1. Section06: Sequences
Chapter 4 and 5

2. General Description "Normal" variables Sequence variables: x = 19
The name "x" is associated with a single value
Sequence variables:
L = (5, 17, 19, -1)
A name which is associated with a group of values.

3. Python sequences Immutable types (Ch. 4) Mutables types (Ch. 5)
Strings: a group of characters
Tuples: a group of other python objects
Mutables types (Ch. 5)
Lists: a group of other python objects
Dictionaries: a group of key-value pairs (the value is a python object), indentified by key.

4. Length of sequences Returns the number of "things" in the sequence.
S = "ABCDEF"
T = (1, 2, 3)
U = (4, 5, (6, 7, 8), 9)
len(S) # 6
len(T) # 3
len(U) # 4

5. Indexing Sequences (except Dictionaries)
Index number identifies a position.
s = "Oranges"
Examples:
>>> s[0] # 'O'
>>> s[2] # 'a'
>>> s[-1] # 's'
>>> s[-2] # 'e'
The indexed value can be used anywhere a "normal" variable / constant would -7 -6 -5 -4 -3 -2 -1 1 2 3 4 5 6 O r a n g e s

6. Slicing Sequences Returns a section of a sequence
Doesn't work for Dictionaries.
>>> s = "Apples-Oranges"
>>> s[1:4] # "ppl"
>>> s[5:9] # "s-Or"
>>> s[-6:-1] # "range"
>>> s[-5:-7] # ""
>>> s[10:] # "nges"
>>> s[:6] # "Apples"
>>> s[1:10:2] # "plsOa"
>>> s[12:8:-1] # "egna"
>>> s[-1::-1] # "segnarO-selppA" (backward)
-14
-13
-12
-11
-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 1 2 3 4 5 6 7 8 9 10 11 12 13 A p l e s - O r a n g

7. for loops A type of repetition, but tied to a sequence. Syntax:
for var in seq:
# for-loop block
The number of items in seq controls the number of iterations.
var:
A variable, created / updated by python
A reference to the "current" item in the sequence.

8. for loop example s = "ABCDE" for c in s: print(c) Outputs: A B C D E
# Doing the same thing with a while loop
i = 0
while i < len(s):
c = s[i]
print(c)
i += 1
s = "ABCDE"
for c in s:
print(c)
Outputs: A B C D E

9. range objects range([start,] end [, step]) Treated as a sequence
Useful when used with for loops
Example:
for i in range(1, 6,2): # range is (1,3,5)
print(i)
s = "ABCDE"
for i in range(len(s)): # range is (0,1,2,3,4)
print(i, s[i])

10. a new random function random.choice(sequence)
Returns a random element of sequence.
s = "ABCDEFG"
print(random.choice(s))
Here's how we could do this ourself:
Indx = random.randint(0,len(s)-1)
X = s[Indx] # Same as x = random.choice(s)

11. Concatenation Combining two sequences into a new one. s1 = "ABCDEF"
s2 = "GHI"
s3 = s1 + s2

12. count method All sequences have this method.
Returns the # of times that value appears
S = "ABCDEFGBH"
S.count("B") # returns 2
S.count("AB") # returns 1
S.count("I") # returns 0

13. in operator
val in seq
Returns True or False if the given value appears in the sequence.
Examples:
S = "ABCD"
"B" in S # True
"X" in S # False
"AB" in S # True

14. Tuples A (possibly) heterogenous group of python objects.
Deliminted by parentheses (when creating)
Examples:
T1 = (1, 2, 3, 4, 5)
T2 = ("ABC", "DEF", "GHI")
T2[1] # "DEF"
T2[1][0] # "D"
T3 = (1.7, "XYZ", 19)
T4 = () # Empty tuple
T5 = (5,) # Tuple with only 1 elem.
T6 = (10, 11, (12, 13), 14) # Nested
T7 = (screen, marioSurf, luigiSurf)

15. Tuples, cont. All the previous operations, work with tuples [Try it!]
Where have we seen tuples already?
Colors (pygame.draw.xxx, screen.fill)
Positions (pygame.draw.xxx)
Window Dimensions (pygame.display.set_mode)
Point-lists (pygame.draw.polygon)
Rectangles (pygame.draw.rect)

16. 2D Tuples A common name for a tuple with tuple (sub-)elements Example:
T[1] + T[2] # (3,2,9,1,-2,10)
T[1] + T[1] # (3,2,9,3,2,9)
T[1][1] + T[2][-1] # 12

17. Tuple un-packing Another way to extract values from a sequence.
x,y,z = T # x is 1, y is 2, z is 3
One catch: #variables on left must be == len(T)

18. Lists Like a tuple, but they are mutable.
You can add elements
You can remove elements
You can change elements
You can share references to the same thing.
Delimited by square brackets.
Note: All sequences use square brackets for slicing / indexing.
The delimiters are only relevant when creating.

19. Lists Adding an element:
H = L + G # H is now [4, 5, 1.0, 2.0, 3.0]
G.append(9.0) # G is now [1.0, 2.0, 3.0, 9.0]
G.insert(0, 11.0) # G is [11.0,1.0,2.0,3.0,9.0]
G.insert(2, 0.0) # G is [11.0,1.0,0.0,2.0,3.0,9.0]
Note: these are both methods of all list objects.

