CS21 Lab 1: First Programs

Due Sunday, February 1, before midnight

Goals

The goals for this lab assignment are:

  • Practice using a text editor such as Visual Studio Code.

  • Write your first python programs!

  • Understand and debug python error messages

  • Use variable assignment to store values

  • Get comfortable with print() and input()

  • Get comfortable with python data types: int, float, str

  • Use type casting with input() to get numeric data

1. Warmup: A first program

The file intro.py is initially empty. Practice using an editor and type in the program below.

Since the point is to start getting used to typing Python code, you should actually type this, rather than using copy/paste.

This part will not be graded. It is an optional (but recommended) warmup to help make sure you understand the workflow before you start trying to come up with your own code.

  • After saving your file, run the program and fix any errors.

Here is the program to type:

"""
This is a sample python program

Author: <Put your name here>
Date: <Put today's date here>
"""

#define the main function
def main():
    name = input("What is your name? ")
    print("Hello")
    print(name)
    print("Nice to meet you!")

# run the body of the main function defined above
main()

1.1. Sample output

Here are two examples of the running program. User input is shown in bold.

$ python3 intro.py

What is your name? Samantha
Hello
Samantha
Nice to meet you!
$ python3 intro.py

What is your name? Katherine Johnson
Hello
Katherine Johnson
Nice to meet you!

1.2. A note on printing strings

Printing Strings in Python

Here’s an example of printing two words separated by a single blank space:

print("Hi " + "there!")

The extra space after Hi and BEFORE the quotation will result in the following output:

Hi there!

You can also use print() with no arguments to print a blank line, e.g.:

print("Hi " + "there!")
print()
print("Welcome to CS21!")

will result in the output

Hi there!

Welcome to CS21!

2. Favorite TV Show and Movie

Write a program called tvmovie.py that asks the user for their favorite TV show and movie. Your program should then print a short response as shown below.

2.1. Sample output

Two examples of the running program are shown below. User input is shown in bold.

test 1
$ python3 tvmovie.py
What is your favorite TV show? Ted Lasso
What is your favorite movie? Good Will Hunting

Ted Lasso is a fun show, and Good Will Hunting is a great movie!
test 2
$ python3 tvmovie.py
What is your favorite TV show? Friends
What is your favorite movie? F1

Friends is a fun show, and F1 is a great movie!

2.2. Requirements

Your program should meet the following requirements:

  1. Ask the user for their favorite TV show and movie, in that order.

  2. Print a short response that includes their favorite TV show and movie.

Your output should match the examples shown above when given the same inputs. Your solution should be contained within a main function that you call at the end of your program

3. Python Debugging

Often a program won’t work as intended the first time we try to run it. We must review and revise the program in a process called debugging to fix syntax or logic errors in our program. The provided program fixme.py should complete the following steps:

  1. Prompt the user for an integer n (it is OK for the program to crash if the user enters something other than an integer)

  2. Compute the value of half of n, and store it in the variable answer

  3. Print the string "half n is " followed by the answer

However when we run the program, we get the following output:

$ python3 fixme.py
  File "fixme.py", line 22
    val = input("Enter an integer n: )
                ^
SyntaxError: unterminated string literal (detected at line 22)

Look closely at the program and the error output and try to fix this first error. Looking at a working example program may help.

Unfortunately, there are more errors in this program. After fixing the first, continue your debugging process to fix the other three. At some point, python may not generate any error messages, but it also may not generate any output at all! Hint: something is missing at the end of your program. Can you add it to fix the program? Compare the bottom of your non-working program to a working example in-class program.

The broken fixme.py as provided is shown below. You should fix the errors in this program.
def main():

    val = input("Enter an integer n: )
    n = int(val)
    answer = 0.5n
    print("half n is " + answer )

3.1. Sample output

When all errors are fixed, the program should run as follows, where user input is in bold:

test 1
$ python3 fixme.py
Enter an integer n: 10
half n is 5.0
test 2
$ python3 fixme.py
Enter an integer n: 9
half n is 4.5

3.2. Requirements

Your program should meet the following requirements:

  1. Run without errors.

  2. Ask the user to enter a number and display the result of half that number.

Your output should match the examples shown above when given the same inputs.

