Lesson 6.1

Introduction

One important feature of any programming language is the ability to write decision branches based on certain inputs or conditions. A decision branch simply means that if a certain condition is met, the program executes a certain way; if the condition isn’t met, the program executes a different way.

For instance, if we were writing a program that helped students sign up for electives, we might list certain classes if the student is in 7-8th grade, but give a different list of classes if the student is in 9-12th grade.

To write decision trees, we need to learn how to write if...else statements in Python, and in order to do that, we need to learn what Boolean statements are.

Boolean Statements

A Boolean statement is a unique data type in Python, and every Boolean has one of two values: True or False. (Booleans are named after mathematician George Boole, who developed a type of symbolic logic that evaluates the true value of complex statements.)

Let’s explore Booleans in the Python interpreter:

>>> 3 > 2
True			# the expression 3 > 2 is a Boolean expression
>>> type(3 > 2)
<class 'bool'>
>>> type(True)	
<class 'bool'>
>>> light = False
>>> type(light)
<class 'bool'>

We can create Boolean expressions by using any of the six basic Boolean operators:

• == (is equal to)
• != (is not equal to)
• > (is greater than)
• < (is less than)
• >= (is greater than or equal to)
• <= (is less than or equal to)

(Note: be very careful not to confuse =, which we use for variable assignment (i.e., score = 20), with ==, which we use to indicate equal value (i.e., 3 == 1 + 2).

>>> 3 == 3
True
>>> 3 != 3
False				
>>> 3 >= 4
False

These six basic Boolean operators are not always adequate for Python, especially when we need to evaluate multiple conditions. We often need to use one of the three compound Booleans operators:

• and

• or

• not

  >>> 3 == 3 and 2 >= 4
  False				# both conditions must be True in order for the expression to be True
  >>> 3 == 3 or 2 >= 4
  True				# only one condition must be True in order for the expression to be True
  >>> not (3 < 4)
  True				# not inverts the value of the expression
  

If…Else Statements

An if statement always contains a Boolean expression. If the Boolean is True, then the code under the if statement executes. If the condition is False, this code is skipped:

>>> if 3 == 3:
				print("Hello, World!")
Hello, World!
>>> if 3 != 3:
  			print("Hello!")
   		# nothing happens because the condition is False

There will be times when we use just an if statement by itself, but often if statements also have an else statement. The else statement does not have a condition and will execute if the if statement does not:

>>> if 3 != 3:
				print("Hello, World!")
    else:
      	print("Goodbye")
Goodbye

There will also be many times when we have to test multiple conditions. To do this, we use an if...elif...else statement, which we will use in our solution for Exercise 3.

Exercise 3

Write a function which is given an exam mark, and it returns a string — the grade for that mark — according to this scheme:

The square and round brackets denote closed and open intervals. A closed interval includes the number, and open interval excludes it. So 79.99999 gets grade C , but 80 gets grade B.

Test your function by printing the mark and the grade for a number of different marks.

Solution to Exercise 3

# grade.py

def scale(num_grade):
    if num_grade >= 90:
        print("A")
    elif num_grade >= 80:
        print("B")
    elif num_grade >= 70:
        print("C")
    elif num_grade >= 60:
        print("D")
    else:
        print("F")

scale(90)   # A
scale(80)   # B
scale(70)   # C
scale(60)   # D
scale(50)   # F
scale(79.999999) # C

Exercise 7

Write a function called is_even(n) that takes an integer as an argument and returns True if the argument is an even number and False if it is odd.

# even_test.py

def is_even(number):
    if number % 2 == 0:
        return ("The number is even.")
    else:
        print("The number is odd.")

is_even(9)
is_even(4)
is_even(2)
is_even(0)

Exercise 8

Now write the function is_odd(n) that returns True when n is odd and False otherwise.

# odd_test.py

def is_odd(number):
    if number % 2 == 1:				# number %2 != 0 would also work as a condition
        print("The number is even.")
    else:
        print("The number is odd.")

is_odd(9)
is_odd(4)
is_odd(1)
is_odd(0)

Homework

1. Read HTCS, 7.1-7.4
2. Work on Project 5