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

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< cpp‎ | container‎ | map

| (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().

### 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

end cend (C++11) | returns an iterator to the end (public member function)
begin cbegin (C++11)(C++14) | returns an iterator to the beginning of a container or array (function template)

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