Debugging Blocks within Blocks

One potential challenge in building custom blocks that use other custom blocks is that a custom block may work fine on its own, but when it's used inside another custom block, something goes wrong.

Here is an example. A programmer intended to draw five squares in a row, like this: , and built this script:

Of course, it required a draw square block with an input size, so the programmer created this definition:

But instead of getting the desired picture of five squares in a row like , the script draws .

Debugging Blocks within Blocks Self-Check

What is the bug?

The repeat block code needs to be more complicated.

Well, that could solve the problem, but there is an easier fix. Not always, but often, when it seems like it's necessary to make things more complicated, there is an easier fix.

The draw square block draws a square of the wrong size.

No, each side is the correct length.

The draw square block won't draw 4 sides in the shape of a square.

No, the square looks correct when it is drawn by itself.

The draw square block actually works correctly!

Only sort of... The draw square block draws a square of the correct size and shape, but causes a problem that the repeat script doesn't expect.

The draw square block doesn't return the sprite to the correct location when it is done.

The sprite's location is correct, but something else is wrong about the way the sprite has been left at the end of draw square.

The draw square block leaves the sprite facing the wrong direction.

Right! Because the draw square block doesn't leave the sprite facing the same way as when it started, the the repeat script moves in different directions each time draw a square draws a square. If there was another at the end of draw square, all would be well.

Consider the following block definition to draw a house; if it uses a correct draw square block, it will draw a house (with a flat roof) and a door at the lower right:

What will be drawn if it uses the buggy draw square code shown at the top of the page?

(The correct picture)

No. The bug definitely has an effect when the two squares are composed. By using a buggy block, draw house becomes buggy itself!

Because draw square is buggy, there is no way to predict what the picture will look like.

Incorrect. This buggy program, like most buggy programs, will predictably do the same wrong thing every time it is run. Carefully "run" the program block by block in your head.

(Flipped over)

Nope. Carefully "run" the program block by block in your head to see what the buggy draw square will do.

Right!