Lecture 5, Tue 01/23

Boolean Expressions, Conditions / Branching

Recorded Lecture: 1_23_24

Boolean Type

• In programming languages, the ability to tell the computer to perform one thing in a situation vs. another thing in another situation enables programmers to customize the program’s flow of execution
• There is a specific type to represent a True or False value called Boolean
• We can program logic in our computer programs based on Boolean logic
• Example:

x = True
y = False

print(x)
print(type(x))
print(y)
print(type(y))

Relational Operators

• Python provides specific operators that will return a Boolean value
• Examples: == (equality), != (inequality), < (less than), <= (less than or equal), > (greater than), >= (greater than or equal)
• Example:

print(4 < 6) # True
print(4 == 4) # True
print(4 != 4) # False
print(4 >= 5) # False
print(4 <= 4) # True

Branching with if Statements

• The basic construct to tell a program to do one thing or another is an if statement

Syntax:
if BOOLEAN_EXPRESSION:
	STATEMENT(S) # note these statements are tab indented

• The steps are to first evaluate the BOOLEAN_EXPRESSION (evaluates to True or False)
  • If True, execute the STATEMENT(S)
  • If False, skip STATEMENT(S) and continue execution
  • Indentation is important! It tells Pytho which statements belong to a specific block
• Example:

milesDriven = 250
print("Should you pull over and fill up your gas tank?")

if milesDriven > 200:
	print("Yes, you need gasoline") # indented (part of the if block)

print("Drive safe") # not part of if block since not indented

Branching With if-else Statements

• What if we want to do something specific i the event that the BOOLEAN_CONDITION is False?
• We can pair if statements with an else portion

Syntax:
if BOOLEAN_EXPRESSION:
	STATEMENT(S) #1
else:
	STATEMENT(S) #2

• The steps are to first evaluate the BOOLEAN_EXPRESSION (evaluates to True or False)
  • If True, only execute STATEMENT(S) #1
  • If False, only execute STATEMENT(S) #2
• Example:

milesDriven = 50
print("Should you pull over and fill up your gas tank?")
if milesDriven > 200:
	print("Yes, you need gasoline")
else:
	print("No, you can keep driving")
print("Drive safe")

• You can nest expressions / functions to form your boolean expression. For example:

if int(input("Enter your age (in years): ")) >= 21:
	print("You can drink alcohol, but do so responsibly")
else:
	print("Can’t drink alcohol yet ... how about some OJ?")

print("Enjoy the party!")

Multi-branch / elif Statements

• An example of using multi-branches to determine class standing based on units

numUnits = int(input("Enter number of units: "))

if numUnits < 50:
	print("Freshman status")
else:
	if numUnits < 90:
		print("Sophomore status")
	else:
		if numUnits < 135:
			print("Junior status")
		else:
			print("Senior status")
print("Done!")

• Note we will only print out one status and can embed multiple if / else statements in a block
• Also note if we had many conditions to check, our code could get messy (leans to the right)
• else: if patterns are common, so Python allows us to combine these together into elif statements.
• We can rewrite the same code in the following (cleaner) way:

numUnits = int(input("Enter number of units: "))

if numUnits < 50:
	print("Freshman status")
elif numUnits < 90:
	print("Sophomore status")
elif numUnits < 135:
	print("Junior status")
else:
	print("Senior status")

print("Done!")

Logical Operators

• Python provides specific operators that use Boolean values to return a Boolean value
• Logical operators include and, or, not
• When using logical or operator
  • True or anything → True
  • False or False → False
• When using logical and operator
  • False and anything → False
  • True and True → True When using the not operator, it turns True → False, and False → True
• High level language example:
  • Think of two given facts like:
    • I am hungry
    • I am cold
  • [I am hungry] or [I am warm] → True or False → True
  • [I am hungry] and [I am warm] → True and False → False
  • not [I am hungry] → not True → False

Detecting Numberical Ranges with Logical Operators

• Logical operators can be useful when detecting valid ranges of numbers
• Example:

x = 10
print((x >= 10) and (x < 15)) # True
print((x < 10) and (x < 15)) # False
print((x < 4) or (x <= 10)) # True
print((x < 15) or (x <= 10)) # True