Howardism: 4. Concepts and Overview of Scratch

4. Concepts and Overview of Scratch

Before we get our knuckles sore, let's take a moment to go over some basic concepts.

Welcome to the Theater

While I suppose that all the world's a stage, I think it is helpful to look at a Scratch project as a play production. Any play is presented on a stage . The stage is where backgrounds are placed and actors carry out their lines. (The stage, in Scratch, is also in charge of "things that happen behind the scenes", but we'll get into that later).

On the stage, our actors perform. In Scratch, actors are called sprites, and I dare say that you may feel your own actors aren't robots of your own creations, but capricious characters with their own motives. Each sprite can have one or more costumes as well as a script . The script contains the directions and actions it should perform.

Like an actor in a play, a sprite can begin off-stage (we call it hiding), and then move across the stage, repeat its lines, and then exit. Unlike in a play, however, the sprite can change costume mid-scene without causing a ruckus.

Image of a puppet theater

First Look at the Environment

If you haven't already done so, now is a good time to start up the Scratch program. Let me highlight a couple of key areas; feel free to ignore the other dials, knobs and whistles for the moment.

On the right side near the top is a large white area (in the screen shot below, I've marked with pink lines). This is the stage. This is where all the stories are told, the art designs are shown, the games are played, and the action happens.

Screenshot of the Scratch Environment

Below the stage is the Sprite Container, listing all of the sprites in the project. When you start a new project, you will have a single sprite, named Sprite1 wearing a cat costume. It just so happens that this sprite is wearing his costume centered on the stage.

Next to the sprites is an icon representing the stage. Clicking on this will allow you to change the backgrounds for the stage.

Occupying the center section is the Sprite Details Area. It displays details based on which sprite (or the stage) is selected. Three tab allows you to change what is displayed:

Scripts. Instructions and directions for the sprite's performance is placed in this area.
Costumes. Each sprite can have one more more costumes. A costume is an image that the sprite can change based on a script. For instance, a sprite can change color, or a script can cycle through costumes in order to animate the sprite walking.
Sounds. Each sprite can have a collection of recordings that it can speak based on instructions from the scripts.

Screenshot of the Sprite Details Area

We'll talk about the details of this section later. I just wanted to get you familiar with the environment.

Overview of Blocks

When the Scripts tab is selected, the instructions for a particular sprite is shown. A sprite's script is where most of the interest lies, for without the script, our play would be quite lifeless. A single action for the sprite to perform is encapsulated in a block.

The blocks come from the section on the far left side of the window, the Blocks Palette area. Scratch comes with a lot of blocks, so the blocks are grouped based on what they do or what they affect.

Motion. Blocks that move and manipulate the placement of a sprite.
Looks. Blocks that affect how the sprite appears; for example, to be larger or smaller or in different costumes.
Sound. Blocks that play music, drum beats or recordings.
Pen. Blocks that allow the sprite to draw on the screen.
Control. Blocks that determine when scripts should run and control other stacks of blocks.
Sensing. Blocks that allow a sprite to sense what is around it; for instance, various colors or other sprites.
Numbers. Blocks that help a sprite perform math operations.
Variables. Blocks that remember things.

Screenshot of the Block Palette Group area.

A block from a particular group has a color that matches the group. This feature is helpful in finding a particular block, as well as the added benefit of making it easier to duplicate the script listings in this book.

Block Types

Blocks come in two flavors, connectable and embeddable (called reporters). Connectable blocks connect into a sequence. An embeddable block is placed within another block and affects how it behaves.

Sample stack illustration. In this example, the sprite will move, say something and wait 1 second.

In the above diagram are three connectable blocks. As you can see, a connectable block is a lot like a LEGO brick in that in can be connected to other blocks to create a stack . Each block has the action it performs written on the brick. Each of these particular blocks has a notch at the top and bottom, which means that the brick can come anywhere in a stack.

Hat Blocks

The other type of connectable block is called a hat block which only occurs at the beginning of a stack. It usually specifies when the stack will be executed.

Sample hat stack illustration. In this example, when the 'Kitty' sprite is clicked, it will play the Meow recording.

Grouping Blocks

Grouping blocks surround a stack of other blocks (they are also called crocodile mouths because of the oral resemblance). A grouping block can be a looping block, in which case, the inner stack is executed more than one time (as in the example below), or it could be conditional, in which case, it may not be executed at all.

Grouping block example illustration. In this case, the inner stack will be repeated 10 times.

Value Blocks

Embeddable blocks come in two flavors: value blocks and conditional blocks. The value blocks contain some value that can be placed into another block to affect how it behaves. For instance, in the following stack, when the "Kitty Cat" sprite is clicked, it changes its size. The size is determined by the value block, pick random which will choose a random number and give this random number to the set size to block.

