1. Introduction to Computer Science
Programming with Python

2. Lists
Chapter 8

3. Lists
As we want to solve more complex problems in Python, we need more powerful variables. Up to now we have been using simple variables to store numbers or strings where we have a single value in a variable. Starting with lists we will store many values in a single variable using an indexing scheme to store, organize, and retrieve different values from within a single variable. We call these multi-valued variables "collections" or "data structures".

4. Programming Algorithms Data Structures
A set of rules or steps used to solve a problem
Data Structures
A particular way of organizing data in a computer – ways to use variables differently

5. What is Not A “Collection”?
Most of our variables have one value in them. When we put a new value in the variable, the old value is overwritten
What if I want to add both numbers 2 and 4 in variable x?

6. A List is a Kind of Collection
A collection allows us to put many values in a single variable
A collection is efficient because we can carry many values around in one convenient package

7. List Constants
List constants are surrounded by square brackets and the elements in the list are separated by commas
A list element can be any Python object, even another list
A list can be empty
In other programming languages, lists are known as arrays

8. We Have Already Used Lists

9. Lists and Definite Loops

10. Looking Inside Lists
Just like strings, we can get at any single elements in a list using an index specified in square brackets

11. Lists Are Mutable
Strings are “immutable” – we cannot change the contents of a string, we must make a new string to make any change
Lists are “mutable” – we can change an element of a list using the index operator

12. How Long is a List?
The len() function takes a list as a parameter and returns the number of elements in the list
Actually len() tells us the number of elements of any set or sequence (such as a string, etc.)

13. Using the Range Function
The range function returns a list of numbers that range from zero to one less than the parameter
We can construct an index loop using for and integer iterator

14. A Tale of Two Loops

15. Manipulating Lists
Chapter 8

16. Concatenating Lists Using +
We can create a new list by adding two existing lists together

17. Lists Can be Sliced Using :
Remember, just like in strings, the second number is “up to but not including”

18. List Methods

19. Building a List from Scratch
We can create an empty list and then add elements using the append method
The list stays in order and new elements are added at the end of the list

20. Is Something in a List?
Python provides two operators that let you check if an item is in a list
These are logical operators that return True or False
They do not modify the list

21. Lists are in Order
A list can hold many items and keeps those items in the order until we do something to change the order
A list can be sorted (i.e., change its order)
The sort method (unlike in strings) means “sort yourself”

22. Built-in Functions
There are a number of functions built into Python that takes lists as parameters
Remember the loops we built? There much simpler.

24. This will keep numbers in memory
This will keep numbers in memory. If we have millions/billions of numbers, this will affect performance

25. Exercise
Instead of using the built-in function sum, implement it yourself and call it whenever you need to sum values

26. Strings and Lists – Best Friends
Split breaks a string into parts and produces a list of strings. We think of these as words. We can access a particular word or loop through all the words

27. Strings and Lists
When you do not specify a delimiter, multiple spaces are treated like one delimiter
You can specify what delimiter character to use in the splitting

28. List’s Functions
remove() – removes the first matching value, not a specified index
>>> a = [0, 2, 3, 2]
>>> a.remove(2)
>>> a [0, 3, 2]
del() – removes the item at a specific index
>>> a = [3, 2, 2, 1]
>>> del a[1]
>>> a [3, 2, 1]

29. List’s Functions
pop() – removes the item at a specific index and returns it (Queue)?
>>> a = [4, 3, 5]
>>> a.pop(1)  3
>>> a [4, 5]
sort() – will sort the list in ascending order
reverse() – reverses the list

30. STOP HERE!
Chapter 8

31. From Stephen.marquard@uct.ac.za Sat Jan 5 09:14:16 2008

32. The Double Split Pattern
Sometimes we split a line one way, and then grab one of the pieces of the line and split that piece again From Sat Jan 5 09:14: words = line.split() = words[1]

33. The Double Split Pattern
Sometimes we split a line one way, and then grab one of the pieces of the line and split that piece again From Sat Jan 5 09:14: words = line.split() = words[1] 

34. The Double Split Pattern
Sometimes we split a line one way, and then grab one of the pieces of the line and split that piece again From Sat Jan 5 09:14: words = line.split() = words[1]  pieces =  [‘Stephen.marquard’, ‘uct.ac.za’]

35. The Double Split Pattern
Sometimes we split a line one way, and then grab one of the pieces of the line and split that piece again From Sat Jan 5 09:14: words = line.split() = words[1]  pieces =  [‘Stephen.marquard’, ‘uct.ac.za’] print(pieces[1])  ‘uct.ac.za’

36. Exercise – Live Demo
In here we will read the file from py4e and split some data to get relevant information

37. Summary Concept of a collection Lists and definite loops
Indexing and lookup
List mutability
Functions: len(), min(), max(), sum()
Slicing lists
List methods: append, remove
Sorting lists
Splitting strings into lists of words
Using split to parse strings