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

From cppreference.com
< cpp‎ | container‎ | map

 
C++
 
Containers library
 
std::map
Member functions
Element access
Iterators
map::beginmap::cbegin
(C++11)
(C++11)
(C++11)
(C++11)
Capacity
Modifiers
(C++23)
(C++17)
(C++11)
(C++11)
(C++17)
(C++17)
(C++17)
Lookup
(C++20)
Observers
Non-member functions
(C++20)
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
Deduction guides (C++17)
 
(1)
iterator begin();
 (until C++11)
iterator begin() noexcept;
 (since C++11)
(2)
const_iterator begin() const;
 (until C++11)
const_iterator begin() const noexcept;
 (since C++11)
const_iterator cbegin() const noexcept;
 (3) (since C++11)

Returns an iterator to the first element of the map.

If the map 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; };
 
//Compare the x-coordinates of two Point pointers
struct PointCmp {
    bool operator()(const Point *lhs, const Point *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
        // If auto &i: mag was used instead, it will 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

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)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/container/map/begin&oldid=135924"