Finding Bugs

Even experienced programmers write programs that don't do what they should. A mistake in a program is called a bug, and finding and fixing it is called debugging. The name supposedly comes from a real moth that was found in 1947 inside one of the first computers, where it was causing a fault.

The best way to find a bug is to run the program and watch closely what happens:

1. Watch the dog and the editor at the same time. The block the dog is carrying out lights up.
2. Find the first place where the dog does something other than it should. When it bumps into a hedge or steps off the path, the program stops, and the block where that happened stays lit.
3. The bug is usually in that block or a little before it. Think about what should have been there.

How to fix the program:

• Delete one block: click it and press the Delete key (or right-click it and choose to delete it). The blocks below it move up.
• Put a block in the middle of the program: drag it between two blocks. The other blocks make room for it.
• Careful when dragging: when you drag a block in the middle of a program, all the blocks below it come along. To pull out just that one block, hold Ctrl (Cmd on a Mac) while dragging.