CHAPTER 4 AND 5 Section06: Sequences

General Description "Normal" 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.

Python sequences Immutable types (Ch. 4)  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.

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

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 Oranges

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) Apples-Oranges

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 is a variable, created / updated by python  A reference to the "current" item in the sequence. Equivalent to: i = 0 while i < len(seq): var = seq[i] i += 1 # for-loop block

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

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

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)

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

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

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

Review A few "quiz-ish" problems:  Write a function which takes a string as an argument and returns true if this string is a palindrome.  Write another function which also takes a string as an argument and returns the odd characters, in reverse sequence.  Example: func2("ABCDEFGHI") would return "HFDB"

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)

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)  Multiple return values from functions

2D Tuples A common name for a tuple with tuple (sub-)elements Example: T = ((15,8,7), (3,2,9), (1,-2,10)) T[0] # (15,8,7) T[0][1] # 8 T[0:2] # ((15,8,7), (3,2,9)) T[0][1:]# (8,7) 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

Tuple un-packing Another way to extract values from a sequence. T = (1, 2, 3) x,y,z = T # x is 1, y is 2, z is 3 One catch: #variables on left must be == len(T) Look familiar?  We've seen this with functions returning multiple values def foo(): x = random.randint(1,10) y = random.randint(1,10) return x, y # Equivalent to return (x, y) a, b = foo() # or… t = foo() a = t[0] b = t[1]

Review Write a function which takes a tuple of numbers as an arugment and returns the average of this tuple:  e.g. print(avg((3, 5, 15, 9))) would print 8.0  Make sure it will work with tuples of any size Use a for loop to "traverse" a tuple of this form: example = (("A", 5), ("joe", 3), ("!", 4)) and produce this output (make it work with tuples of any size): AAAAA joejoejoe !!!!

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.

Lists Adding an element: L = [4, 5] G = [1.0, 2.0, 3.0] 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.

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.

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] You can only do this to mutable objects. Even though tuples allow indexing, they don't allow changing.  A work around: T = (1.1, 2.2, 3.3, 4.4, 5.5) T = (9.9,) + T[1:] T = (T[0],) + (10.1, 11.1, 12.12) + T[3:] Note: has the same effect for lists, but we're re-creating the tuple (could be slow for large tuples)

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

Lists & Mutability, cont. This is important because assigning a new variable = to the mutable object simply copies the reference. AddressValue This is L …… …… This is the actual list data (4 bytes per float) …… >>> L = [1.2, 3.7, 5.0, 9.9, 10.3] >>> K = L This is K …… >>> K[2] = 1000 AddressValue This is L …… …… This is the actual list data (4 bytes per float) …… >>> print(K) [1.2, 3.7, 1000, 9.9, 10.3] >>> print(L) [1.2, 3.7, 1000, 9.9, 10.3]

Mutable objects as function parameters Recall: when you have a function call, the argument is copied to the parameter. This is still true, but with mutable objects (e.g. Lists), you're copying the reference. This means that when we change elements of the list in the function those changes are also reflected in the argument (in the caller). We'll take advantage of this…a lot.

Mutable objects as parameters Example: def func(a): """ a is a list of integers """ for i in range(len(a)): a[i] -= 1 L = [3.1, 2.9, 7.6, 4.0, 1.3, 10.4] H = [0.0, 12.3, 6.5] func(L) func(H) print(L) # Prints [2.1, 1.9, 6.6, 3.0, 0.3, 9.4] Print(H) # Prints [-1.0, 11.3, 5.5]

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

A bigger example Lame space-invaders clone  Bullets  Enemies moving in a group Data Structures  A list holding player data [x-pos, bullets]  A list of invaders [[x1,y1], [x2,y2], [x3,y3], …] Functions  drawInvaders(surf, L)  drawPlayer(surf, P)  handleCollisions(L, P) ...