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

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
Operations on uninitialized storage
(C++17)
(C++17)
(C++17)
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
Operations on uninitialized storage
Return types
 
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.

Contents

[edit] 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

[edit] Return value

An iterator equal to last.

[edit] Complexity

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

[edit] 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).

[edit] Possible implementation

First version
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{};
Second version
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{};

[edit] 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

[edit] See also

copies a range, replacing elements satisfying specific criteria with another value
(niebloid) [edit]
replaces all values satisfying specific criteria with another value
(function template) [edit]