

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

### Parameters

first, last | - | the range of elements to process
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 `ForwardIt`, 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
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- `UnaryPredicate` must meet the requirements of Predicate.

### Return value

(none)

### 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.

### Notes

Because the algorithm takes `old_value` and `new_value` by reference, it can have unexpected behavior if either is a reference to an element of the range `[first, last)`.

### Possible implementation

First version
    template<class ForwardIt, class T>
    void replace(ForwardIt first, ForwardIt last,
                 const T& old_value, const T& new_value)
    {
        for (; first != last; ++first) {
            if (*first == old_value) {
                *first = new_value;
            }
        }
    }
Second version
    template<class ForwardIt, class UnaryPredicate, class T>
    void replace_if(ForwardIt first, ForwardIt last,
                    UnaryPredicate p, const T& new_value)
    {
        for (; first != last; ++first) {
            if(p(*first)) {
                *first = new_value;
            }
        }
    }

### Example

The following code at first replaces all occurrences of 8 with 88 in a vector of integers. Then it replaces all values less than 5 with 55.

Run this code

    #include <algorithm>
    #include <array>
    #include <iostream>
    #include <functional>

    int main()
    {
        std::array<int, 10> s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};

        std::replace(s.begin(), s.end(), 8, 88);

        for (int a : s) {
            std::cout << a << " ";
        }
        std::cout << '\n';

        std::replace_if(s.begin(), s.end(),
                        std::bind(std::less<int>(), std::placeholders::_1, 5), 55);
        for (int a : s) {
            std::cout << a << " ";
        }
        std::cout << '\n';
    }

Output:

    5 7 4 2 88 6 1 9 0 3
    5 7 55 55 88 6 55 9 55 55

### See also

replace_copy replace_copy_if | copies a range, replacing elements satisfying specific criteria with another value (function template)

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