Lesson 9: Writing Simple Programs

You now have all the primary tools of a Python programmer: Variables, Input/Output, Operators, Conditionals, and Loops. But having a hammer and a saw doesn't mean you know how to build a house. In this lesson, we focus on the Process of writing a program.

Lesson Overview

In this lesson, we will cover:

• Decomposition: How to break a big problem into small pieces.
• Pseudocode: Writing code in "human" first.
• The DRY Principle: "Don't Repeat Yourself."
• Case Study: Building a "Tip Calculator" from scratch.

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1. Problem Decomposition

When faced with a coding task, don't just start typing. Ask yourself:

1. What is the Input? (e.g., The total bill amount).
2. What is the Process? (e.g., Calculate tax and tip).
3. What is the Output? (e.g., The final amount per person).

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2. Using Pseudocode

Pseudocode is a plain English version of your code. It helps you design logic without worrying about syntax.

Example for a Guest List program:

Ask for the guest's name.
If name is on the allowed list:
    Print "Welcome!"
Else:
    Print "Identify yourself!"

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3. The DRY Principle (Don't Repeat Yourself)

If you find yourself copying and pasting the same code three times, there is probably a better way to do it using a Loop or a later concept called Functions. DRY code is easier to maintain and has fewer bugs.

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4. Case Study: The "Split the Bill" App

Let's build a program that calculates how much each person should pay at dinner, including tip.

The Plan:

1. Input: Total bill, Tip percentage, Number of people.
2. Process:
   • Find the tip amount (bill * tip_percent / 100).
   • Add tip to bill.
   • Divide total by number of people.
3. Output: Final cost per person.

The Code:

print("--- Welcome to the Bill Splitter ---")

# Step 1: Gather Input
total_bill = float(input("What was the total bill? $"))
percentage = int(input("How much tip would you like to give (10, 15, or 20)? "))
people = int(input("How many people to split the bill? "))

# Step 2: Perform Calculations
tip_amount = total_bill * (percentage / 100)
final_total = total_bill + tip_amount
price_per_person = final_total / people

# Step 3: Show Output
print(f"\nFinal Bill (with tip): ${final_total:.2f}")
print(f"Each person should pay: ${price_per_person:.2f}")

Note: :.2f inside the f-string tells Python: "Show only 2 digits after the decimal point."

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Practice Exercise: The Temperature Converter

Create a file named temp_converter.py. Your program should:

1. Ask the user for a temperature in Celsius.
2. Ask if they want to convert to Fahrenheit or Kelvin.
3. Use conditional statements to perform the correct math:
   • Fahrenheit: (Celsius * 9/5) + 32
   • Kelvin: Celsius + 273.15
4. Print the result clearly.

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Quick Knowledge Check

1. What are the three stages of every computer program?
2. How does pseudocode help you as a beginner?
3. What does the DRY principle stand for?
4. Why is problem decomposition important?

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Key Takeaways

• Break big problems into small, manageable steps.
• Plan your logic in plain English (pseudocode) before coding.
• Avoid repeating code whenever possible.
• Always consider your Input, Process, and Output.

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What’s Next?

Every programmer makes mistakes. In Lesson 10, we’ll look at Common Beginner Mistakes and learn how to debug them like a pro so you don't get stuck!