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Python 02: Branching Programs

Introduction: This document presents learning steps for Python 02. In Python 02, you will learn basics of Python structures to add decision points to your programs. A branching program is a flow of sequential instructions with branching statements. By the end this week, you will be able to implement a program where a user can enter simple input, the program can make choices based on some conditions and after calculation, results can be printed.

Note: Exercises of this learning path can be done using:

  1. Online Python Editor OPyEditor: The final program should be stored on your local machine.
  2. Local Python Package (see Step-01): Using BRef-01: Appendix B Python can be installed on your local machine.

Materials:

The activities are designed based on these following references:

  • BRef-01: Book, Bill Lubanovic; "Introducing Python: Modern Computing in Simple Packages"; Available here
  • OPyEditor: Online Editor for Programming; "Online Python (with shell and file storing functionalities)"; Available here

Path:

Follow these steps:

Step-01: Set Up

Goals:

After taking this step, you will be able to:
	1. implement and run your Python programs on your local machine.

What to Learn?

  1. Using BRef-01: Appendix B perform the following tasks:
    1. Install Python on your machine.
    2. Open a terminal (command window) and check the version of your Python. Which command did you use?
    3. Using OPyEditor implement a program that prints a statement of a defined variable, like Hello Python!. Save the file on your local machine within a folder created by you. Using your terminal (command line) execute your first Python program. Which command do you need to execute a Python program?

Exercises:

  1. Create a file named print_input.py. Open it using an editor of your own choice. Enter your code in it to ask the user to input a text. Print that text. Run the file using the command line.
  2. Read BRef-01: Chapter 01, Section Running Python and runPython shell. Excute quit(). What do you observe?
  3. StartPython shell and execute num = input('Enter a number:'). Enter a number and print the value of num. There are two different ways to print the value of num. Try both at shell. Which one works in OPyEditor? Do you recognise differences between programming using python shell and an editor? Read BRef-01: Chapter 01, Section Running Python including subsections.

Note: After this step, you can try both python shell and editor to practice. It is recommended to use python shell for small experiments and use programming within editors (local or online) for writing a full program.

Step-02: Programs need to decide.

Goals:

After taking this step, you will be able to:
	1. interpret and implement boolean expressions.
	2. implement Python programs with conditional statements.

What to Learn?

  1. Using BRef-01: Chapter 04 discuss and experiment the following questions:
    1. What is a comment? How can you specify a comment in Python?
    2. What are: boolean values, boolean expressions, comparison operators?
    3. What is a conditional statement in Python? What is correct syntax for a correct if-else statament? What is a body of a if-else statement?

Exercises:

  1. Create a variable with the value True. Print it. Change the value to False. Print it.
  2. Create a variable
    • named a with the value True, another
    • named b with the value False. Use print to check the output of print(a == b).
    • Do the same for the other comparison operators, !=, <, >, <=, >=.
  3. Repeat the second exercise using an if-statement. Print yes or no, for true false.
    • Write a comment above the if-statement explaining with it does.
  4. Implement a program in which the user is asked for input. Save the input of the user in a variable. Print yes if the input contains the character e, no if not.
  5. Think of an useful situation where you need to check something with a if-statement within another if-statement (nested if-statements). Code it and write a comment to explain why it needs a nested if.
  6. Finish all the exercises listed in BRef-01-Chapter 04: Things to Do.

Step-03: Strings are essential.

Goals:

After taking this step, you will be able to:
	1. interpret and implement basic operations of strings: concatenation (combining), duplicating, scanning and slicing.

What to Learn?

  1. Using BRef-01: Chapter 05 discuss and experiment the following questions:
    1. How can you combine several strings? Implement an example.
    2. Can you multiply a number with a string? What is the result? Implement an example.
    3. How can you get character 5 of a given string? How can you get the first character?
    4. How can you get a substring from a given string?

Exercises:

  1. We can use a + or a , to combine two strings within a print. What's the difference?
  2. Using a program, ask the user for input. Print the first and last character from the input.
  3. Student numbers start with 0. Implement a program where the user enters a student number and your program must check if it starts with 0.
  4. Ask the user to input a number. If it is lower than 10 print it, otherwise only print the first character.
  5. Write down the complete alphabet in a variable. Split it halfway over two different variables. Join them back together in the wrong order and print it.
  6. Explain in your own words with a f style string is?
  7. Look up the Spanish alphabet and put it in a variable. Put the Dutch alphabet in a variable. Print yes if the letter k is at the same position at both, print no if not. Use an if statement.
  8. Ask the user for a text input. Change both the first and last characters to uppercase and print it.
  9. Practice the exercises listed in BRef-01-Chapter 05: Things to Do.

Learning Activities:

Code Analysis

  1. Given the following problem statement, one of the students has submitted a solution. The submitted solution is not working correctly.

    • Without executing the submitted code, check the implementation, analyze and find the issue(s).
    • Use the following link to run the code step by step and visualize the execution. Python Execution Visualizer
    • This step-by-step execution should confirm the issues you have found in the code.
    • After listing the issues, propose how the code must be fixed. Fix the code and again use step-by-step execution to see that the issues are fixed.

    Problem Statement

    Develop a program that reads a four-digit integer from the user and displays the sum of the digits in the number.

    Input example:

    3141

    Output example:

    3+1+4+1=9

    Incorrect Solutions
    Solution 1:

    fourdigit_num = int(input("Input a four digit number: "))
x  = fourdigit_num // 1000
x1 = (fourdigit_num - x * 1000) // 100
x2 = (fourdigit_num - x * 1000 - x1 * 100) // 10
x3 = fourdigit_num - x * 1000 - x1 * 100 - x2 * 10
print("Sum:",x+x1+x2+x3)

    Solution 2:

    numstr = input("enter a 4 digit number")
sum = 0
text = ""
for i in range(len(numstr)):
   sum += int(numstr[i])
   text = text + numstr[i]
print(text)

Supporting Topics

  1. Perform a free (re-)search and explore answers for the following questions:

    1. What is an Operating System and why do we need it?
    2. Consider your own computer and OS (Operating System). How do you interact with the OS: Graphical User Interface (GUI) or commands?

  2. Perform a free (re-)search and carry on the following tasks:

    1. Open a terminal / a command prompt. What is the current folder?
    2. How can you make a list of the contents of the current folder?
    3. Using the command line, create a folder and name it workspace.
    4. Use command prompts and change your current folder such that you are in the folder workspace.
    5. Create a folder basecamp and within basecamp create week01.
    6. You have used an online editor to implement your solutions for the given problems of this week. Save one of the solutions within ../basecamp/week01/. Name the file properly. Make sure that the extension of the file is .py.
    7. Is it possible to execute the downloaded Python program using command line? Try it. Note: You may receive an error because Python is not installed on your machine. In this case, you can try this next week.
    8. Use the command line to create an empty Python file with a proper name.
    9. Use the command line to create another folder named week02 within ./basecamp/. Copy (using the command line) your empty Python file within week02.
    10. Within the folder of week01 use the command line and create a file named summary.txt. Open the file (using commands), write down a summary of commands you have learned and used so far and save the file.