EECS 110: Lec 17: Review for the Final Exam Aleksandar Kuzmanovic Northwestern University

General Info Wednesday, June 4, 8:30-10:30am, Tech L361 To be done individually Closed book One 8.5” by 11” sheet of paper permitted Please do not discuss the exam with others until everyone has taken it. There are six questions. Each question is worth 20 points. Hence, you can acquire 120 points in total. Those who get > 90 points will get an A. Those who get >80 points and <90 points will get a B, etc. 2

Final Exam 6 questions: 1.Misc 2.Loops and recursion 3.Mutation 4.2D in Python 5.Dictionaries 6.Classes and Objects 3

What does this code do? print('It keeps on') while True: print('going and') print('Phew! I\'m done!’) Q2 (and Q1): Loops ( while and for ) and Recursion

Extreme Looping Anatomy of a while loop: print('It keeps on') while True: print('going and') print('Phew! I\'m done!') “while” loop the loop keeps on running as long as this test is True This won't print until the while loop finishes - in this case, never!

What do these two loops print? n = 0 for c in 'forty-two': if c not in 'aeiou': n += 1 print(n) ?? n = 3 while n > 1: print(n) if n%2 == 0: n = n/2 else: n = 3*n + 1

What do these two loops print? n = 0 for c in 'forty-two': if c not in 'aeiou': n += 1 print(n) 7 n = 3 while n > 1: print(n) if n%2 == 0: n = n/2 else: n = 3*n + 1 ??

What do these two loops print? n = 0 for c in 'forty-two': if c not in 'aeiou': n += 1 print(n) 7 n = 3 while n > 1: print(n) if n%2 == 0: n = n/2 else: n = 3*n + 1 3

What do these two loops print? n = 0 for c in 'forty-two': if c not in 'aeiou': n += 1 print(n) 7 n = 3 while n > 1: print(n) if n%2 == 0: n = n/2 else: n = 3*n

What do these two loops print? n = 0 for c in 'forty-two': if c not in 'aeiou': n += 1 print(n) 7 n = 3 while n > 1: print(n) if n%2 == 0: n = n/2 else: n = 3*n

What do these two loops print? n = 0 for c in 'forty-two': if c not in 'aeiou': n += 1 print(n) 7 n = 3 while n > 1: print(n) if n%2 == 0: n = n/2 else: n = 3*n

What do these two loops print? n = 0 for c in 'forty-two': if c not in 'aeiou': n += 1 print(n) 7 n = 3 while n > 1: print(n) if n%2 == 0: n = n/2 else: n = 3*n

What do these two loops print? n = 0 for c in 'forty-two': if c not in 'aeiou': n += 1 print(n) 7 n = 3 while n > 1: print(n) if n%2 == 0: n = n/2 else: n = 3*n

What do these two loops print? n = 0 for c in 'forty-two': if c not in 'aeiou': n += 1 print(n) 7 n = 3 while n > 1: print(n) if n%2 == 0: n = n/2 else: n = 3*n

for e! for x in range(8): print('x is', x) print('Phew!') x is assigned each value from this sequence the BODY or BLOCK of the for loop runs with that x Code AFTER the loop will not run until the loop is finished LOOP back to step 1 for EACH value in the list

Two kinds of for loops Element-based Loops L = [ 42, -10, 4 ] x i 0 12 Index-based Loops sum = 0 for x in L: sum += x sum = 0 for i in range(len(L)) : sum += L[i] L[i]

Q2: Recursion vs. Loops Solve a problem using: (a) recursion (b) loops

Example 1a: fac(N) with recursion Let recursion do the work for you. def fac(N): if N <= 1: return 1 else: return N * fac(N-1) You handle the base case – the easiest possible case to think of! Recursion does almost all of the rest of the problem! Exploit self-similarity Produce short, elegant code Less work !

Example 1b: fac with for (v1) def fact( n ): answer = 1 for x in range(n): answer = answer * x return answer

def fact( n ): answer = 1 for x in range(1,n+1): answer = answer * x return answer Example 1b: fac with for (v2)

def sum(L): """ input: a list of numbers, L output: L's sum """ if len(L) == 0: return 0.0 else: return L[0] + sum(L[1:]) Base Case if the input has no elements, its sum is zero Recursive Case if L does have an element, add that element's value to the sum of the REST of the list… This input to the recursive call must be "smaller" somehow… Example 2a: sum(L) with recursion

Example 2b: sum(L) with for def sum( L ): """ returns the sum of L's elements """ sum = 0 for x in L: sum = sum + x return sum Finding the sum of a list: Accumulator!

