std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin, (3) - Linux Man Pages

std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin,: std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin,

NAME

std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin, - std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin,

Synopsis


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 container.


If the container is empty, the returned iterator will be equal to end().


range-begin-end.svg

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,     },
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


end  returns an iterator to the end
cend (public member function)