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std::ranges::shift_left, std::ranges::shift_right

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, 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
Constrained numeric operations
Operations on uninitialized storage
Return types
 
Defined in header <algorithm>
Call signature
template< std::permutable I, std::sentinel_for<I> S >

constexpr ranges::subrange<I>

    shift_left( I first, S last, std::iter_difference_t<I> n );
(1) (since C++23)
template< ranges::forward_range R >

requires std::permutable<ranges::iterator_t<R>>
constexpr ranges::borrowed_subrange_t<R>

    shift_left( R&& r, ranges::range_difference_t<R> n );
(2) (since C++23)
template< std::permutable I, std::sentinel_for<I> S >

constexpr ranges::subrange<I>

    shift_right( I first, S last, std::iter_difference_t<I> n );
(3) (since C++23)
template< ranges::forward_range R >

requires std::permutable<ranges::iterator_t<R>>
constexpr ranges::borrowed_subrange_t<R>

    shift_right( R&& r, ranges::range_difference_t<R> n );
(4) (since C++23)

Shifts the elements in the range [first, last) or r by n positions. The behavior is undefined if [first, last) is not a valid range.

1) Shifts the elements towards the beginning of the range:
  • If n == 0 || n >= last - first, there are no effects.
  • If n < 0, the behavior is undefined.
  • Otherwise, for every integer i in [0, last - first - n), moves the element originally at position first + n + i to position first + i. The moves are performed in increasing order of i starting from 0.
3) Shifts the elements towards the end of the range:
  • If n == 0 || n >= last - first, there are no effects.
  • If n < 0, the behavior is undefined.
  • Otherwise, for every integer i in [0, last - first - n), moves the element originally at position first + i to position first + n + i. If I models bidirectional_iterator, then the moves are performed in decreasing order of i starting from last - first - n - 1.
2, 4) Same as (1) or (3) respectively, but uses r as the range, as if using ranges::begin(r) as first and ranges::end(r) as last.

Elements that are in the original range but not the new range are left in a valid but unspecified state.

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 - the beginning of the original range
last - the end of the original range
r - the range of elements to shift
n - the number of positions to shift

[edit] Return value

1-2) {first, /*NEW_LAST*/}, where NEW_LAST is the end of the resulting range and equivalent to:
  • first + (last - first - n), if n is less than last - first;
  • first otherwise.
3-4) {/*NEW_FIRST*/, last}, where NEW_FIRST is the beginning of the resulting range and equivalent to:
  • first + n, if n is less than last - first;
  • last otherwise.

[edit] Complexity

1-2) At most ranges::distance(first, last) - n assignments.
3-4) At most ranges::distance(first, last) - n assignment or swaps.

[edit] Notes

ranges::shift_left / ranges::shift_right has better efficiency on common implementations if I models bidirectional_iterator or (better) random_access_iterator.

Implementations (e.g. MSVC STL) may enable vectorization when the iterator type models contiguous_iterator and swapping its value type calls neither non-trivial special member function nor ADL-found swap.

Feature testing macro: __cpp_lib_shift

[edit] Example

#include <algorithm>
#include <iostream>
#include <string>
#include <type_traits>
#include <vector>
 
struct S
{
    int value{0};
    bool specified_state{true};
 
    S(int v = 0) : value{v} {}
    S(S const& rhs) = default;
    S(S&& rhs) { *this = std::move(rhs); }
    S& operator=(S const& rhs) = default;
    S& operator=(S&& rhs) {
        if (this != &rhs) {
            value = rhs.value;
            specified_state = rhs.specified_state;
            rhs.specified_state = false;
        }
        return *this;
    }
};
 
template <typename T>
std::ostream& operator<< (std::ostream& os, std::vector<T> const& v)
{
    for (const auto& s : v) {
        if constexpr (std::is_same_v<T, S>)
            s.specified_state ? os << s.value << ' ' : os << ". ";
        else if constexpr (std::is_same_v<T, std::string>)
            os << (s.empty() ? "." : s) << ' ';
        else
            os << s << ' ';
    }
    return os;
}
 
int main()
{
    std::cout << std::left;
 
    std::vector<S>            a{1, 2, 3, 4, 5, 6, 7};
    std::vector<int>          b{1, 2, 3, 4, 5, 6, 7};
    std::vector<std::string>  c{"α", "β", "γ", "δ", "ε", "ζ", "η"};
 
    std::cout << "vector<S> \tvector<int> \tvector<string>\n";
    std::cout << a << "  " << b << "  " << c << '\n';
 
    std::ranges::shift_left(a, 3);
    std::ranges::shift_left(b, 3);
    std::ranges::shift_left(c, 3);
    std::cout << a << "  " << b << "  " << c << '\n';
 
    std::ranges::shift_right(a, 2);
    std::ranges::shift_right(b, 2);
    std::ranges::shift_right(c, 2);
    std::cout << a << "  " << b << "  " << c << '\n';
 
    std::ranges::shift_left(a, 8);  // has no effect: n >= last - first
    std::ranges::shift_left(b, 8);  // ditto
    std::ranges::shift_left(c, 8);  // ditto
    std::cout << a << "  " << b << "  " << c << '\n';
 
//  std::ranges::shift_left(a, -3);  // UB
}

Possible output:

vector<S>       vector<int>     vector<string>
1 2 3 4 5 6 7   1 2 3 4 5 6 7   α β γ δ ε ζ η
4 5 6 7 . . .   4 5 6 7 5 6 7   δ ε ζ η . . .
. . 4 5 6 7 .   4 5 4 5 6 7 5   . . δ ε ζ η .
. . 4 5 6 7 .   4 5 4 5 6 7 5   . . δ ε ζ η .

[edit] See also

moves a range of elements to a new location
(niebloid) [edit]
moves a range of elements to a new location in backwards order
(niebloid) [edit]
rotates the order of elements in a range
(niebloid) [edit]
shifts elements in a range
(function template) [edit]