# 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 (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 do not participate in overload resolution unless

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)

### 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.

The expression *first = new_value must be valid. | (until C++20)
new_value must be writable to first. | (since C++20)

### Return value

(none)

### Complexity

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

1,2) exactly `N` comparisons with old_value using `operator==`
3,4) exactly `N` applications of the predicate p

### 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

replace
    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;
    }
replace_if
    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 <functional>
    #include <iostream>

    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

### 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`), but the value type of `ForwardIt` is not always `T` and `T` is not always writable to `ForwardIt` | required *first = new_value to be valid instead

### See also

replace_copy replace_copy_if | copies a range, replacing elements satisfying specific criteria with another value (function template)
ranges::replace ranges::replace_if (C++20)(C++20) | replaces all values satisfying specific criteria with another value (niebloid)

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