std::unordered_map::begin, std::unordered_map::cbegin
Da cppreference.com.
iterator begin() noexcept; |
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const_iterator begin() const noexcept; |
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const_iterator cbegin() const noexcept; |
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Restituisce un iteratore al primo elemento del container.
Se il container è vuoto, l'iteratore restituito sarà uguale a end().
Parametri
(nessuno)
Valore restituito
Iteratore al primo elemento
Complessità
Costante
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
Vedi anche
restituisce un iteratore fino alla fine Original: returns an iterator to the end The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. (metodo pubblico) | |