Unit 1 Lab 3: Building Your Own Blocks, Page 2

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 (or control-click) on it and select "edit."
- To add an input, click (the last) plus sign in the hat block, type an input name (size), and click "OK."
- To use the inputed data, drag the input name (size) out of the hat block and place it in your script.
Use the animation to get the idea. You can read the details (in yellow below) to learn more.

If you don't see the + marks, uncheck "Plain prototype labels" in the Settings menu.
The + marks: 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 there. 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 an input name, like size or side length, that makes the input's meaning clear. 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. That's 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 block name from draw square to draw square picture, 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:

The variable will contain whatever value the user of your block puts in its input slot. So if the user says draw square (87) then the value of size will be 87.
To use the size variable, drag the variable where you want to use it. In this case, that's in the move block. If you ever need another copy of a variable, just drag another down.

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

Check your work. When you click OK (or Apply) 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.
Edit your draw triangle, draw house, and draw row of houses blocks so that you can draw triangles, houses, and rows of houses of any size you specify as input.

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 size or side 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 an important technique later in the course.

This is an example of abstraction, one of the central ideas of this course and of computer science in general.

Giving Your Block an Input Variable