[Term Entry] C++ Deque: at()

content/cpp/concepts/deque/terms/at/at.md

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+---
+Title: '.at()'
+Description: 'Accesses an element at a specified index in a deque with bounds checking.'
+Subjects:
+  - 'Computer Science'
+  - 'Game Development'
+Tags:
+  - 'Containers'
+  - 'Deques'
+  - 'Methods'
+  - 'STL'
+CatalogContent:
+  - 'learn-c-plus-plus'
+  - 'paths/computer-science'
+---
+
+The C++ **`.at()`** [function](https://www.codecademy.com/resources/docs/cpp/functions) accesses the element at a given index in a deque while performing bounds checking. If the index is out of range, the function throws an `std::out_of_range` [exception](https://www.codecademy.com/resources/docs/cpp/exceptions). This makes it a safer alternative to the subscript operator `[]`, which offers no safety checks.
+
+## Syntax
+
+```pseudo
+deque.at(pos)
+```
+
+**Parameters:**
+
+- `pos`: A zero-based index of the element to access. Must be within the valid range of the deque.
+
+**Return value:**
+
+Returns a reference to the element at the given position.
+
+**Exceptions:**
+
+Throws `std::out_of_range` if `pos` is invalid.
+
+## Example 1: Accessing Elements Safely
+
+In this example, `.at()` retrieves elements from valid positions in the deque:
+
+```cpp
+#include <iostream>
+#include <deque>
+using namespace std;
+
+int main() {
+  deque<int> numbers = {10, 20, 30, 40};
+
+  cout << "Element at index 2: " << numbers.at(2) << endl;
+  cout << "Element at index 0: " << numbers.at(0) << endl;
+
+  return 0;
+}
+```
+
+The output of this code is:
+
+```shell
+Element at index 2: 30
+Element at index 0: 10
+```
+
+The `.at()` function returns elements at positions 2 and 0 with built-in bounds checking.
+
+## Example 2: Handling Out-of-Range Access
+
+In this example, accessing index 5 triggers an exception because the index is outside the deque's valid range:
+
+```cpp
+#include <iostream>
+#include <deque>
+using namespace std;
+
+int main() {
+  deque<string> names = {"Ava", "Mira", "Leo"};
+
+  try {
+    cout << names.at(5) << endl;
+  } catch (const out_of_range& e) {
+    cout << "Error: " << e.what() << endl;
+  }
+
+  return 0;
+}
+```
+
+The output of this code is:
+
+```shell
+Error: deque::_M_range_check: __n (which is 5)>= this->size() (which is 3)
+```
+
+## Codebyte Example
+
+This example demonstrates safe element access and shows how `.at()` behaves when an invalid index is used:
+
+```codebyte/cpp
+#include <iostream>
+#include <deque>
+using namespace std;
+
+int main() {
+  deque<char> letters = {'A', 'B', 'C', 'D'};
+
+  cout << "First element: " << letters.at(0) << endl;
+  cout << "Third element: " << letters.at(2) << endl;
+
+  try {
+    cout << letters.at(10) << endl;
+  } catch (const out_of_range& e) {
+    cout << "Caught exception: " << e.what() << endl;
+  }
+
+  return 0;
+}
+```
+
+## Frequently Asked Questions
+
+### 1. What is the use of deque in C++?
+
+A deque (double-ended queue) stores elements in a dynamic sequence where insertions and deletions at both the front and back are efficient. It supports random access like a vector but provides faster operations at the beginning of the structure.
+
+### 2. What is a std::deque?
+
+`std::deque` is a standard container that provides a dynamic array-like structure with fast operations at both ends. It is implemented as segmented memory blocks, which allows it to grow without relocating all elements like a vector.
+
+### 3. How to check if deque is empty in C++?
+
+A deque can be checked for emptiness using the `.empty()` method, which returns `true` if it contains no elements and `false` otherwise. This check is constant time and works for all standard containers.