Lists - Intro to the Data Structure & Common Operations¶
Date published: 2018-02-08
Category: Python
Subcategory: Beginner Concepts
Tags: lists
What are Lists¶
Lists are mutable sequence types to store a collection of items. Mutable is the ability to alter the items in place without re-creating a new list.
Python lists can be collections of items you have on your computer such as:
- browser bookmarks
- songs in a playlist
- history of videos watched on YouTube
Construct a List¶
All lists are assigned a name and enclosed in square brackets.
List below is assigned to the variable name math_grades and contains a collection of scores for math exams for a single student.
There are 4 items in the list below.
math_grades = [88, 91, 82, 84]
math_grades
[88, 91, 82, 84]
We can further verify math_grades is a list by checking its type.
type(math_grades)
list
Change an element in a list (mutability)¶
Since lists are mutable, we can change elements in the existing list data structure.
We can change the last grade to be 100. Re-assign the item at index 3 to be `100.
math_grades[3] = 100
math_grades
[88, 91, 82, 100]
Lists Can Hold Multiple Object Types¶
Lists can contain multiple types of objects such as:
- numeric types:
- floats (example: 3.14)
- ints (example: 3)
- sequence types:
- strings (example: "pass")
- lists (example: [1, 2, 3])
- tuples (example: (8, "pass"))
- and more!
various_grades = [82, "pass", ["great", "ok", "very good"]]
various_grades
[82, 'pass', ['great', 'ok', 'very good']]
Lists Are Ordered Sequences of Elements¶
The order of items in a list matters. Let's take a look at math_grades.
math_grades
[88, 91, 82, 84]
88 is the score on the first exam, 91 on the second exam and so forth.
Index Items in a List¶
Index a list to retrieve a single or multiple items in a list. An index number is always an integer value and the first item in a list is index number 0.
For the list math_grades, here's the index breakdown:
| index | value |
|---|---|
| 0 | 88 |
| 1 | 91 |
| 2 | 82 |
| 3 | 84 |
Retrieve the item at index 0.
math_grades[0]
88
Retrieve the item at index 1.
math_grades[1]
91