Example 3: uniquify(L) Problem: Part A Use loops (no recursion!) to write a Python function uniquify(L), which takes in any list L and returns a list of the distinct elements in the list L. The order of the elements may be preserved, but they do not have to be. For example, >>> uniquify( [ 42, 'spam', 42, 5, 42, 5, 'spam', 42, 5, 5, 5 ] ) [ 'spam', 42, 5 ] >>> L = range(4) + range(3) >>> uniquify(L) [ 3, 0, 1, 2 ]

Example 3a: uniquify(L) def uniquify(L): s = [] for i in range(len(L)): n=0 for j in range(i+1,len(L)): if L[i] == L[j]: n=1 if n == 0: s=s+[L[i]] return s

Example 3: uniquify(L) Problem: Part B Use recursion (no loops!) to write a Python function uniquify(L), which takes in any list L and returns a list of the distinct elements in the list L. The order of the elements may be preserved, but they do not have to be. For example, >>> uniquify( [ 42, 'spam', 42, 5, 42, 5, 'spam', 42, 5, 5, 5 ] ) [ 'spam', 42, 5 ]

Example 3b: uniquify(L) def uniquify(L): if len(L) == 0: return [] else: if L[0] in L[1:]: return uniquify(L[1:]) else: return L[0:1] + uniquify(L[1:])

Changeable types: dictionary Unchangeable types: list tuple string int float bool Mutable vs. Immutable data Question 3: Mutation

Functions and (immutable) Variables def fact(a): result = 1 while a > 0: result *= a a -= 1 return result >>> x = 5 >>> y = fact(x) >>> x ??

Functions and (immutable) Variables def fact(a): result = 1 while a > 0: result *= a a -= 1 return result >>> x = 5 >>> y = fact(x) >>> x 5

Functions and (immutable) Variables def swap(a, b): temp = a a = b b = temp >>> x = 5 >>> y = 10 >>> swap(x, y) >>> print(x, y) ?? x y a b temp

Functions and (immutable) Variables def swap(a, b): temp = a a = b b = temp >>> x = 5 >>> y = 10 >>> swap(x, y) >>> print(x, y) 5, 10 x y a b temp

Functions and Mutable Types def swap(L, i1, i2): temp = L[i1] L[i1] = L[i2] L[i2] = temp >>> MyL = [2, 3, 4, 1] >>> swap(myL, 0, 3) >>> print(myL) ?? RAM MyL L i1 i

Functions and Mutable Types def swap(L, i1, i2): temp = L[i1] L[i1] = L[i2] L[i2] = temp >>> MyL = [2, 3, 4, 1] >>> swap(myL, 0, 3) >>> print(myL) [1,3,4,2] RAM MyL L i1 i

The conclusion You can change the contents of lists in functions that take those lists as input. Those changes will be visible everywhere. (actually, lists or any mutable objects) (immutable objects are safe, however)

Example 1 def zeroOdd1( L ): for i in range(len(L)): if L[i] % 2 == 1: L[i] = 0 def zeroOdd2( L ): for i in L: if i % 2 == 1: i = 0 >>> L = [1, 2, 3, 4, 5] >>> zeroOdd1(L) >>> L ?? >>> L = [1, 2, 3, 4, 5] >>> zeroOdd2(L) >>> L

Example 1 def zeroOdd1( L ): for i in range(len(L)): if L[i] % 2 == 1: L[i] = 0 def zeroOdd2( L ): for i in L: if i % 2 == 1: i = 0 >>> L = [1, 2, 3, 4, 5] >>> zeroOdd1(L) >>> L [0,2,0,4,0] >>> L = [1, 2, 3, 4, 5] >>> zeroOdd2(L) >>> L ??

