04 Checkpoint: Variable Scope

Purpose

Check your understanding of parameters, arguments, and local variable scope by fixing a program that won’t run because the programmer tried to use variables that were defined in a different function.

Helpful Documentation

Problem Statement

Diagram of a right circular cone with radius r and height h
A right circular cone with radius r and height h

The following example Python program is supposed to ask the user for the radius and height of a right circular cone and then compute and print the volume of that cone. The code is almost correct, but it contains a common mistake. The programmer who wrote it assumed that the cone_volume function could use the radius and height variables that are defined in the main function. Of course, this assumption is incorrect because as the preparation content for this lesson explains, variables defined inside a function have local scope and can be used only inside their containing function.



"""Compute and print the volume of a right circular cone."""

# Import the standard math module so that
# math.pi can be used in this program.
import math


def main():
    # Call the cone_volume function to compute
    # the volume of an example cone.
    ex_radius = 2.8
    ex_height = 3.2
    ex_vol = cone_volume()

    # Print several lines that describe this program.
    print("This program computes the volume of a right")
    print("circular cone. For example, if the radius of a")
    print(f"cone is {ex_radius} and the height is {ex_height}")
    print(f"then the volume is {ex_vol:.1f}")
    print()

    # Get the radius and height of the cone from the user.
    radius = float(input("Please enter the radius of the cone: "))
    height = float(input("Please enter the height of the cone: "))

    # Call the cone_volume function to compute the volume
    # for the radius and height that came from the user.
    vol = cone_volume()

    # Print the radius, height, and
    # volume for the user to see.
    print(f"Radius: {radius}")
    print(f"Height: {height}")
    print(f"Volume: {vol:.1f}")


def cone_volume():
    """Compute and return the volume of a right circular cone."""
    volume = math.pi * radius**2 * height / 3
    return volume


# Start this program by
# calling the main function.
main()




    
Assignment

    
Do the following:

    
    
        
  1. Using VS Code, open a new file. Copy and paste all of
        the example program above into the new file. Save the new file
        as cone_volume.py
    2. 

        
  2. Run your cone_volume.py program. What
        error message do you see in the terminal frame?
    3. 

        
  3. Fix the cone_volume function which begins
        on line 37 by
        adding two parameters to its header. What parameter names should
        you use?

    
        Hint: the parameter names at
        line 37 and the
        variable names at
        line 39
        must be the same.
    4. 

        
  4. Fix the call to the cone_volume function
        that is in main at
        line 13 by adding
        two arguments. Because the cone_volume function has
        two parameters (you added the two parameters in the previous
        step), each call to the cone_volume function must
        have two arguments. What variable names should you use for the
        arguments?

    
        Hint: the only variables that you can use must be defined inside
        main above line 13.
    5. 

        
  5. Fix the call to the cone_volume function
        that is in main at
        line 28 by adding
        two arguments. What variable names should you use for the
        arguments?

    
        Hint: the only variables that you can use must be defined inside
        main above line 28.
    6. 

        
  6. Save your program and run it again. Did it run
        correctly? If not, fix the mistakes until it runs
        correctly.
    7. 
    


    
Testing Procedure

    
Verify that your program works correctly by following each
    step in this testing procedure:

    
    
        
  1. After you finish the steps of this assignmnet, run your
        program and enter the inputs shown below. Ensure that your
        program’s output matches the output below.


    > python cone_volume.py
This program computes the volume of a right circular cone.
For example, if the radius of a cone is 2.8 and
the height is 3.2, then the volume is 26.3

Please enter the radius of the cone: 5
Please enter the height of the cone: 8.2
Radius: 5.0
Height: 8.2
Volume: 214.7
    
        
    2. 
    


    
Sample Solution

    
When your program is finished, view the sample solution for this assignment to
    compare your solution to that one. Before looking at the sample
    solution, you should work to complete this checkpoint program.
    However, if you have worked on it for at least an hour and are still
    having problems, feel free to use the sample solution to help you
    finish your program.


    
Ponder

    
The original program that you copied and pasted to start this
    assignment didn’t work because the programmer tried to use the
    radius and height variables inside the
    cone_volume function. However, the radius
    and height variables are defined inside the
    main function and therefore have local scope and cannot
    be used outside of main. To fix the program, you added
    two parameters to the cone_volume function and then
    added two arguments to each call of the cone_volume
    function.


    
Why was local variable scope invented by computer scientists? How
    does local variable scope make a program easier to write and
    understand?


    
Submission

    
When complete, report your progress in the associated I‑Learn
    quiz.