std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin, std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::cbegin
From cppreference.com
< cpp | container | unordered map
| 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_map.
If the unordered_map is empty, the returned iterator will be equal to end().
Parameters
(none)
Return value
Iterator to the first element.
Complexity
Constant.
Example
Run this code
#include <cmath> #include <iostream> #include <unordered_map> struct Node { double x, y; }; int main() { Node nodes[3] = {{1, 0}, {2, 0}, {3, 0}}; // mag is a map mapping the address of a Node to its magnitude in the plane std::unordered_map<Node*, double> mag = { { nodes + 0, 1 }, { nodes + 1, 2 }, { nodes + 2, 3 } }; // Change each y-coordinate from 0 to the magnitude for (auto iter = mag.begin(); iter != mag.end(); ++iter) { auto cur = iter->first; // pointer to Node cur->y = mag[cur]; // could also have used cur->y = iter->second; } // Update and print the magnitude of each node for (auto iter = mag.begin(); iter != mag.end(); ++iter) { auto cur = iter->first; mag[cur] = std::hypot(cur->x, cur->y); std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is "; std::cout << iter->second << '\n'; } // Repeat the above with the range-based for loop for (auto i : mag) { auto cur = i.first; cur->y = i.second; mag[cur] = std::hypot(cur->x, cur->y); std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is "; std::cout << mag[cur] << '\n'; // Note that in contrast to std::cout << iter->second << '\n'; above, // std::cout << i.second << '\n'; will NOT print the updated magnitude } }
Possible output:
The magnitude of (3, 3) is 4.24264 The magnitude of (1, 1) is 1.41421 The magnitude of (2, 2) is 2.82843 The magnitude of (3, 4.24264) is 5.19615 The magnitude of (1, 1.41421) is 1.73205 The magnitude of (2, 2.82843) is 3.4641
See also
| (C++11) |
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) |