Debugging Watch and Text Output

1. Debugging Watch and Text Output Alice

2. Debugging Techniques • Several techniques are helpful in eliminating errors (bugs) in programs. • careful design • incremental development • In this session we look at two additional techniques: • Watch • Text output

3. Watch • Variables are elusive and a bit magical because they exist somewhere in the murky depths of the computer's memory and we can't see what is going on inside them. • To help you "see" the inner workings of a variable, Alice provides a watch -- a small window where the value of a variable can be displayed at runtime.

4. Creating a Watch • Select the object in the Object tree and then the variable you want to watch. • Then right click on the variable and select watch this variable. • In this example, we selected the corvette and then right clicked on its direction variable.

5. Demo • Every time the value changes, it is immediately updated in the window. • Setting a watch on a variable thus allows you to see whether the value is changing as expected. If not, you can then change your code to make it work correctly.

6. Text Output • Using a print instruction is another way to keep an eye on the value stored in a variable at runtime. • The print instruction causes the animation to be displayed in a split window. • One pane displays the animation • A second pane is a text console where text output is displayed.

7. Demo:Creating a print instruction

8. Modified methods • Here is the modified left method: • Note that a print instruction is added to both the left and right turn methods.

9. Demo • The major difference between a watch window and text output is that text output provides a history of the values as the program runs.