Objective

In this unit, our objective is to gain a thorough understanding of controlling program flow using if, else, and elif statements in Python. These are integral parts of any programming language that let us decide what portions of the code should execute based on specific conditions. By the end of this unit, you'll be able to use these conditional statements effectively to guide the decision-making process within your programs.

Understanding Control Flow

Control flow is the order in which individual statements, instructions, or function calls are executed within a program. In Python, control flow is regulated by conditional statements, loops, and function calls.

if Statements

if statements in Python allow the program to execute specific code if a certain condition is met. The condition is always a boolean expression that evaluates to either True or False.

x = 10
if x > 5:
    print("x is greater than 5")

else and elif Statements

else is used when we want to execute a block of code when the if condition is not met. elif (short for else if) is used when we need to check for multiple conditions.

x = 10
if x > 15:
    print("x is greater than 15")
elif x == 10:
    print("x is equal to 10")
else:
    print("x is less than 15")

Nested if Statements

We can put an if statement inside another if statement. This is called nesting. Nested if statements can be useful when we need to test for multiple conditions and execute different actions for each condition.

NOTE: Be sure to align your indentation correctly or else you will end up with a statement that should belong to a nested if inadvertently get executed in the enclosing block.

x = 10
if x > 0:
    if x % 2 == 0:
        print("x is a positive even number")
    else:
        print("x is a positive odd number")
else:
    print("x is not a positive number")

Project: Draw a Shape with Color Based on the Number of Sides

In this project, we'll be using conditional statements to draw a shape with a certain color based on the number of sides it has.

Here's the Python code for the project:

import turtle

# Create a new turtle screen and set its background color
screen = turtle.Screen()
screen.bgcolor("white")

# Initialize a turtle object
t = turtle.Turtle()

# Ask the user for a number of sides
num_sides = int(input("Enter the number of sides for your shape (between 3 and 5): "))

# Choose color based on number of sides
if num_sides == 3:
    t.color("red")
elif num_sides == 4:
    t.color("blue")
elif num_sides == 5:
    t.color("green")
else:
    print("Invalid input! Please enter a number between 3 and 5.")
    quit()

# Draw the shape
for _ in range(num_sides):
    t.forward(100)
    t.right(360/num_sides)

# Wait until the window is closed
turtle.done()

As in previous projects, we create a window and initialize a Turtle object. We then use the input() and int() functions to get the number of sides from the user and convert it to an integer.

Using conditional statements, we then assign different colors to the turtle based on the number of sides entered by the user: red for a triangle, blue for a square, and green for a pentagon. If the user inputs a number outside the range of 3 to 5, we inform them of the invalid input and end the program.

After setting the color, we then draw the shape according to the number of sides. We do this by moving the turtle forward a certain distance and then rotating it by a certain angle. The angle is calculated as 360 (the total degree of a circle) divided by the number of sides. This ensures the correct angle for each shape.

In the next unit, we'll continue our exploration of control flow statements and introduce the match-case statement.