unordered_map begin() in C++ (original) (raw)

Last Updated : 3 Dec, 2021

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:

unorderedmap.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;

}

Syntax for first element in unordered_map bucket:

unorderedmap.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;

}