[Term Entry] C++ Unordered-sets: swap()

content/cpp/concepts/unordered-set/terms/swap/swap.md

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+---
+Title: 'swap()'
+Description: 'Exchanges the contents of two unordered sets in constant time.'
+Subjects:
+  - 'Code Foundations'
+  - 'Computer Science'
+Tags:
+  - 'Containers'
+  - 'Sets'
+  - 'STL'
+CatalogContent:
+  - 'learn-c-plus-plus'
+  - 'paths/computer-science'
+---
+
+The **`swap()`** method exchanges the contents of one [`unordered_set`](https://www.codecademy.com/resources/docs/cpp/unordered-set) with another. This operation runs in constant time by swapping internal structures rather than moving individual elements.
+
+After the swap, both containers take ownership of each other's elements. Iterators and references remain valid but now refer to elements stored in the opposite container.
+
+The two `unordered_set` containers do not need to have the same size, but they must be of the same type.
+
+## Syntax
+
+The `swap()` method can be used in the following syntax:
+
+1/. Member Function
+
+```pseudo
+unordered_set_first.swap(unordered_set_second);
+```
+
+2/. Non-Member Function
+
+```pseudo
+swap(unordered_set_first, unordered_set_second);
+```
+
+**Parameters:**
+
+- `unordered_set_second`: Another `unordered_set` with the same template parameters(element type, hash, predicate, allocator) as `unordered_set_first`.
+
+**Return value:**
+
+This method returns nothing (`void`).
+
+## Example
+
+In this example, the contents of two `unordered_set` containers are exchanged using the member `swap()` function:
+
+```cpp
+#include <iostream>
+#include <unordered_set>
+using namespace std;
+
+int main() {
+  unordered_set<int> setA = {1, 2, 3};
+  unordered_set<int> setB = {10, 20};
+
+  cout << "Before swap:\n";
+  cout << "setA contents: ";
+  for (int x : setA) cout << x << " ";
+  cout << "\n";
+
+  cout << "setB contents: ";
+  for (int x : setB) cout << x << " ";
+  cout << "\n";
+
+  setA.swap(setB);
+
+  cout << "\nAfter swap:\n";
+  cout << "setA contents: ";
+  for (int x : setA) cout << x << " ";
+  cout << "\n";
+
+  cout << "setB contents: ";
+  for (int x : setB) cout << x << " ";
+  cout << "\n";
+
+  return 0;
+}
+```
+
+The expected output of this code is:
+
+```shell
+Before swap:
+setA contents: 3 2 1
+setB contents: 20 10
+
+After swap:
+setA contents: 20 10
+setB contents: 3 2 1
+```
+
+> **Note:** The output order may vary because `unordered_set` does not maintain a defined element order.
+
+## Codebyte Example
+
+In this example, the code swaps the contents of two `unordered_set` containers and prints their updated elements:
+
+```codebyte/cpp
+#include <iostream>
+#include <unordered_set>
+using namespace std;
+
+int main() {
+  unordered_set<string> fruits = {"apple", "banana"};
+  unordered_set<string> colors = {"red", "blue", "green"};
+
+  cout << "Before swap:\n";
+  cout << "fruits: ";
+  for (const auto& f : fruits) cout << f << " ";
+  cout << "\n";
+
+  cout << "colors: ";
+  for (const auto& c : colors) cout << c << " ";
+  cout << "\n\n";
+
+  fruits.swap(colors);
+
+  cout << "After swap:\n";
+  cout << "fruits: ";
+  for (const auto& f : fruits) cout << f << " ";
+  cout << "\n";
+
+  cout << "colors: ";
+  for (const auto& c : colors) cout << c << " ";
+  cout << "\n";
+
+  return 0;
+}
+```