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.

  1. 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.

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    // 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;
      
        // Initilisation
        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;
    }

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    Output:

    France Paris
    
  2. 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.

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    // 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;
      
        // Initilisation
        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;
    }

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    Output:

    U.S. Washington
    


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