std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin, std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::cbegin
iterator begin() noexcept; |
(since C++11) | |
const_iterator begin() const noexcept; |
(since C++11) | |
const_iterator cbegin() const noexcept; |
(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
#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, 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) |
© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/container/unordered_map/begin
