unordered_map begin() in C++

The unordered_map::begin() is a built-in function in C++ STL which returns an iterator pointing to the first element in the unordered_map container or in any of its bucket. 

• Syntax for first element in unordered_map container: 

unordered_map.begin()

Parameters: This function does not accepts any parameters. 
Return Value: The function returns an iterator pointing to the first element in the unordered_map container. 
Note: In an unordered map, there is no specific element which is considered as the first element. 
Below program illustrate the above function. 

CPP

// CPP program to demonstrate the
// unordered_map::begin() function
// when first element of the container
// is to be returned as iterator
#include <bits/stdc++.h>
using namespace std;

int main()
{

    // Declaration
    unordered_map<std::string, std::string> mymap;

    // Initialisation
    mymap = { { "Australia", "Canberra" },
              { "U.S.", "Washington" },
              { "France", "Paris" } };

    // Iterator pointing to the first element
    // in the unordered map
    auto it = mymap.begin();

    // Prints the elements of the first element in map
    cout << it->first << " " << it->second;

    return 0;
}

Output:

France Paris

• Syntax for first element in unordered_map bucket: 

unordered_map.begin( n )

Parameters: The function accepts one mandatory parameter n which specifies the bucket number whose first element’s iterator is to be returned. 
Return Value: The function returns an iterator pointing to the first element in the n-th bucket. 
Below program illustrate the above function. 

CPP

// CPP program to demonstrate the
// unordered_map::begin() function
// when first element of n-th container
// is to be returned as iterator
#include <bits/stdc++.h>
using namespace std;

int main()
{

    // Declaration
    unordered_map<std::string, std::string> mymap;

    // Initialisation
    mymap = { { "Australia", "Canberra" },
            { "U.S.", "Washington" }, { "France", "Paris" } };

    // Iterator pointing to the first element
    // in the n-th bucket
    auto it = mymap.begin(0);

    // Prints the elements of the n-th bucket
    cout << it->first << " " << it->second;

    return 0;
}

Output:

U.S. Washington