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

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< cpp‎ | container‎ | unordered map

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().

### 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 + 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

### 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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