4.14. Group Work - Conditionals and Logic

It is best to use a POGIL approach with the following. In POGIL students work in groups on activities and each member has an assigned role. For more information see https://cspogil.org/Home.

Note

If you work in a group, have only one member of the group fill in the answers on this page. You will be able to share your answers with the group at the bottom of the page.

Content Learning Objectives

After completing this activity, students should be able to:

• Evaluate boolean expressions with comparison operators (<, >, <=, >=, ==, !=).

• Explain the syntax and meaning of if/else statements and indented blocks.

• Evaluate boolean expressions that involve comparisons with and, or, and not.

Process Skill Goals:

During the activity, students should make progress toward:

• Evaluating complex logic expressions based on operator precedence (Critical Thinking).

4.14.1. Comparison Operators

In Python, a comparison (e.g., 100 < 200) will yield a Boolean value of either True or False. Most data types (including int, float, str, list, and tuple) can be compared using the following operators:

Less than (<), less than or equal (<=), greater than (>), greater than or equal (>=), equal (==), and not equal (!=).

Run this code to see what it prints. Each line of code may or may not print something.

print(type(True)) print(type(3 < 4)) print(3 < 4) three = 3 four = 4 print(three == four) check = three > four print(check) print(type(check)) three = four print(three == four) # print(three = four)

Q-2: What would be printed if the commented line above was uncommented?

• True
• Incorrect! Although the variable "three" does equal "four", the == operator has a return value, while the = operator does not. Try again.
• False
• Incorrect! "three" has been set equal to "four", but this is irrelevant because the = operator does not have a return value. Try again.
• class 'bool'
• Incorrect! "print(type())" prints output that looks like that, not just "print()". Try again.
• TypeError
• Correct! "three = four" (with a single equals sign) doesn't return anything, so it cannot be printed and a TypeError occurs.

Q-4: Match examples from the previous code block to their appropriate term. Incorrect - keep trying!
• check
• Boolean variables
• <, ==, >
• Boolean operators
• 3 < 4, three == four, three > four
• Boolean expressions

Q-5: Which of these Boolean expressions evaluates to False?

• 5 != 6
• Incorrect! 5 does not equal 6, which makes this expression True. Try again.
• 2 + 2 != 5
• Incorrect! 4 does not equal 5, so this expression is True. Try again.
• 4 + 6 != 11 - 1
• Correct! 10 equals 10, so the statement "10 != 10" is False.

4.14.2. if/else Statements

An if statement makes it possible to control what code will be executed in a program, based on a condition. For example:

Run this code to see what it prints.

number = int(input("Enter an integer: ")) if number < 0: print(number, "is negative") else: print(number, "is a fine number") print("Until next time...")

Python uses indentation to define the structure of programs. The line indented under the if statement is executed only when number < 0 is True. Likewise, the line indented under the else statement is executed only when number < 0 is False.

Statements that are indented under an if/else statement are executed based on the status of the if’s condition. Statements indented at the same level as the if/else statement later in the program are always executed. If you indent code incorrectly or inconsistently, a SyntaxError: unexpected indent may be in your future.

Modify this code to print (number) is 10 if number equals 10, and (number) is not 10 otherwise.

number = int(input("Enter an integer: ")) # add your code here

Q-10: True or False: An if statement must always be followed by an else statement.

• True
• Incorrect! An if statement does not necessarily need to be followed by an else statement. Try again.
• False
• Correct! An else statement must follow an if statement, however.

4.14.3. Boolean Operations

Expressions may include Boolean operators to implement basic logic. If all three operators appear in the same expression, Python will evaluate not first, then and, and finally or. If there are multiple of the same operator, they are evaluated from left to right.

Run this code to see what it prints.

a = 3 b = 4 c = 5 print(a < b and b < c) print(a < b or b < c) print(a < b and b > c) print(a < b or b > c) print(not a < b) print(a > b or not a > c and b > c)

Q-12: What data type would be the result of a < b? What about the result of a < b and b < c? Use the values of a, b, and c from the code block above.

• bool, bool
• Correct! The type of each is bool; both are Boolean expressions.
• True, bool
• Incorrect! True is not a data type. Try again.
• True, True
• Incorrect! True is not a data type. Try again.
• bool, True
• Incorrect! True is not a data type. Try again.

Q-13: What would be the value of a < b? What about the value of a < b and b < c? Use the values of a, b, and c from the code block above.

• True, True
• Correct! The value of each statement is True.
• True, False
• Incorrect! 4 is less than 5, so "b < c" is True. Try again.
• False, False
• Incorrect! 3 is less than 4 and 4 is less than 5. Try again.
• False, True
• Incorrect! 3 is less than 4, so "a < b" is True. Try again.

Q-14: If two True Boolean expressions are compared using the and operator, what is the resulting Boolean value? What if you compare two False expressions instead?

• True, True
• Incorrect! "and" only returns True if the expressions on both sides are True. Try again.
• True, False
• Correct! "and" only returns True if the expressions on both sides are True and returns False in any other situation.
• False, False
• Incorrect! "and" only returns True if the expressions on both sides are True. Try again.
• False, True
• Incorrect! "and" only returns True if the expressions on both sides are True. Try again.

Q-15: If two True Boolean expressions are compared using the or operator, what is the resulting Boolean value? What if you compare two False expressions instead?

• True, True
• Incorrect! "or" only returns True if the expressions on one or both sides are True. Try again.
• True, False
• Correct! "or" only returns True if the expressions on one or both sides are True and returns False if both sides are false.
• False, False
• Incorrect! "or" only returns True if the expressions on one or both sides are True. Try again.
• False, True
• Incorrect! "or" only returns True if the expressions on one or both sides are True. Try again.

Q-16: If a True and a False Boolean expression are compared using the and operator, what is the resulting Boolean value? What if you use the or operator instead?

• True, True
• Incorrect! "and" only returns True if the expressions on both sides are True. Try again.
• True, False
• Incorrect! "or" returns True if the expressions on one or both sides are True. Try again.
• False, False
• Incorrect! "or" returns True if the expressions on one or both sides are True. Try again.
• False, True
• Correct! "and" needs both sides to be True, while "or" only needs of of them.

Suppose you wanted to print the sum of x and y only when both x and y are positive. Write a block of code to achieve this that uses only one if statement.

x = int(input("Enter an integer: ")) y = int(input("Enter an integer: ")) # add your code here

Rewrite your code from the previous code block using the not operator. Your answer should yield the same result as before, not the opposite, and still only use one if statement. Hint: you’ll need to change the > signs!

x = int(input("Enter an integer: ")) y = int(input("Enter an integer: ")) # add your code here

Suppose that you instead wanted to print the sum of x and y except when both x and y are positive. Write a block of code to achieve this that only uses one if statement.

x = int(input("Enter an integer: ")) y = int(input("Enter an integer: ")) # add your code here

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