20. Lists, cont. Removing an element L = [1.3, 9.2, 5.8, 1.7, 1.3, 2.1]
Using the remove method
L.remove(9.2) # L is [1.3,5.8,1.7,1.3,2.1]
L.remove(1.3) # L is [5.8, 1.7, 1.3, 2.1]
L.remove(L[0]) # L is [1.7, 1.3, 2.1]
L.remove(2.2) # ERROR!
Using the del operator
del L[0]
Note: del can be used to "un-define" any variable, not just mutable seq. variables.

21. Lists, cont. Changing elements L = [1.1, 2.2, 3.3, 4.4, 5.5]
L[0] = 9.9 # L is [9.9, 2.2, 3.3, 4.4, 5.5]
L[1:3] = [10.1, 11.11, 12.12]
print(L) # [9.9, 10.1, 11.1, 12.12, 4.4, 5.5]

22. Lists & Mutability
Mutability means we can change values of a mutable object after it’s created.
L = [1.2, 3.7, 5.0, 9.9, 10.3]
This is changing the list:
L[2] = 4.2
This is re-creating the list (we can do this with immutable types):
L = L[:2] + [4.2,] + L[3:]

23. Lists & Mutability, cont.
Mutability also means a variable of a mutable type is a reference (or pointer) to the memory location.
L = [1.2, 3.7, 5.0, 9.9, 10.3]
Address
Value
This is L
1024
8000 …
This is the actual
1.2
list data.
8004
3.7
(4 bytes per float)
8008
5.0
8012
9.9
8016
10.3

24. Lists & Mutability, cont.
This is important because assigning a new variable = to the mutable object simply copies the reference.
L = [1.2, 3.7, 5.0, 9.9, 10.3]
Address
Value
This is L
1024
8000 …
This is the actual
1.2
list data.
8004
3.7
(4 bytes per float)
8008 99
8012
9.9
8016
10.3
Address
Value
This is L
1024
8000 …
This is the actual
1.2
list data.
8004
3.7
(4 bytes per float)
8008
5.0
8012
9.9
8016
10.3
K = L
K[2] = 1000
print(K)
[1.2, 3.7, 1000, 9.9, 10.3]
print(L)
[1.2, 3.7, 1000, 9.9, 10.3]
This is K
12100
8000 …

25. Lists & Mutability We’ll see other mutable objects in python:
Dictionaries
Objects (from Object-Oriented Programming)

26. Event (input) handling
Part III
Event (input) handling

27. Input and Windowed-OS’s
Windowed OS’s (Windows, OSX, GUI-linux’s) send messages (events) when the user does something “on” the window:
Move mouse
Press a key
Close the window
Resize the window
Minimize / maximize
The window notifies the program when these first happen.
The application can respond to these (event handling)

28. Input and Windowed OS’s
An application can also use device-polling to get the current state of an input device.
“Is the ‘a’ key currently pressed?
Where is the mouse cursor?

29. Event handling in pygame
The pygame.event.get() function returns a list of new event objects.
Each object has a .type attribute (a variable)
You can compare it to pygame.XYZ variables to determine its type.
Depending on the type, the event object may have other
Calling pygame.event.get() empties the event queue for your window.

30. Examples Write a program which:
Can be closed by clicking the ‘X’ or by pressing escape.
Can be resized / fullscreen-ed
Draws a square at a random location when a key is pressed.
Moves a circle when the arrow keys are pressed.

31. Device Polling in pygame
Emptying the event queue saves input device state to various module variables
pygame.mouse.xyz
pygame.keyboard.xyz
pygame.joystick.xyz
You can call functions in these modules to determine the current state of the device.
If you’re not doing event handling, you must call pygame.event.pump() to process events.

32. Device polling important func’s
Mouse
pygame.mouse.get_pos()
pygame.mouse.get_pressed()
Keyboard
pygame.key.get_pressed()
Joysticks: …

33. Example Write a program which:
Draws a circle at the mouse cursor’s location
That can’t go off-screen
Draws a square controllable by the arrow keys.
Quits by pressing escape.

34. A few pygame tidbits Clock objects Sounds / Music Font objects
tick
get_fps
Sounds / Music
loading and playing
Font objects
rendering
blitting
Images (sprites)
loading and blitting
Off-screen Surfaces

35. Back to sequences…

36. Examples Example1. Star-field Example2. Space-invaders start
Moving
Changing colors
Example2. Space-invaders start
Moving spaceship
Fire key
Moving (& Fading) bullets
Power-up: stream of bullets
Example3. Space-invaders, pt II
Line of aliens
Hit-detection
Sounds

37. Dictionary A sequence of key-value pairs
Key is usually a string or integer
Value can be any python object
There is no order to the elements
So, there isn't a first or a last element
Python uses its own algorithm to order elements (for efficient access)
You access values by using the key.

38. Dictionary operations
Creating an empty dictionary:
D = {}
Creating a dictionary with string-float pairs
D2 = {"ABC":42.9, "DEF":13.8, "GHI":-19.1}
Getting a list of keys or values
D2.keys() # Returns ["ABC", "GHI", "DEF"] or something like this.
Getting a list of values
D2.values() # Returns [42.9, -19.1, 13.8]

39. Dictionary Index'ing Using an existing element
print(D2["ABC"])
Changing a value for an existing key
D2["ABC"] = 100.3
Adding a new value
D2["XYZ"] = 0.0
Note how this looks the same as modifying.

40. Dictionary examples [Using integers as a key]
[Load a sequence of sounds]