Unit 1 Lab 4: Building and Debugging Your Own Blocks

Your draw square block draws squares of only one particular size—the size built into its script. You can improve your block to draw squares of any size by giving your block an input.

Edit your draw square block to give it an input. To edit a block, right-click on it (or control-click or two-finger-click) and select "edit." Use the video to get the idea. You can read the detailed instructions (in yellow below) to learn more about what you are doing.

This is the first time we've hinted that they might be using a touchscreen/touchpad. We should maybe be consistent about this? Define "secondary click" carefully once and then just use it? PG: This is getting overwhelmingly complicated and just begs to be obsolete when the next technology comes in. Right-click, control-click, two-finger-click....

The + signs: In the Block Editor, hover your cursor over the words and plus signs (+) in the hat block. Clicking on a word lets you change the word; clicking on a + lets you put a new word in that position. Your new input will go after the title, so click the + at the end.

See also Snap Manual pages 19-20 and, for more advanced info, pages 27-31.
Naming the Input: Choose a name for the input that makes its meaning clear to any reader: size or side length would be good. Type that name in the input field.

You have two choices about how to treat that new text:
1. Input name is automatically selected for you, and that is what you want now, so just leave it selected.
2. But if you ever want to change a block's title, you could click Title text (gray in this picture) and type the new words. (For example, to change the name from draw square to draw square picture you'd click on "Title text" and type the word "picture.")
Your block header has changed: there is now a variable named size (or whatever name you chose) after the title:
To use the size variable in the draw square script, drag the variable down into the move block. Whenever you need a new copy of a variable, just drag another down.

This is exactly what you did when you used the variable in the for block.
Check your work: When you click "OK" in the block editor, you'll see that the draw square block in the scripting area now takes an input. Run this script with a few different inputs to check that it works:

If you have extra copies of a block or variable or script that you want to get rid of, you can drag them all the way to the palette at the left of the snap window.

If you made the house block on the previous page, edit your triangle and house blocks so that you can draw houses of any size you specify as input.
Create a spell backwards block that takes an input word or phrase and spells it backward. You may want your for block to use in order to know when to stop spelling.

You may wish to look back at the work with polygons you did in Lab 3. How many degrees altogether? How many turns?

Create a draw polygon block that takes two inputs:
- a number of sides (call the variable sides)
- the length of each side (call the variable length)
Be sure to test out your solution with several polygons with different lengths and numbers of sides.

This draw polygon block generalizes the pattern of draw square, draw triangle, draw hexagon, and so on. It does so by adding an input that expresses the difference from one block to another, namely, the number of sides. Adding an input to generalize a pattern is going to be a very important technique later in the course. This is an important example of abstraction, one of the central ideas of this course and of computer science in general.
You may wish to review how you nested squares in squares. Create a new procedure called nested polygons with two or three levels of nesting and inputs that tell how many sides and what side length to use.

Adding an Input