C++ Unordered_map Library - begin() Function

Description

The C++ function std::unordered_map::begin() returns an iterator pointing to the first element in one of its buckets.

Declaration

Following is the declaration for std::unordered_map::begin() function form std::unordered_map header.

C++11

local_iterator begin(size_type n);
const_local_iterator begin(size_type n) const;

Parameters

n − Bucket number, which must be less than bucket_count.

Return value

If object is constant qualified then method returns constant iterator otherwise it returns non-constant reference.

Time complexity

Constant i.e. O(1)

Example

The following example shows the usage of std::unordered_map::begin() function.

#include <iostream>
#include <unordered_map>

using namespace std;

int main(void) {
   unordered_map<char, int> um = {
            {'a', 1},
            {'b', 2},
            {'c', 3},
            {'d', 4},
            {'e', 5}
            };

   for (int i = 0; i < um.bucket_count(); ++i) {
      cout << "Bucket " << i << " contains:" << endl;

      for (auto it = um.begin(i); it != um.end(i); ++it)
         cout << it->first << " = " << it->second << endl;
   }

   return 0;
}

Let us compile and run the above program, this will produce the following result −

Bucket 0 contains:
b = 2
Bucket 1 contains:
c = 3
Bucket 2 contains:
d = 4
Bucket 3 contains:
e = 5
Bucket 4 contains:
Bucket 5 contains:
Bucket 6 contains:
a = 1