Operation Manual

Example 3: A Simple Game
Using Scratch for simple animation is one thing, but the software also allows users to read inputs from the keyboard to introduce
interactivity. By combining some simple animation controls to the previously described program, you can create a simple game—
and, at the same time, introduce the concepts of sprite collision, if statements and input.
For this example, start a new Scratch project—remembering to save the previous example, if you haven’t already done so—and
begin by dragging a move 10 steps block to the Scripts area. This time, rather than telling the code blocks to execute when the
flag icon is clicked, go to the Control block palette and drag a when space key pressed block above the move block.
As the name suggests, the when space key pressed block looks for input from the user—in this instance, the spacebar being
pressed—and uses that as the trigger for executing a list of blocks. The block works at any time—if you press the spacebar
now, the sprite will obey its instructions and move 10 steps to the right.
The when space key pressed block also differs from the when [flag icon] clicked block in another important way: it
can be customised. Click the down-arrow button next to the word space to see a list of all the keys the block can watch, and
then select right arrow from the list to change the block into a when right arrow key pressed block.
A game in which the player can move in only one direction isn’t much fun, so drag a new when space key pressed block
into the Scripts area. This can’t link to the existing block list—you can only have a single trigger block—so start a new list
somewhere further down. As before, use the down-arrow button next to the word space to customise the block, turning it into a
when left arrow key pressed block. Finally, switch the block palette back to Motion mode and connect a move 10
steps block beneath the new when left arrow key pressed block before changing it to read move -10 steps.
If you press the left and right arrow keys now, you’ll see that the cat moves according to your input (see Figure 10-7): pressing
the left arrow moves the cat 10 steps to the left (although, as far as Scratch is concerned, it’s moving minus 10 steps to the right),
and pressing the right arrow moves the cat 10 steps to the right.
Figure 10-7: Using input blocks to control the motion of a sprite in Scratch
Now that the sprite can be moved by the player, it’s time to give the sprite something to do. As this is just a very simple game,
the goal should also be simple: to collect some food. Click the Choose New Sprite From File button, found in the middle of the
three buttons above the Sprite palette at the bottom-right of the Scratch window.
A dialogue box will appear asking you to pick a sprite: double-click on the Things folder, and then double-click on the Cheesy-
Puffs sprite (shown in Figure 10-8). This will place the sprite as a new entry in the Sprite palette, giving you a new object to
control in the game.