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

 iterator begin() noexcept; (1) (since C++11) const_iterator begin() const noexcept; (2) (since C++11) const_iterator cbegin() const noexcept; (3) (since C++11)

Returns an iterator to the first element of the unordered_map.

If the unordered_map is empty, the returned iterator will be equal to end(). ## Contents

(none)

### Return value

Iterator to the first element.

Constant.

### Example

#include <cmath>
#include <iostream>
#include <unordered_map>

struct Node { double x, y; };

int main()
{
Node nodes = {{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 + 0, 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