C++ count()

The count() method in C++ checks whether a given key exists in a std::unordered_set. Since this container stores only unique elements, count() will always return one of these two values:

• 1: If the element is found in the set.
• 0: If the element is not found in the set.

This method is commonly used as a fast, O(1) average time complexity way to check for element existence.

Syntax

unordered_set_name.count(key);

Parameters:

• key (const Key&): The value of the element to search for. Must be of the same type as the elements stored in the unordered_set.

Return value:

Returns an integer. 1 if the element exists, 0 otherwise.

Example

This example demonstrates using count() to check for the presence of elements within a set of strings:

#include <iostream>
#include <string>
#include <unordered_set>

int main() {
  std::unordered_set<std::string> inventory = {
    "Sword",
    "Shield",
    "Potion"
  };

  std::cout << "Inventory contains:\n";
  for (const auto& item : inventory) {
    std::cout << "- " << item << "\n";
  }

  // Check for an existing element
  if (inventory.count("Sword")) {
    std::cout << "\n'Sword' is present (Count: " << inventory.count("Sword") << ").\n";
  }

  // Check for a missing element
  if (inventory.count("Axe") == 0) {
    std::cout << "'Axe' is not present (Count: " << inventory.count("Axe") << ").\n";
  }

  return 0;
}

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The output of the code is:

Inventory contains:
- Potion
- Shield
- Sword

'Sword' is present (Count: 1).
'Axe' is not present (Count: 0).

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Codebyte Example

Run the codebyte below to check for the presence of an item in a set of integers:

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