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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:

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 {};

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

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