std::map<Key,T,Compare,Allocator>::begin,std::map<Key,T,Compare,Allocator>::cbegin (3) - Linux Manuals

std::map<Key,T,Compare,Allocator>::begin,std::map<Key,T,Compare,Allocator>::cbegin: std::map<Key,T,Compare,Allocator>::begin,std::map<Key,T,Compare,Allocator>::cbegin

NAME

std::map<Key,T,Compare,Allocator>::begin,std::map<Key,T,Compare,Allocator>::cbegin - std::map<Key,T,Compare,Allocator>::begin,std::map<Key,T,Compare,Allocator>::cbegin

Synopsis

iterator begin(); (until C++11)
iterator begin() noexcept; (since C++11)
const_iterator begin() const; (until 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 <iostream>
  #include <map>

  int main() {
    std::map<int, float> num_map;
    num_map[4] = 4.13;
    num_map[9] = 9.24;
    num_map[1] = 1.09;
    // calls a_map.begin() and a_map.end()
    for (auto it = num_map.begin(); it != num_map.end(); ++it) {
      std::cout << it->first << ", " << it->second << '\n';
    }
  }

Output:

  1, 1.09
  4, 4.13
  9, 9.24

Example using a custom comparison function

// Run this code

  #include <cmath>
  #include <iostream>
  #include <map>

  struct Point { double x, y; };

  typedef Point * PointPtr;
  //Compare the x-coordinates of two Point pointers
  struct PointCmp {
      bool operator()(const PointPtr &lhs, const PointPtr &rhs) const {
          return lhs->x < rhs->x;
      }
  };

  int main() {
      //Note that although the x-coordinates are out of order, the
      // map will be iterated through by increasing x-coordinates
      Point points[3] = { {2, 0}, {1, 0}, {3, 0} };

      //mag is a map sending the address of node to its magnitude in the x-y plane
      //Although the keys are pointers-to-Point, we want to order the map by the
      // x-coordinates of the points and NOT by the addresses of the Points. This
      // is done by using the PointCmp class's comparison method.
      std::map<Point *, double, PointCmp> mag({
          { points, 2 },
          { points + 1, 1 },
          { points + 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
      }
  }

Output:

  The magnitude of (1, 1) is 1.41421
  The magnitude of (2, 2) is 2.82843
  The magnitude of (3, 3) is 4.24264
  The magnitude of (1, 1.41421) is 1.73205
  The magnitude of (2, 2.82843) is 3.4641
  The magnitude of (3, 4.24264) is 5.19615