Building a Simple Logo Interpreter

Welcome to your first major project! In this tutorial, we’ll create a simple interpreter for the Logo programming language. Logo is an excellent choice for beginners because it introduces programming concepts in a visual and interactive way. With Logo, you can control a virtual turtle to draw shapes and patterns by giving it commands.

Why Build a Logo Interpreter?

Creating a Logo interpreter allows us to:

1. Practice breaking down a problem into smaller, manageable chunks.
2. Combine several programming concepts like variables, loops, and functions.
3. Gain experience building a meaningful program from scratch.
4. Visualize the results of our code in a fun and interactive way.

Let’s get started!

Step 1: Understanding the Problem

Before we write any code, let’s understand what we want our Logo interpreter to do.

Core Features:

1. Basic Commands: Move the turtle forward, backward, turn left, and turn right.
2. Pen Control: Put the pen down to draw and lift it to move without drawing.
3. Loops: Repeat a set of commands multiple times.
4. Procedures: Define reusable functions.

Example Commands:

• FORWARD 100: Move the turtle forward by 100 units.
• RIGHT 90: Turn the turtle 90 degrees to the right.
• REPEAT 4 [FORWARD 100 RIGHT 90]: Draw a square.

Step 2: Break the Problem into Chunks

To build our interpreter, we need to:

1. Parse Commands: Convert user input into something the program can understand.
2. Execute Commands: Make the turtle perform actions based on the parsed commands.
3. Draw Graphics: Visualize the turtle’s movements.
4. Add Advanced Features: Implement loops and procedures.

Step 3: Writing the Code

We’ll write this program in Python for simplicity. You can follow along step-by-step.

Complete Code Listing

Here is the complete code for our simple Logo interpreter. You can also find this code on our GitHub repository:

# Simple Logo Interpreter

import turtle

def execute_command(command):
    parts = command.split(maxsplit=1)
    action = parts[0].upper()

    if action == "FORWARD":
        distance = int(parts[1])
        t.forward(distance)
    elif action == "RIGHT":
        angle = int(parts[1])
        t.right(angle)
    elif action == "LEFT":
        angle = int(parts[1])
        t.left(angle)
    elif action == "PENDOWN":
        t.pendown()
    elif action == "PENUP":
        t.penup()
    elif action == "REPEAT":
        repeat_parts = parts[1].split("[", 1)
        count = int(repeat_parts[0])
        commands = repeat_parts[1].strip("]")

        for _ in range(count):
            for subcommand in commands.split(";"):
                execute_command(subcommand.strip())
    else:
        print(f"Unknown command: {action}")

# Set up the turtle
screen = turtle.Screen()
screen.title("Logo Interpreter")

t = turtle.Turtle()
t.speed(1)  # Slow down the turtle for better visualization

# Example commands
def main():
    commands = [
        "PENDOWN",
        "FORWARD 100",
        "RIGHT 90",
        "FORWARD 100",
        "RIGHT 90",
        "FORWARD 100",
        "RIGHT 90",
        "FORWARD 100",
        "REPEAT 3 [FORWARD 50; RIGHT 120]",
    ]

    for cmd in commands:
        execute_command(cmd)

    screen.mainloop()

if __name__ == "__main__":
    main()

Step 4: Testing Your Program

Try out different commands to see your turtle in action! Here are some examples:

• Draw a triangle: REPEAT 3 [FORWARD 100; RIGHT 120]
• Draw a square: REPEAT 4 [FORWARD 100; RIGHT 90]

Step 5: Advanced Features (Optional)

In an advanced version of this course, we’ll:

1. Add support for defining procedures like:

   TO SQUARE
   REPEAT 4 [FORWARD 100; RIGHT 90]
   END
   

2. Build a GUI for easier input and visualization.
3. Introduce error handling for invalid commands.

Stay tuned for the advanced course!

Conclusion

Congratulations! You’ve built a simple Logo interpreter. Along the way, you practiced breaking down a problem, writing code step-by-step, and creating a fun, interactive program. Keep experimenting with new shapes and commands to see what your turtle can do!