1. Lists COMPSCI 105 S2 2015 Principles of Computer Science

2. Lists  Lists are a built-in type in Python  Use square brackets to signify a list  Lists can contain any type of data, or any mixture of data my_list1 = [1, 2, 3] my_list2 = ['Hello', 'Is', 'there', 'anybody', 'out', 'there?'] my_list3 = [1, 5.899, 'Hello'] my_list4 = [4, 2, 6, 9, 3] my_list1 = [1, 2, 3] my_list2 = ['Hello', 'Is', 'there', 'anybody', 'out', 'there?'] my_list3 = [1, 5.899, 'Hello'] my_list4 = [4, 2, 6, 9, 3] my_list 426 9 3 Lecture 02COMPSCI 1052

3. List functions  Numerous list functions are supported  Use help(list) to find out the functions  Examples: >>> x = [1, 2, 3] >>> len(x) >>> x = [1, 2, 3] >>> len(x) Lecture 02COMPSCI 1053 3 3 [1, 2, 3, 4] [1, 2, 3, 5] >>> x += [5] >>> x + [4] >>> 3 in x True 1 1 >>> x[0] x 123 x 123 5

4.  append(item)  insert(i, item)  pop()  pop(i)  sort()  reverse()  del( my_list[i] )  index(item)  count(item)  remove(item) Functions of lists 4COMPSCI 105 Lecture 02

5.  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 Exercise 5COMPSCI 105 Lecture 02

6. 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]] my_list 0 1 2 3 123 0 1 2 456 0 1 2 7 0 89 0 1 Lecture 02COMPSCI 1056

7. Exercise 4  Draw a diagram showing how the following list is structured in memory: my_list = [[1, 2], [[3, 4], [5, 6, 7]]] Lecture 02COMPSCI 1057

8. Slices of sequences  A piece of a sequence can be obtained using the following syntax  sequence_name[x:y]  where x is the index of the first element and y is the index after the last element >>> name = 'Andrew' >>> name[0:0] >>> name = 'Andrew' >>> name[0:0] Lecture 02COMPSCI 1058 '' 'A' 'ndr' >>> name[0:1] >>> name[1:4]

9. Slice step value  Actually, the syntax allows for a third value, used to define the step size between elements included in the slice. If a value if omitted, it defaults to [start:end:1]  If the step size is negative, it starts at the end and steps backward towards the start. >>> name = 'Andrew' >>> name[:4:2] >>> name = 'Andrew' >>> name[:4:2] >>> name = 'Andrew' >>> name[::-1] >>> name = 'Andrew' >>> name[::-1] nameAndrew Positive Index 012345 Negative index -6-5-4-3-2 Lecture 02COMPSCI 1059 'ad' 'werdnA'

10. For loops  Used to iterate through a sequence numbers = [2, 3, 5, 7, 11] for i in numbers: print(i) numbers = [2, 3, 5, 7, 11] for i in numbers: print(i) name = "Andrew" for c in name: print(c) name = "Andrew" for c in name: print(c) Lecture 02COMPSCI 10510 2 3 5 7 11 2 3 5 7 11 AndrewAndrew AndrewAndrew Example01.py

11. While loops  Used to execute code when the end condition is unknown name = "Andrew Luxton-Reilly" i = 0 while name[i] != ' ': i += 1 print('Space is at position:', i) name = "Andrew Luxton-Reilly" i = 0 while name[i] != ' ': i += 1 print('Space is at position:', i) Space is at position: 6 Lecture 02COMPSCI 10511 Example01.py

12. Range  Range is a special object in Python  Used to generate integer numbers within a range of values  Can iterate through the range for x in range(0, 5): print(x) for x in range(0, 5): print(x) Lecture 02COMPSCI 10512 0123401234 0123401234 Example01.py

13. List comprehensions  A list can be created using instructions that appear within the square brackets  The general format is as follows:  Examples:  To convert a string to a list of characters:  To generate all the even numbers between 1 and 20 my_list = [x for x in range(0, 10)] [expression for variable in sequence] my_list = [c for c in 'Andrew'] my_list = [n * 2 for n in range(1, 10)] Lecture 02COMPSCI 10513

14. Exercise 5  Write a list comprehension that generates all the odd numbers between 1 and 50 Lecture 02COMPSCI 10514

15.  We can extend the syntax for a list comprehension to include a condition:  The general format is as follows:  Examples:  Generate a list of the non-vowel letters that appear in a string: List comprehensions that use conditions my_list = [x for x in range(0, 10) if x % 2 == 0] [expression for variable in sequence if condition] Lecture 02COMPSCI 10515 >>> name = 'Andrew Luxton-Reilly' >>> vowels = 'aeiou‘ >>> my_list = [c for c in name if c not in vowels] >>> name = 'Andrew Luxton-Reilly' >>> vowels = 'aeiou‘ >>> my_list = [c for c in name if c not in vowels] ['A', 'n', 'd', 'r', 'w', ' ', 'L', 'x', 't', 'n', '-', 'R', 'l', 'l', 'y']

16.  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) Features of lists Lecture 02COMPSCI 10516