Practice final exam answers

CS 121B, Spring 2014

1

def report(lis):
    desirable = 0
    borderline = 0
    high = 0
    # invar:  desirable, borderline, and high contain counts of the 
    #   readings in those ranges for all readings seen so far
    for chol in lis:
        if chol < 200:
            desirable = desirable + 1
        elif chol < 240:
            borderline = borderline + 1
        else:
            high = high + 1
    total = desirable + borderline + high
    return "{:.1%} desirable, {:.1%} borderline, {:.1%} high".format(
        desirable/total, borderline/total, high/total)

2

def countInput(prompt):
    print(prompt)
    lines = 0
    words = 0
    chars = 0
    # invar:  lines, words, and chars hold the counts of lines, 
    #   words, and characters seen so far
    nextline = input()
    while nextline != '':
        lines = lines + 1
        words = words + len(nextline.split())
        chars = chars + len(nextline) + 1
        nextline = input()
    print("Summary:  {:d} words, {:d} characters".format(words,chars))
    return lines

3

# iterative solution

def replaceD(lis, sdict):
    retval = []
    # invar:  retval holds all words seen so far, after replacing words that 
    #   are keys in sdict with those keys' values, capitalized
    for word in lis:
        if word in sdict:
            retval = retval + [sdict[word].upper()]
        else:
            retval = retval + [word]
    return retval


# recursive solution

def replaceD(lis, sdict):
    if lis == []:
        return []
    # assert:  at least one element in lis
    elif lis[0] in sdict:
        return [sdict[lis[0]].upper()] + replaceD(lis[1:], sdict)
    # assert:  first element is not a key in sdict
    else:
        return [lis[0]] + replaceD(lis[1:], sdict)

4

def nnn(lis):  # any possibly nested list
    if lis == []:
        return []
    elif type(lis[0]) == int or type(lis[0]) == float:
        return [-lis[0]] + nnn(lis[1:])
    elif type(lis[0]) == list:
        return [nnn(lis[0])] + nnn(lis[1:])
    else:
        return [lis[0]] + nnn(lis[1:])

5

Mapper:

# IN keys and values:
#   key: holds a line number, colon, then chapter name
#   value: a line of text
# OUT keys and values:
#   key: a word from that text, in lower case
#   value:  chapter name, space, line number for that word

def mapper(key, value):
    lis = key.split(':')
    outval = lis[1] + ' ' + lis[0] 
    for word in value.split():
        Wmr.emit(word.lower(), outval)

Reducer:

# IN keys and values:
#   key: a word from that text, in lower case
#   value:  chapter name, space, line number for that word
# OUT keys and values:  
#   key: holds a word from that text, in lower case  
#   value: chapter name, space, line number, comma, and space for
#      each occurrence of that word in lines of input text

def reducer(key, iter):
    accum = ''
    # invar:  accum holds values seen so far, each followed by comma and space
    for val in iter:
        accum = accum + val + ', '
    Wmr.emit(key, accum)

6

6 a

class Firm:
    '''Represents a business firm
       State variables:
         name -- String, name of this firm
         size -- non-negative integer, the number of employees in this firm.'''
    
    def __init__(self, name, size):
        self.name = name
        self.size = size
        
    def getName(self):
        return self.name

    def getSize(self):
        return self.size

    def setName(self, newname):
        prev = self.name
        self.name = newname
        return prev

6 b

class Corp(Firm):
    '''Represents an incorporated business firm
       State variables:
         id -- Integer, an id for this firm.
         nextId -- Integer, the lowest unused value for a firm {\tt id},
           initially 1.  (CLASS VARIABLE)'''

    nextId = 1

    def __init__(self, name, size):
        Firm.__init__(self, name, size)
        self.id = Corp.nextId
        Corp.nextId = Corp.nextId + 1

    def __str__(self):
        return "{:d}. {:s} ({:d} employees)".format(
            self.id, self.name, self.size)

Alternative implementation of __str__()

    def __str__(self):
        return str(self.id) + '. ' + self.name + ' (' + \
            str(self.size) + ' employees)'

7

    import turtle
    win = turtle.Screen()
    tom = turtle.Turtle()

    tom.penup()
    tom.goto(25,60)
    tom.pendown()
    tom.right(90)
    tom.forward(20)
    tom.left(90)
    tom.forward(50)
    tom.left(90)
    tom.forward(20)
    tom.left(90)
    tom.color('red')
    tom.forward(50)
    tom.penup()
    tom.color('black')
    tom.goto(45,45)
    tom.pendown()
    tom.write("Tools")

8

def quarterGray(img):
    img2 = image.EmptyImage(img.getWidth(), img.getHeight())
    for row in range(img.getHeight()):
        for col in range(img.getWidth()):
            pixel = img.getPixel(col, row)
            if row < img.getHeight()/2 and col < img.getWidth()/2:
                mean = int((pixel[0] + pixel[1] + pixel[2])/3)
                pixel = image.Pixel(mean,mean,mean)
            img2.setPixel(col, row, pixel)
    return img2
    

9

9 a

A function call is a language expression indicating that the instructions of a function should be carried out. Example:

   math.sqrt(7)

9 b

An object is a programming structure that consists of named memory locations (state variables) together with operations on the values in those memory locations (methods). Example:

  acct = Account(500)

defines an Account object having as state variables balance and initBal, both initialized at 500

9 c

A constructor is a specialized function for creating a new object and assigning initial values to that object's state variables. Example:

  acct = Account(500)

for the Account example discussed in class.