std::map<Key,T,Compare,Allocator>::begin, std::map<Key,T,Compare,Allocator>::cbegin
From cppreference.com
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().
Parameters
(none)
Return value
Iterator to the first element
Complexity
Constant
Example
Run this code
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
See also
returns an iterator to the end (public member function) |