4. Python Math Operators

To familiarize yourself with Python’s math operators, complete the program called math_demo.py that shows the result of various math operations on integer inputs. The operators for each are shown below:

  • Addition (+)

  • Subtraction (-)

  • Multiplication (*)

  • Division (/)

  • Floor division (//) - sometimes called "integer division" or "whole division"

Complete the program in the empty math_demo.py file provided. The program should print the result of math operations shown above. To do so, your program should prompt the user to type in the two numbers num1 and num2. Both num1 and num2 should be chosen to be small positive integer values.

4.1. Sample Output

Two sample runs are shown below. User input is shown in bold.

test 1
$ python3 math_demo.py
This program tests some python math operators.
Enter the first positive integer value: 2
Enter the second positive integer value: 5

2 + 5 = 7
2 - 5 = -3
2 * 5 = 10
2 / 5 = 0.4
2 // 5 = 0
test 2
$ python3 math_demo.py
This program tests some python math operators.
Enter the first positive integer value: 4
Enter the second positive integer value: 3

4 + 3 = 7
4 - 3 = 1
4 * 3 = 12
4 / 3 = 1.3333333333333333
4 // 3 = 1

4.2. Requirements

Your program should meet the following requirements:

  1. Ask the user for two positive integer values.

  2. Show the result of combining these two integers using addition, subtraction, multiplication, division, and floor division, in that order.

Your output should match the examples shown above when given the same inputs. Your solution should be contained within a main function that you call at the end of your program

5. Cost Calculator

For your last program this week, you’ll write a cost calculator program, cost.py. Your program should prompt for the following:

  1. An item description (string)

  2. An item quantity (int)

  3. A cost per item (float)

Your program should compute a total cost and print out a short summary as shown below. Note: you are not required to format decimal cost to round to the nearest penny. We haven’t learned how to do that yet.

$ python3 cost.py
Enter an item description: Apples
Enter quantity of item: 3
Enter cost per item in dollars: 0.75

Apples (Qty: 3) Total cost = $2.25
$ python3 cost.py
Enter an item description: Widgets
Enter quantity of item: 271
Enter cost per item in dollars: 1.37

Widgets (Qty: 271) Total cost = $371.27000000000004

5.1. Requirements

Your program should meet the following requirements:

  1. Ask the user for the item description, item quantity, and price per item.

  2. Compute and display the total cost of the items as specified in the description above.

Your output should match the examples above when given the same inputs. Your solution should be contained within a main function that you call at the end of your program.

5.2. Optional Rounding

As an optional challenge, you can use Python’s built-in round() function to round your results to the nearest penny. To use it, you can call round with two parameters: the number to round and the number of digits you want to round to (two for pennies).

For example, if you wanted to round pi to two decimal places:

pi = 3.141592
pi_rounded = round(pi, 2)
print("pi rounded to two decimal places is: " + str(pi_rounded))

Which produces:

pi rounded to two decimal places is: 3.14

Note that converting the result of round to a string with str() is necessary for concatenation (adding the result to a string).

Notice that after rounding to two decimal places, if the hundredths place is a 0, Python won’t print that last 0. We haven’t seen how to fix that yet so for this lab, that is the expected output.

6. Answer the Questionnaire

After each lab, please complete the short Google Forms questionnaire. Please select the right lab number (Lab 01) from the dropdown menu on the first question.

Once you’re done with that, you should run handin21 again.

Submitting lab assignments

Remember to run handin21 to turn in your lab files! You may run handin21 as many times as you want. Each time it will turn in any new work. We recommend running handin21 after you complete each program or after you complete significant work on any one program.

Logging out

When you’re done working in the lab, you should log out of the computer you’re using.

First quit any applications you are running, including your vscode editor, the browser and the terminal. Then click on the logout icon (logout icon or other logout icon) and choose "log out".

If you plan to leave the lab for just a few minutes, you do not need to log out. It is, however, a good idea to lock your machine while you are gone. You can lock your screen by clicking on the lock xlock icon. PLEASE do not leave a session locked for a long period of time. Power may go out, someone might reboot the machine, etc. You don’t want to lose any work!