2 IF-ELSE AND FLOW CONTROL

Boolean Values

The Boolean data type has only two values: True and False. They can be stored in variables and used in expressions, just like values of other data types. For each of the following, answer “yes” if it is a Python Boolean value and “no” if it is not.

1. 1. False

2. 2. 'True'

3. 3. false

4. 4. True

5. 5. 'false'

6. 6. true

Comparison Operators

Comparison operators, also called relational operators, compare two values and evaluate down to a Boolean True or False. For each of the following, answer “yes” if it is a Python comparison operator and “no” if it is not.

1. 7. =

2. 8. <

3. 9. =>

4. 10. =!

5. 11. !=

6. 12. ==

7. 13. >

8. 14. <=

To test your understanding of Python’s comparison operators, answer the following questions.

1. 15. What is the difference between the < and <= operators?

2. 16. What is the difference between the = and == operators?

3. 17. Why does 42 == 42.0 evaluate to True?

4. 18. Why does 42 == '42' evaluate to False?

5. 19. What happens if you enter 42 < 'hello' into the interactive shell?

Boolean Operators

The three Boolean operators (and, or, and not) are used to compare Boolean values. Like comparison operators, they evaluate expressions down to a Boolean True or False value.

Draw the truth tables for the Boolean operators in questions 20 through 22.

1. 20. and

2. 21. or

3. 22. not

What do the following expressions evaluate to?

1. 23. 2 + 2 > 4 or True

2. 24. True and 2 + 2 >= 4

3. 25. True and (True or False)

4. 26. (False or True) and True

5. 27. True and not False

6. 28. not (False or True)

7. 29. not False or True

8. 30. True and True and True and True and False

9. 31. False or False or False or True or False

You can use Boolean operators in an expression along with the comparison operators to evaluate the value of a variable. Answer the following questions.

1. 32. Say the variable is_raining is set to either True or False. Describe what the assignment statement is_raining = not is_raining does.

2. 33. If the variable name has the value 'Alice', which expression is correct: the expression name == 'Alice' or name == 'Bob' or the expression name == 'Alice' or 'Bob'?

Components of Flow Control

Flow control statements often start with a part called the condition and are always followed by a block of code called the clause. A flow control statement decides what to do based on whether its condition is True or False, and almost every flow control statement uses a condition. Python code can be grouped together in blocks, which you can identify by their level of indentation.

1. 34. When does a new block begin?

2. 35. Can a block be inside another block?

3. 36. A new block is expected after statements that end with what character?

4. 37. When does a block end?

5. 38. What is the program execution?

Questions 39 through 41 relate to the following program (which is labeled with line numbers):

1. name = 'Alitza'
2. if name == 'Dolly':
3.     print('Hello, Dolly!')
4. print('Done')

1. 39. How many blocks are in this program?

2. 40. On what line does the first block begin?

3. 41. On what line does the first block end?

Flow Control Statements

The most common type of flow control statement is the if statement. An if statement’s clause (that is, the block following the if statement) will execute if the statement’s condition is True and be skipped if the condition is False. The else and elif statements can follow if statements with other instructions.

Answer “yes” if the following are valid if statements, given a variable named eggs that has the value 12. Answer “no” if they are not valid statements.

1. 42. if eggs = 12:

2. 43. if eggs > 12

3. 44. if:

4. 45. if eggs == 12:

5. 46. if eggs != 'hello':

6. 47. if eggs < 12:

Answer “yes” if the following are valid else statements, given that they follow a statement if eggs == 12: and its if block. Answer “no” if they are not valid statements.

1. 48. else:

2. 49. else if eggs != 12:

3. 50. else

4. 51. else if not:

5. 52. else not:

Answer “yes” if the following are valid elif statements, given that they follow a statement if eggs == 12: and its if block. Answer “no” if they are not valid statements.

1. 53. elif:

2. 54. elif eggs != 12:

3. 55. else if eggs != 12:

4. 56. elif eggs == 12:

Examine the following flawed program:

password = 'swordfish'
if password == 'rosebud':
    print('Access granted.')
else:
    print('Access denied.')
elif password == 'swordfish':
    print('That is the old password.')

1. 57. Why does this program cause an error?

2. 58. How many elif statements (each with an elif block of code following it) can follow an if statement and an if block of code?

Fixing the Safe Temperature Program

The following program reports whether a given temperature is safe. It asks the user to enter a temperature in two parts. First, they should enter C or F to indicate the Celsius or Fahrenheit scale; second, they should enter the number of degrees. If the temperature is between 16 and 38 degrees Celsius (inclusive of 16 and 38) or between 60.8 and 100.4 degrees Fahrenheit (inclusive of 60.8 and 100.4), the program prints Safe. Outside of these temperature ranges, the program prints Dangerous.

This program has bugs, however. Rewrite the code to fix the errors. You may assume the user always enters valid inputs and not, say, X for the scale or hello for the number of degrees.

print('Enter C or F to indicate Celsius or Fahrenheit:')
scale = input()
print('Enter the number of degrees:')
degrees = int(input())
if scale == 'C':
    if degrees >= 16 or degrees <= 38:
        print('Dangerous')
    else:
        print('Dangerous')
elif scale == 'F':
    if degrees > 60.8 and degrees >= 100.4:
        print('Safe')
    else:
        print('Dangerous')

Test this program by entering a temperature in both the safe and dangerous ranges and in both the Celsius and Fahrenheit scales.

Save this program in a file named safeTemp.py.

Single-Expression Safe Temperature

It’s possible to write the safe temperature logic of the previous program in a single condition. Fill in the blank in the following program with this condition to make it work in the same way as the previous program:

print('Enter C or F to indicate Celsius or Fahrenheit:')
scale = input()
print('Enter the number of degrees:')
degrees = int(input())
if ____:
    print('Safe')
else:
    print('Dangerous')

This condition will be rather long. As a hint, you’ll need to have separate parts for Celsius and Fahrenheit, combined by an or operator. It should look something like this: (scale == 'C' and ____) or (scale == 'F' and ____).

Test this program by entering a temperature in both the safe and dangerous ranges and in both the Celsius and Fahrenheit scales.

Save this program in a file named safeTempExpr.py.

Fizz Buzz

Fizz Buzz is a common programming challenge that goes like this. Write a program that accepts an integer from the user. If the integer is divisible by 3, the program should print Fizz. If the integer is divisible by 5, the program should print Buzz. If the integer is divisible by 3 and 5, the program should print Fizz Buzz. Otherwise, the program should print the number the user entered. The output of this program should look something like this:

Enter an integer:
18
Fizz

Or this:

Enter an integer:
25
Buzz

Or this:

Enter an integer:
15
Fizz Buzz

Or this:

Enter an integer:
37
37

Here are some hints to help you write this program:

• Use the modulo operator to determine whether a number is divisible. If the condition number % 3 == 0 is True, then number is divisible by 3.
• Be sure to check whether the number is divisible by both 3 and 5 before checking whether the number is divisible by either 3 or 5. Otherwise, the number 15 won’t cause the program to print Fizz Buzz.

Save this program in a file named fizzBuzzNumber.py.