# std::ranges::replace, std::ranges::replace_if

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
template< std::input_iterator I, std::sentinel_for<I> S, class T1, class T2, class Proj = std::identity > requires std::indirectly_writable<I, const T2&> && std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>, const T1*> constexpr I replace( I first, S last, const T1& old_value, const T2& new_value, Proj proj = {} ); | (1) | (since C++20)
template< ranges::input_range R, class T1, class T2, class Proj = std::identity > requires std::indirectly_writable<ranges::iterator_t<R>, const T2&> && std::indirect_binary_predicate<ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T1*> constexpr ranges::borrowed_iterator_t<R> replace( R&& r, const T1& old_value, const T2& new_value, Proj proj = {} ); | (2) | (since C++20)
template< std::input_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > requires std::indirectly_writable<I, const T&> constexpr I replace_if( I first, S last, Pred pred, const T& new_value, Proj proj = {} ); | (3) | (since C++20)
template< ranges::input_range R, class T, class Proj = std::identity, std::indirect_unary_predicate< std::projected<ranges::iterator_t<R>, Proj>> Pred > requires std::indirectly_writable<ranges::iterator_t<R>, const T&> constexpr ranges::borrowed_iterator_t<R> replace_if( R&& r, Pred pred, const T& new_value, Proj proj = {} ); | (4) | (since C++20)

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 std::invoke(proj, *i) == old_value to compare.
3) Replaces all elements for which the predicate pred evaluates to true, where evaluating expression is std::invoke(pred, std::invoke(proj, *i)).
2,4) Same as (1,3), but uses r as the range, as if using ranges::begin(r) as first and ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions.

### Parameters

first, last | - | the range of elements to process
r | - | the range of elements to process
old_value | - | the value of elements to replace
new_value | - | the value to use as a replacement
pred | - | predicate to apply to the projected elements
proj | - | projection to apply to the elements

### Return value

An iterator equal to last.

### Complexity

Exactly ranges::distance(first, last) applications of the corresponding predicate comp and any projection proj.

### Notes

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

### Possible implementation

replace
    struct replace_fn
    {
        template<std::input_iterator I, std::sentinel_for<I> S, class T1, class T2,
                 class Proj = std::identity>
        requires std::indirectly_writable<I, const T2&> && std::indirect_binary_predicate<
                     ranges::equal_to, std::projected<I, Proj>, const T1*>
        constexpr I
            operator()(I first, S last, const T1& old_value, const T2& new_value,
                       Proj proj = {}) const
        {
            for (; first != last; ++first)
                if (old_value == std::invoke(proj, *first))
                    *first = new_value;
            return first;
        }

        template<ranges::input_range R, class T1, class T2, class Proj = std::identity>
        requires std::indirectly_writable<ranges::iterator_t<R>, const T2&> &&
                 std::indirect_binary_predicate<ranges::equal_to,
                 std::projected<ranges::iterator_t<R>, Proj>, const T1*>
        constexpr ranges::borrowed_iterator_t<R>
            operator()(R&& r, const T1& old_value, const T2& new_value, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), old_value,
                           new_value, std::move(proj));
        }
    };

    inline constexpr replace_fn replace {};
replace_if
    struct replace_if_fn
    {
        template<std::input_iterator I, std::sentinel_for<I> S, class T,
                 class Proj = std::identity, std::indirect_unary_predicate<
                     std::projected<I, Proj>> Pred>
        requires std::indirectly_writable<I, const T&>
        constexpr I
            operator()(I first, S last, Pred pred, const T& new_value, Proj proj = {}) const
        {
            for (; first != last; ++first)
                if (!!std::invoke(pred, std::invoke(proj, *first)))
                    *first = new_value;
            return std::move(first);
        }

        template<ranges::input_range R, class T, class Proj = std::identity,
                 std::indirect_unary_predicate<std::projected<ranges::iterator_t<R>, Proj>> Pred>
        requires std::indirectly_writable<ranges::iterator_t<R>, const T&>
        constexpr ranges::borrowed_iterator_t<R>
            operator()(R&& r, Pred pred, const T& new_value, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::move(pred),
                           new_value, std::move(proj));
        }
    };

    inline constexpr replace_if_fn replace_if {};

### Example

Run this code

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

    int main()
    {
        auto print = [](const auto& v)
        {
            for (const auto& e : v)
                std::cout << e << ' ';
            std::cout << '\n';
        };

        std::array p {1, 6, 1, 6, 1, 6};
        print(p);
        std::ranges::replace(p, 6, 9);
        print(p);

        std::array q {1, 2, 3, 6, 7, 8, 4, 5};
        print(q);
        std::ranges::replace_if(q, [](int x) { return 5 < x; }, 5);
        print(q);
    }

Output:

    1 6 1 6 1 6
    1 9 1 9 1 9
    1 2 3 6 7 8 4 5
    1 2 3 5 5 5 4 5

### See also

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

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