C++ .begin()

In C++, the .begin() method returns an iterator that points to the first element in the unordered_set container. Because unordered_set does not maintain sorted order, the element returned by .begin() is simply the first element in its internal bucket structure, not the “smallest” or “first” in any logical sense.

Syntax

mySet.begin(n);

Parameters:

• n (Optional): The index of the bucket whose first element is to be accessed (must be in the range 0 to mySet.bucket_count() - 1).

Return value:

• .begin(): Returns an iterator that refers to the first element in the container. If the container is empty, the iterator equals .end().
• .begin(n): Returns a local iterator that refers to the first element in bucket n. If the bucket is empty, the iterator equals .end(n).

Example

The following example demonstrates using .begin() to iterate through an unordered_set:

#include <iostream>
#include <unordered_set>

int main() {
  std::unordered_set<int> numbers = {10, 20, 30, 40, 50};

  std::cout << "Elements in the unordered_set: ";
  for (auto it = numbers.begin(); it != numbers.end(); ++it) {
    std::cout << *it << " ";
  }

  return 0;
}

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This outputs the elements in the unordered_set (order may vary):

Elements in the unordered_set: 50 40 30 20 10

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Codebyte Example

The following runnable example shows how to use .begin() with an unordered_set:

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