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

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

Command to display std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin, manual in Linux: $ man 3 std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::begin,

NAME

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

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