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

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:

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

#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

(C++20)(C++20)
copies a range, replacing elements satisfying specific criteria with another value
(niebloid)
replaces all values satisfying specific criteria with another value
(function template)

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