Deque count()

content/python/concepts/deque/terms/count/count.md

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+---
+Title: 'count()'
+Description: 'Returns the number of occurrences of a specified element in a deque.'
+Subjects:
+  - 'Computer Science'
+  - 'Data Structures'
+Tags:
+  - 'Collections'
+  - 'Deque'
+  - 'Python'
+CatalogContent:
+  - 'learn-python-3'
+  - 'paths/computer-science'
+---
+
+The **`count()`** method of a [`collections.deque`](https://www.codecademy.com/resources/docs/python/collections-module/deque) object counts how many times a given value appears in the deque. It provides a direct way to determine the frequency of a specific element within the deque container.
+
+## Syntax
+
+```pseudo
+deque_object.count(value)
+```
+
+**Parameters:**
+
+- `value`: The element whose occurrences are to be counted in the deque.
+
+**Return value:**
+
+Returns an integer representing the number of elements in the deque that are equal to `value`.
+
+## Example 1: Using `count()` to find repeated values
+
+In this example, the `count()` method is used to find how many times the value `3` appears in a deque:
+
+```py
+from collections import deque
+
+dq = deque([1, 2, 3, 3, 4, 3, 5])
+print(dq.count(3))
+```
+
+The output of this code is:
+
+```shell
+3
+```
+
+## Example 2: Counting non-existent elements returns zero
+
+In this example, the method counts an element not present in the deque, returning zero:
+
+```py
+from collections import deque
+
+dq = deque([10, 20, 30, 40])
+print(dq.count(100))
+```
+
+The output of this code is:
+
+```shell
+0
+```
+
+## Codebyte Example: Using `count()` with string elements
+
+In this example, the `count()` method is used to compute the frequency of a specific item in a deque of strings representing colours:
+
+```codebyte/python
+from collections import deque
+
+colours = deque(['red', 'blue', 'red', 'green', 'blue', 'red'])
+target_colour = 'red'
+count_red = colours.count(target_colour)
+print(f"The colour {target_colour!r} appears {count_red} times in the deque.")
+```
+
+## Frequently Asked Questions
+
+### 1. What is the time complexity of the `count()` method in Python deque?
+
+The `count()` method has `O(n)` time complexity, where n is the number of elements in the deque. This is because the method must iterate through all elements in the deque to count occurrences of the specified value.
+
+### 2. What does `count()` return if the element is not found in the deque?
+
+The `count()` method returns 0 if the specified element is not present in the deque. It does not raise an exception or return None, it simply indicates that there are zero occurrences of that value.
+
+### 3. How does `count()` determine equality when counting elements?
+
+The `count()` method uses the equality operator `(==)` to compare elements. For custom objects, it will use the object's `__eq__()` method if defined. Two objects are considered equal if `obj1 == obj2` evaluates to True.