Introduction to Computer Science with Scratch and App Inventor

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Click on the word procedure to change the name of the procedure. We will call our procedure MoveMole. In programming when you are naming things, you want to make sure you use descriptive names. This will help you remember how your program is working! Now we just need to fill in the MoveMole procedure with the commands that move the mole the way we want it to. How will we tell the mole where to go? Do you remember the X and Y properties of the mole sprite? We can access those properties and change them using blocks! We can tell the mole to go to a new location by setting (changing) the value of the X and Y properties. If you click on the My Blocks tab, you will see that Mole is one of the options. If you click on Mole, you will see a list of blocks that allow you to manipulate the Mole sprite using code! Here, we want to change the X and Y properties of the mole. Scroll all the way to the bottom of the menu of Mole blocks. You will see blocks that say set Mole.X to and set Mole.Y to. Mole.X means “the X property of the sprite Mole”. So these are the blocks that we want. Drag one set Mole.X to block and one set Mole.Y to into the MoveMole procedure. Now we need to figure out what we want the new X and Y values for the Mole to be. We want the mole to reappear at a new random location on the screen. Just like Scratch, App Inventor has some built in tools that allow us to generate random numbers. However, it is a little trickier than in Scratch to set the parameters for the random number. To generate the random location on the screen, we will use several different blocks from the Math menu which is located in the Built-In tab – we will need call random fraction and some of the simple calculation blocks. mbrace Game Development Workbook Page 40
We know that our X and Y locations can be anywhere on the canvas. Since we set our canvas to be 300 pixels by 300 pixels, we know that the X and Y coordinates range from 0 to 300. The call random fraction block calls a procedure that generates a random number between 0 and 1. This is a built in procedure which means we don’t have to write it ourselves, we can just call it. If we multiply 300 (the width of the canvas) by 0, we get 0 (the smallest possible X coordinate). If we multiply 300 by 1, we get 300 (the largest possible X coordinate). So, that means that if we multiply 300 by any number between 0 and 1, we are guaranteed to get a valid X coordinate. We can do the same thing for the height of the canvas and Y coordinates. In code, that would look like this: You can get the MyCanvas.Width and MyCanvas.Height blocks from the MyCanvas menu on the My Blocks tab. Notice the difference between the set Mole.X to block and the MyCanvas.Width block. Blocks that say set ___ to and are darker blue allow you to update the value of a property. Blocks that just have the name of the property (like MyCanvas.Width) and are lighter blue-gray just get the value of a property. You can see on the Mole menu of blocks that both types of blocks are available for each property. There is a set Mole.X to block that allows you to change the value of the Mole.X property and a Mole.X block that allows you to find out what the current value of that property is. In programming, blocks like set Mole.X to are called mutators, since you are changing or mutating the value of a property. Blocks like Mole.X are called accessors since you are accessing the value of a property. Now we are almost done with our MoveMole procedure, but there is one small change to make. Right now it is possible for our Mole to appear only partially on the screen. We can fix this issue by updating our procedure like this: Why does this work? Here, we worked out the whole procedure for moving the mole in one step, but often in programming, you will build a procedure in small pieces over time. You will test each piece as you go. Remember that the way to learn programming is just to play and try new things. If your program doesn’t work the way you expect it to, just undo the last step you did and try something else! This process of trial and error is one of the things that makes programming FUN! mbrace Game Development Workbook Page 41
Page 1 and 2: Introduction to Computer Science wi
Page 3 and 4: LESSON 1: Getting Started with Scra
Page 5 and 6: I decided to pick a dinosaur. You c
Page 7 and 8: The direction that your sprite move
Page 9 and 10: Let’s add some yellow wait blocks
Page 11 and 12: Changing the Background on the Stag
Page 13 and 14: To use an if block to control the m
Page 15 and 16: Costumes and Animation Now that you
Page 17 and 18: Other useful blocks are the say and
Page 19 and 20: ? Now we know how to tell when the
Page 21 and 22: Here is one script that works. To m
Page 23 and 24: After you create a variable, you wi
Page 25 and 26: Winning, Losing, and Game Play Our
Page 27 and 28: You might have noticed that when th
Page 29 and 30: Click the checkbox next to timer to
Page 31 and 32: Scratch Review Now you have built y
Page 33 and 34: App Inventor Interface The App Inve
Page 35 and 36: The first thing that we will do is
Page 37 and 38: To add the mole to our program, we
Page 39: Your Blocks Editor Screen should lo
Page 43 and 44: Our mole shows up in the emulator,
Page 45 and 46: The last thing we need is the value
Page 47 and 48: Team Project Worksheet We have lear

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