Example 1 def zeroOdd1( L ): for i in range(len(L)): if L[i] % 2 == 1: L[i] = 0 def zeroOdd2( L ): for i in L: if i % 2 == 1: i = 0 >>> L = [1, 2, 3, 4, 5] >>> zeroOdd1(L) >>> L [0,2,0,4,0] >>> L = [1, 2, 3, 4, 5] >>> zeroOdd2(L) >>> L [1,2,3,4,5]

Example 2 What are the values of A, B, C and D at the indicated points? def mystery1(L1, N, C): for i in L1: if i == N: C += 1 return C def mystery2(C): for i in range(len(C)): C[i] *= 2 >>> A = [22, 10, 30] >>> B = 22 >>> C = 0 >>> D = mystery1(A, B, C) >>> mystery2(A) 1 2 1) 2)

Example 2 What are the values of A, B, C and D at the indicated points? def mystery1(L1, N, C): for i in L1: if i == N: C += 1 return C def mystery2(C): for i in range(len(C)): C[i] *= 2 >>> A = [22, 10, 30] >>> B = 22 >>> C = 0 >>> D = mystery1(A, B, C) >>> mystery2(A) >>> A 1 1) 2) A = [22, 10, 30] B = 22 C = 0 D = 1 2

Example 2 What are the values of A, B, C and D at the indicated points? def mystery1(L1, N, C): for i in L1: if i == N: C += 1 return C def mystery2(C): for i in range(len(C)): C[i] *= 2 >>> A = [22, 10, 30] >>> B = 22 >>> C = 0 >>> D = mystery1(A, B, C) >>> mystery2(A) >>> A 1 2 1) 2) A = [22, 10, 30] B = 22 C = 0 D = 1 A = [44, 20, 60]

Example 3: avoiding mutation Problem: Write function x(L), …., The list L should NOT mutate, i.e., it should stay the same.

Example 3: avoiding mutation def x(L): # some function … L2=copyList(L) … # manipulate L2, not L def copyList(L): L2 = [] for x in L: L2 += [x] return L2

Handling rectangular arrays … list A A[0] A[1] A[2] How many rows does A have, in general ? How many columns does A have, in general ? A[2][3] A[0][0] Question 4: 2D in Python

Rectangular arrays Handling rectangular arrays … list A A[0] A[1] A[2] How many rows does A have, in general ? How many columns does A have, in general ? A[2][3] A[0][0] len(A)

Rectangular arrays Handling rectangular arrays … list A A[0] A[1] A[2] How many rows does A have, in general ? How many columns does A have, in general ? A[2][3] A[0][0] len(A) len(A[0])

Which one works? How could we create a rectangular array (of default data, 0 ), given its height and width ? or A = [ [0]*width ]*height A = [ [0]*height ]*width

Which one works? How could we create a rectangular array (of default data, 0 ), given its height and width ? A = [ [0]*width ]*height A = [ [0]*height ]*width

What's really going on? A = [ [0]*width ]*height inner = [0]*width A = [inner]*height

Creating a 2d array def create2dArray( width, height ): """ does just that """ A = [] # start with nothing for row in range( height ): for col in range( width ):

Creating a 2d array def create2dArray( width, height ): """ does just that """ A = [] # start with nothing for row in range( height ): A = A + [[]] for col in range( width ): A[row] = A[row] + [0] return A

Example 1: What 2d loops would create a checkerboard image? def checkerboard(): # loop over each pixel for col in range(width): for row in range(height):

Example 1: What 2d loops would create a checkerboard image? def checkerboard(): # loop over each pixel for col in range(width): for row in range(height): if (col/20 + row/20)%2 == 0: image.plotPoint( col, row )

Example 2: Write a function that returns the number of all-zero rows

def num_zero_rows(L): width = len(L[0]) height = len(L) n = 0 for row in range(height): k = 0 for col in range(width): if L[row][col] != 0: k = 1 if k == 0: n += 1 return n

In Python a dictionary is a set of key - value pairs. It's a list where the index can be any immutable-type key. >>> d = {} >>> d[1988] = 'dragon' >>> d[1989] = 'snake' >>> d {1988: 'dragon', 1989: 'snake'} >>> d[1988] 'dragon' >>> d[1987] key error Question 5: Dictionaries

Lists vs. Dictionaries In Python a dictionary is a set of key - value pairs. It's a list where the index can be any immutable-type key. >>> d = {} >>> d[1988] = 'dragon' >>> d[1989] = 'snake' >>> d {1988: 'dragon', 1989: 'snake'} >>> d[1988] 'dragon' >>> d[1987] key error creates an empty dictionary, d 1988 is the key 'dragon' is the value 1989 is the key 'snake' is the value Anyone seen this before? Retrieve data as with lists… or almost !

More on dictionaries Dictionaries have lots of built-in methods: >>> d = {1988: 'dragon', 1989: 'snake'} >>> d.keys() [ 1989, 1988 ] >>> 1988 in d True >>> 1969 in d False >>> d.pop( 1988 ) 'dragon' delete a key (and its value) check if a key is present get all keys

A family dictionary?

