Introduction to Computer Science

Quiz 2 Study Guide

This page contains a list of things you should know before taking Quiz 2.

Booleans

We introduced booleans, a type for the “truth values” True and False. You should know how to create True and False values from various Python operators and how to describe conditions using Python expressions.

Exercises

For each of the following expressions, is the result True or False? Why?

• 5 > 4
• 4 >= 4
• 13 % 2 == 1
• 5 != 3
• True and False
• True or True
• False and False
• (3 > 4) or (4 < 5)
• (5 in range(5)) or (3 in “3”)

For each of the following conditions, write a Python expression which is True when that condition holds.

• A variable x contains the value 9.
• A variable y contains a number which is strictly greater than the number contained in a variable z.
• A variable s contains the empty string.
• A variable s contains a string that appears somewhere inside of the string in a variable t.
• A variable s contains no more than ten characters.
• The sum of the contents of the variables x and y is no greater than 10.

if statements

We introduced branching control flow in the form of if statements. You should know how if statements are executed; you should also be familiar with the associated elif and else blocks.

Exercises

Predict (without the use of a computer) what each of the following programs will print.

• for n in range(0,10):
    if n > 4:
      print n
    else:
      print "Small"
  

• for word in ["These","are","words","from","a","sentence"]:
    if len(word) > 4:
      print word
  

• for n in range(1,21):
    if n % 3 == 0:
      print 3
    elif n % 5 == 0:
      print 5
    if n % 7 == 0:
      print 7
  

Debugging

As with the last quiz, you should know how to find mistakes in small programs. Given a description of what a program should do and the program itself, you should be able to identify how it can be fixed. (Hint: there may be more than one thing wrong with a program.)

Exercises

For each of the following descriptions, a program is provided which is an incorrect implementation. Identify how that implementation can be corrected to conform with the description.

#1

Description

This program was intended to print all of the squares between 1 and 25 (inclusive).

Program

for n in range(5):
  print n*n

#2

Description

This program was intended to print only the vowels from the string provided by the user.

Program

phrase = raw_input("Please enter a phrase: ")
output = ""
for c in phrase:
  if c == ["a","e","i","o","u"]:
    output += c
print output

#3

Description

This program was intended to print all of the numbers between 1 and 1000 (inclusive) which are divisible by both 3 and 5.

Program

for n in range(1,1001):
  if n % 3 and 5 == 0:
    print n

#4

Description

This program sums the cubes of the numbers between 1 and 10 and then prints the result.

Program

total = 0
for n in range(1,11):
  total = n ** 3
print total