What is the unordered_map::begin() function in C++?

In this shot, we will learn how to use the unordered_map::begin() function, which is available in the <unordered_map> header file in C++.

The unordered_map::begin() function is used to return an iterator pointing to the first element in the unordered map. There are two variations of this function:

Bucket number: It specifies the bucket number in the map and returns the iterator pointing to the first key-value pair in the specified bucket. The bucket number should be less than the total number of buckets in the map.
Without passing any parameter: In this case, the function will return an iterator pointing to the first key-value pair in the map.

Syntax

The syntax of the unordered_map::begin() function is as follows:

iterator begin();
iterator begin (size_type n);

Parameter

The function accepts the bucket number as an optional parameter.

Return

If the bucket number is not specified, then it will return the iterator pointing to the first key-value pair in the map. Otherwise, it will return an iterator pointing to the first key-value pair in the specified bucket.

Code

Let’s have a look at the code now.

#include <iostream>
#include <unordered_map>
using namespace std;
int main()
{
  unordered_map<int,string> umap ={
                            {12, "unordered"},
                            {16, "map"},
                            {89, "in"},
                            {66," C++"}
                          };
  cout <<"The first key-value pair is: ";
  cout << umap.begin()->first << "-> " << umap.begin()->second; 
  cout << endl;
  
  for (int i = 0; i < umap.bucket_count(); ++i) 
  {
    cout << "Bucket " << i << " contains:";
    for (auto it = umap.begin(i); it != umap.end(i); ++it) 
      cout << "(" << it->first << "," << it->second << ")\n ";
    cout << endl;
  }
  return 0;
}

Explanation

In lines 1 and 2, we imported the required header files.
In line 5, we made a main() function.
From lines 7 to 12, we initialized an unordered map with integer type keys and string type values.
In line 15, we printed the first key-value pair from the map using the unordered_map::begin() function without passing the optional parameter.
In line 18, we ran a loop to obtain the bucket count and traverse to all the buckets.
In line 20, we displayed the bucket number.
In line 21, we ran a loop to iterate over all the elements in the bucket using the unordered_map::begin() function. unordered_map::begin() is used to get the iterator pointing to the first element.
In line 22, we displayed the key value pairs present inside each bucket.

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