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

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

NAME

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

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, 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


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