1. "Debugging" a Program

Usually the first time you write a program it doesn't work the way you want it to. There are several types of possible errors that might be involved:

• A logic error is a mistake in the algorithm or program that causes it to behave incorrectly or unexpectedly.
• A syntax error is a mistake in the program where the rules of the programming language are not followed.
• A run-time error is a mistake in the program that occurs during the execution of a program. Programming languages define their own runtime errors.

Logical errors are the most common error type in RoboBlocky's block based coding while syntax and run-time errors are more likely to occur in text based programming languages.

Getting rid of a program's errors so that it runs correctly is known as debugging the program ("getting the bugs out").

There are a number of methods we can use to find and correct errors:

• Trying the program with test cases where the results are known
• Tracing each step in the program by hand and doing the calculations on paper to see where the errors might be
• Using special programs that can analyze the program with the errors and map or visualize what is going on
• Using special programs called debuggers that analyze the program and identify errors
• Adding extra display or output statements to the program to display the internal values of the program as it runs

In this lesson we'll see how we can use a step-by-step approach to identify errors, similar to hand tracing.

2. Step-by-Step to Understand or Debug a Program

Sometimes when you are trying to debug a program, or simply figure out how a program works, it's helpful to have it run one code block at a time. Doing so can give you a better understanding of the logic and sequence of the program.

Going through code "step-by-step" is also very helpful when you are writing your own code and it's not doing what you want it to do. In other words, there are some mistakes in it somewhere. Mistakes and errors in programming are called "bugs." The process of finding the errors and correcting them is called debugging. 

3. The RoboBlocky Step Button

The RoboBlocky  Step  button (next to the  Run  button) is very useful in learning how a program works or in trying to debug your own programs:

The  Step  button allows you to run the code one block at a time. As each block is executed its color in the Workspace changes to orange, so that you can tell what part of the code is being run. This enables you to more easily see what each code block is doing and also spot and correct bugs.

You can also monitor the current position and angular orientation of the robot after each code block is executed by viewing the Position and Angle values in the Robot Settings section:

Watch the video at top right to learn more and see a demonstration.

4. Temporarily Disable Blocks to Aid in Debugging

When you are debugging a program, it can be very useful to temporarily disable code blocks that are already in your code, because by disabling one or more code blocks, you can see if they were causing the problem. When you disable a block, RoboBlocky ignores it when running the program. Disabling a block is preferable to deleting a block, because if the block isn't causing the problem, it's easy to enable it again. It's like turning a block on and off.

To learn how to disable code blocks, see the lesson on Debug Using Disable Block Option.

5. Check the Robot Position Step by Step

Another useful feature when you are debugging your code is to check the robot position at each step. As noted above, you can do this using the Position and Angle values in the Robot Settings section. For more information, see the lesson on Robot Initial and Current Positions and Angles.

6. Help Each Other!

It is important to learn to spot and fix bugs yourself. But sometimes when your program is not working and you are having trouble finding the bug, it is helpful to have someone else look at it. Sometimes they will be able to spot the problem.

Also, be willing to help others too. When we write computer programs we often do it as part of a team. Each member of the team helps the others so that the final program works well. Everybody makes a contribution. 

7. Unplugged Coding Activity: Go to the Library

Click the  Projects  button at top right for a suggested real-life debugging activity to do.