A family dictionary… T = {'abe' :['homer','herb'], 'jackie':['marge','patty','selma'], 'homer' :['hugo','bart','lisa','maggie'], 'marge' :['hugo','bart','lisa','maggie']} keys can be any immutable type values can be any type at all… T['abe'] How to get 'selma' from T ?

A family dictionary… T = {'abe' :['homer','herb'], 'jackie':['marge','patty','selma'], 'homer' :['hugo','bart','lisa','maggie'], 'marge' :['hugo','bart','lisa','maggie']} keys can be any immutable type values can be any type at all… T['abe'] How to get 'selma' from T ? ['homer','herb']

A family dictionary… T = {'abe' :['homer','herb'], 'jackie':['marge','patty','selma'], 'homer' :['hugo','bart','lisa','maggie'], 'marge' :['hugo','bart','lisa','maggie']} keys can be any immutable type values can be any type at all… T['abe'] How to get 'selma' from T ? ['homer','herb'] T['jackie'][2] (T['jackie'] is a list)

Example 1: favChild def favChild( person, Tree ): """ person is a name (a string) Tree is a dictionary of children returns person's favorite child """ if person in Tree: Kids = Tree[person] Kids.sort() return Kids[0] else: return 'no children' Who is favored ? Side effects ?

Example 2: addChild def addChild( person, Tree, jr ): """ adds person's new child to Tree ""“ For example, >>> addChild( 'lisa', T, 'abejr' )

def addChild( person, Tree, jr ): """ adds person's new child to Tree ""“ if person in Tree: kids = Tree[person] kids += [jr] For example, >>> addChild( 'lisa', T, 'abejr' ) Example 2: addChild

EXAMPLE 3: Based on favChild, write favGChild to return the first grandchild alphabetically - or return 'no one' if there are none. def favChild( person, Tree ): if person in Tree: Kids = Tree[person] Kids.sort() return Kids[0] else: return 'no children'

EXAMPLE 3: Based on favChild, write favGChild to return the first grandchild alphabetically - or return 'no one' if there are none. def favChild( person, Tree ): if person in Tree: Kids = Tree[person] Kids.sort() return Kids[0] else: return 'no children' def favGChild( person, Tree ): gChildren = [] if person in Tree : for child in Tree[person]: if child in Tree: gChildren += Tree[child] if gChildren == []: return 'no one' else: gChildren.sort() return gChildren[0]

An object-oriented programming language allows you to build your own customized types of variables. (1) A class is a type of variable. (2) An object is one such variable. There will typically be MANY objects of a single class. Question 6: Classes and Objects

Objects An object is a data structure (like a list), except (1) Its data elements have names chosen by the programmer. (2) Data elements are chosen & organized by the programmer (3) An object can have behaviors built-in by the programmer.

Objects An object is a data structure (like a list), except (1) Its data elements have names chosen by the programmer. (2) Data elements are chosen & organized by the programmer (3) An object can have behaviors built-in by the programmer. usually called "methods" instead of functions

class Date: """ a blueprint (class) for objects that represent calendar days """ def __init__( self, mo, dy, yr ): """ the Date constructor """ self.month = mo self.day = dy self.year = yr def __repr__( self ): """ used for printing Dates """ s = "%02d/%02d/%04d" % (self.month, self.day, self.year) return s The Date class

The Date Class this is an object of type Date >>> d = Date(1,1,2014) >>> d 1/1/2014 this is a CONSTRUCTOR … the repr esentation of a particular object of type Date

class Date: def __init__( self, mo, dy, yr ): def __repr__(self): def isLeapYear(self): def copy(self): """ returns a DIFFERENT object w/SAME date! """ def equals(self, d2): """ returns True if they represent the same date; False otherwise """ More Date

class Date: def __init__( self, mo, dy, yr ): def __repr__(self): def isLeapYear(self): def copy(self): """ returns a DIFFERENT object w/SAME date! ""“ return Date(self.month, self.day, self.year) def equals(self, d2): """ returns True if they represent the same date; False otherwise """ More Date

class Date: def __init__( self, mo, dy, yr ): def __repr__(self): def isLeapYear(self): def copy(self): """ returns a DIFFERENT object w/SAME date! ""“ return Date(self.month, self.day, self.year) def equals(self, d2): """ returns True if they represent the same date; False otherwise ""“ return self.month == d2.month and self.day == d2.day and self.year == d2.year More Date

Good luck with the Exam!