Introduction to Lists in Computer Science: Creation, Manipulation, and Comprehensions

1. COMPSCI 105Principles of Computer Science Lecture 02 - Lists

2. Lists • Creating a list • In memory my_list = [] my_list COMPSCI 105 - Principles of Computer Science

3. Lists • Adding an element • In memory my_list = my_list + [4] my_list 4 COMPSCI 105 - Principles of Computer Science

4. Lists • Joining lists • In memory my_list = my_list + [2, 6, 9, 3] my_list 4 2 6 9 3 COMPSCI 105 - Principles of Computer Science

5. Lists • Accessing an element of the list • In memory print(my_list[3]) my_list 4 2 6 9 3 2 3 0 1 4 COMPSCI 105 - Principles of Computer Science

6. Exercise • Write a function that called sum_list that sums the elements in a given list COMPSCI 105 - Principles of Computer Science

7. Functions of lists • append(item) • insert(i, item) • pop() • pop(i) • sort() • reverse() • del( my_list[i] ) • index(item) • count(item) • remove(item) COMPSCI 105 - Principles of Computer Science

8. Exercise • Write a function that determines if a list is a palindrome. • Your function should return true if the contents of the list are the same regardless of whether the elements are accessed in forward or reverse order. • >>> is_palindrome([1, 2, 3, 2, 1]) • True COMPSCI 105 - Principles of Computer Science

9. Lists of lists • Since a list can contain anything, it can of course contain a list • In memory my_list = [[1, 2, 3], [4, 5, 6], [7], [8, 9]] 1 2 3 2 0 1 4 5 6 2 0 1 7 0 8 9 0 1 my_list 2 3 0 1 COMPSCI 105 - Principles of Computer Science

10. Exercise • Draw a diagram showing how the following list is structured in memory: my_list = [[1, 2], [[3, 4], [5, 6, 7]]] COMPSCI 105 - Principles of Computer Science

11. List comprehensions • A list can be created using instructions that appear within the square brackets • The general format is as follows: [expression for variable in sequence] my_list = [x for x in range(0, 100)] COMPSCI 105 - Principles of Computer Science

12. Examples • To convert a string to a list of characters: • To generate all the even numbers between 1 and 100 my_list = [c for c in 'Andrew'] my_list = [n * 2 for n in range(1, 50)] COMPSCI 105 - Principles of Computer Science

13. Exercise • Write a list comprehension that generates all the odd numbers between 1 and 50 (inclusive) • Given a list containing strings as follows: • days = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday'] • Use a list comprehension to generate a list containing the reverse of each string. • rdays = COMPSCI 105 - Principles of Computer Science

14. Exercise • Generating a list of random numbers • Given that a random number between X and Y inclusive can be obtained using the code: • import random • number = random.randint(X, Y) • Write a list comprehension to generate a list of 100 random values between 0 and 1 to simulate tossing a coin. COMPSCI 105 - Principles of Computer Science

15. List comprehensions that use conditions • We can extend the syntax for a list comprehension to include a condition: • The general format is as follows: [expression for variable in sequence if condition] my_list = [x for x in range(0, 100) if x % 2 == 0] COMPSCI 105 - Principles of Computer Science

16. Examples • Generate a list of the non-vowel letters that appear in a string: • name = 'Andrew Luxton-Reilly' • vowels = 'aeiou' my_list = [c for c in name if c not in vowels] COMPSCI 105 - Principles of Computer Science

17. Features of lists • Information in a list is stored contiguously in memory • location of the information can be calculated • location = start of the list + index * size of each element • Efficiency issues • It takes the same time to access any of the elements • Slow to move elements around (i.e. add and delete elements from within the list) COMPSCI 105 - Principles of Computer Science