Difference between revisions of "cpp/container/unordered set/begin"
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< cpp | container | unordered set
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Latest revision as of 05:10, 12 January 2021
| iterator begin() noexcept; |
(1) | (since C++11) |
| const_iterator begin() const noexcept; |
(2) | (since C++11) |
| const_iterator cbegin() const noexcept; |
(3) | (since C++11) |
Returns an iterator to the first element of the unordered_set.
If the unordered_set is empty, the returned iterator will be equal to end().
Contents |
[edit] Parameters
(none)
[edit] Return value
Iterator to the first element.
[edit] Complexity
Constant.
Notes
Because both iterator and const_iterator are constant iterators (and may in fact be the same type), it is not possible to mutate the elements of the container through an iterator returned by any of these member functions.
[edit] Example
Run this code
#include <iostream> #include <unordered_set> struct Point { double x, y; }; int main() { Point pts[3] = {{1, 0}, {2, 0}, {3, 0}}; // points is a set containing the addresses of points std::unordered_set<Point*> points = { pts, pts + 1, pts + 2 }; // Change each y-coordinate of (i, 0) from 0 into i^2 and print the point for (auto iter = points.begin(); iter != points.end(); ++iter) { (*iter)->y = ((*iter)->x) * ((*iter)->x); // iter is a pointer-to-Point* std::cout << "(" << (*iter)->x << ", " << (*iter)->y << ") "; } std::cout << '\n'; // Now using the range-based for loop, we increase each y-coordinate by 10 for (Point* i : points) { i->y += 10; std::cout << "(" << i->x << ", " << i->y << ") "; } }
Possible output:
(3, 9) (1, 1) (2, 4) (3, 19) (1, 11) (2, 14)
[edit] See also
| returns an iterator to the end (public member function) | |
| (C++11)(C++14) |
returns an iterator to the beginning of a container or array (function template) |
