## Python Lists This section covers the Python `List` data type, a fundamental structure for storing and manipulating collections of data. It addresses the problem of managing a large number of related variables efficiently. ### What is a List? A **list** in Python is an ordered and *alterable* (mutable) data structure that can hold a collection of items. Instead of creating numerous individual variables (e.g., `student1_mark`, `student2_mark`, etc.), you can store all related values in a single list. * **Purpose:** To store multiple values in a single variable. * **Characteristics:** * **Ordered:** Items have a defined order, and that order will not change unless you modify it. * **Mutable:** You can change, add, and remove items in a list after it has been created. * **Allows Duplicates:** A list can contain items with the same value. ### Creating and Accessing Lists A list is defined using square brackets `[]`. * **Initialization:** You can create an empty list or initialize it with values. ```python # An empty list my_list = [] # A list initialized with values SngL_Dgt_Odd_Nums = [1, 3, 5, 7, 9] ``` * **Accessing Elements:** Individual elements are accessed using their *index*, which is their position in the list. **Indexing starts at 0.** ```python # Access the element at index 2 print(SngL_Dgt_Odd_Nums[2]) # Output: 5 ``` * **Negative Indexing:** Python also supports negative indexing, where `-1` refers to the last item, `-2` to the second last, and so on. ### Basic List Functions Python provides several built-in functions that can operate on lists. | Function | Description | Example (using `nums = [1, 3, 5, 7, 9]`) | Result | | :------- | :------------------------------------------------ | :--------------------------------------- | :----- | | `len()` | Returns the total number of elements in the list. | `len(nums)` | 5 | | `min()` | Returns the smallest element in the list. | `min(nums)` | 1 | | `max()` | Returns the largest element in the list. | `max(nums)` | 9 | | `sum()` | Returns the sum of all elements in the list. | `sum(nums)` | 25 | ### Modifying and Inspecting Lists Lists have built-in *methods* (functions attached to the list object) to modify them or retrieve information. * **`insert(index, element)`**: Inserts an `element` at a specified `index`. * **`append(element)`**: Adds an `element` to the *end* of the list. This is useful for building lists dynamically, for example, within a loop. * **`index(element)`**: Returns the index of the **first occurrence** of the specified `element`. If an element appears multiple times, this method will only return the index of the first one it finds. * **`pop(index)`**: Removes the element at the specified position. ### List Slicing Slicing allows you to access a specific range of elements in a list. ### Populating Lists with Random Numbers Lists can be populated dynamically using loops and external modules like `random`. 1. **Import:** Use `from random import randint` to access the `randint()` function. 2. **Initialize:** Create an empty list, e.g., `Rnums = []`. 3. **Loop and Append:** Use a `for` loop to generate random numbers and add them to the list using the `append()` method. *Code and sample outputs:* | Code | Possible Outputs | | :--- | :--- | | ```python 1. from random import randint 2. Rnums = [] 3. for i in range(10): 4. Rnums.append(randint(1, 99)) 5. print(Rnums) ```
**Fig 3.29** | `[66, 1, 6, 15, 83, 8, 73, 15, 30, 7]`
`[94, 49, 96, 16, 97, 57, 24, 5, 87, 10]`
`[79, 36, 92, 53, 86, 94, 36, 68, 34, 29]`
*(Each run produces a different random list.)* | ### Searching in a List with Membership Operators The `in` operator is a simple and readable way to check if an element exists in a list. It returns a Boolean value (`True` or `False`). * **Syntax:** `element in list_name`