[Term Entry] C++ Deque: size() (#7975)

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Author: mamtawardhani

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

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+---
+Title: 'size()'
+Description: 'Returns the number of elements in a deque container.'
+Subjects:
+  - 'Computer Science'
+  - 'Game Development'
+Tags:
+  - 'Containers'
+  - 'Deques'
+  - 'Methods'
+  - 'STL'
+CatalogContent:
+  - 'learn-c-plus-plus'
+  - 'paths/computer-science'
+---
+
+The **`size()`** function returns the number of elements currently stored in a [`std::deque`](https://www.codecademy.com/resources/docs/cpp/deque) container. It has constant time complexity (`O(1)`) and is marked noexcept since C++ 11.
+
+## Syntax
+
+```pseudo
+deque_object.size();
+```
+
+**Parameters:**
+
+This function does not take any parameters.
+
+**Return value:**
+
+Returns a value of type `size_type` (an unsigned integral type) representing the number of elements in the deque.
+
+## Example 1
+
+In this example, the size method is used to check the element count of an initially empty deque, then after [`push_back`](https://www.codecademy.com/resources/docs/cpp/deque/push-back) operations:
+
+```cpp
+#include <deque>
+#include <iostream>
+
+int main() {
+  std::deque<int> d;
+  std::cout << "Initial size: " << d.size() << "\n";
+
+  for (int i = 0; i < 5; ++i) {
+    d.push_back(i);
+  }
+  std::cout << "Size after push_back 5 elements: " << d.size() << "\n";
+
+  return 0;
+}
+```
+
+The output of this code is:
+
+```shell
+Initial size: 0
+Size after push_back 5 elements: 5
+```
+
+## Example 2
+
+In this example, the size method is used after insert and pop operations to illustrate dynamic changes in element count:
+
+```cpp
+#include <deque>
+#include <iostream>
+
+int main() {
+  std::deque<int> d = {1, 2, 3};
+  std::cout << "Initial size: " << d.size() << "\n";
+
+  d.pop_front();
+  d.pop_back();
+  std::cout << "Size after two pops: " << d.size() << "\n";
+
+  d.insert(d.begin(), 10);
+  std::cout << "Size after one insert at front: " << d.size() << "\n";
+
+  return 0;
+}
+```
+
+The output of this code is:
+
+```shell
+Initial size: 3
+Size after two pops: 1
+Size after one insert at front: 2
+```
+
+## Codebyte Example
+
+In this example, the `size()` method is repeatedly checked in a loop until the deque becomes empty:
+
+```codebyte/cpp
+#include <iostream>
+#include <deque>
+
+int main() {
+  std::deque<char> letters = {'A', 'B', 'C', 'D', 'E'};
+
+  while (!letters.empty()) {
+    std::cout << "Current size: " << letters.size() << " front element: " << letters.front() << "\n";
+    letters.pop_front();
+  }
+
+  std::cout << "Final size after emptying: " << letters.size() << "\n";
+  return 0;
+}
+```
+
+## Frequently Asked Questions
+
+### 1. What does `size()` do in C++?
+
+The `size()` function in C++ returns the number of elements present in a container, such as a `std::deque`, `std::vector`, or `std::string`. It gives the current length of the container in constant time (`O(1)`) without modifying it.
+
+### 2. What is a deque function in C++?
+
+A deque (double-ended queue) in C++ is a Standard Template Library (STL) container that allows insertion and deletion of elements from both the front and back efficiently.
+
+### 3. How to get the size of a deque in C++?
+
+You can get the number of elements in a deque using the `size()` member function.