Sample hat stack illustration. In this case, when the sprite is clicked, the size will change based on coming up with a random number from 10% to 200%.

Conditional Blocks

Conditional blocks evaluate something and return either true or false . These blocks are distinguishable from value blocks by having pointy sides. In the following example, the if block has a conditional block that compares if the sprite is touching a particular color, and if so, it executes the commands inside. In this case, it hides the sprite.

Sample hat stack illustration. In this example, if the sprite is touching another sprite, it will move back and apologize.

Conclusion

In this chapter, we were introduced to the concepts and ideas of Scratch. You can't stop now, since we just scratched the surface. Now that you have the concepts in mind, jump to the next chapter and let's start building your first project.

Continue to the next chapter on building your first Scratch project, or return to the previous chapter.

Tell others about this article:

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	1. Introduction 2. Overview - What is Scratch Good For? 3. Downloading and Installing Scratch 5. Getting Started with a First Project 10. Making Music 11. Moving Sprites Around 12. Interacting Sprites 13. Passing Messages with Broadcasts Read other entries in Book...	All the world's a stage, And all the men and women merely players; They have their exits and their entrances, And one man in his time plays many parts, His acts being seven ages. –William Shakespeare As You Like It 4. Concepts and Overview of Scratch Before we get our knuckles sore, let's take a moment to go over some basic concepts. Welcome to the Theater While I suppose that all the world's a stage, I think it is helpful to look at a Scratch project as a play production. Any play is presented on a stage . The stage is where backgrounds are placed and actors carry out their lines. (The stage, in Scratch, is also in charge of "things that happen behind the scenes", but we'll get into that later). On the stage, our actors perform. In Scratch, actors are called sprites, and I dare say that you may feel your own actors aren't robots of your own creations, but capricious characters with their own motives. Each sprite can have one or more costumes as well as a script . The script contains the directions and actions it should perform. Like an actor in a play, a sprite can begin off-stage (we call it hiding), and then move across the stage, repeat its lines, and then exit. Unlike in a play, however, the sprite can change costume mid-scene without causing a ruckus. First Look at the Environment If you haven't already done so, now is a good time to start up the Scratch program. Let me highlight a couple of key areas; feel free to ignore the other dials, knobs and whistles for the moment. On the right side near the top is a large white area (in the screen shot below, I've marked with pink lines). This is the stage. This is where all the stories are told, the art designs are shown, the games are played, and the action happens. Below the stage is the Sprite Container, listing all of the sprites in the project. When you start a new project, you will have a single sprite, named `Sprite1` wearing a cat costume. It just so happens that this sprite is wearing his costume centered on the stage. Next to the sprites is an icon representing the stage. Clicking on this will allow you to change the backgrounds for the stage. Occupying the center section is the Sprite Details Area. It displays details based on which sprite (or the stage) is selected. Three tab allows you to change what is displayed: Scripts. Instructions and directions for the sprite's performance is placed in this area. Costumes. Each sprite can have one more more costumes. A costume is an image that the sprite can change based on a script. For instance, a sprite can change color, or a script can cycle through costumes in order to animate the sprite walking. Sounds. Each sprite can have a collection of recordings that it can speak based on instructions from the scripts. We'll talk about the details of this section later. I just wanted to get you familiar with the environment. Overview of Blocks When the Scripts tab is selected, the instructions for a particular sprite is shown. A sprite's script is where most of the interest lies, for without the script, our play would be quite lifeless. A single action for the sprite to perform is encapsulated in a block. The blocks come from the section on the far left side of the window, the Blocks Palette area. Scratch comes with a lot of blocks, so the blocks are grouped based on what they do or what they affect. Motion. Blocks that move and manipulate the placement of a sprite. Looks. Blocks that affect how the sprite appears; for example, to be larger or smaller or in different costumes. Sound. Blocks that play music, drum beats or recordings. Pen. Blocks that allow the sprite to draw on the screen. Control. Blocks that determine when scripts should run and control other stacks of blocks. Sensing. Blocks that allow a sprite to sense what is around it; for instance, various colors or other sprites. Numbers. Blocks that help a sprite perform math operations. Variables. Blocks that remember things. A block from a particular group has a color that matches the group. This feature is helpful in finding a particular block, as well as the added benefit of making it easier to duplicate the script listings in this book. Block Types Blocks come in two flavors, connectable and embeddable (called reporters). Connectable blocks connect into a sequence. An embeddable block is placed within another block and affects how it behaves. In the above diagram are three connectable blocks. As you can see, a connectable block is a lot like a LEGO brick in that in can be connected to other blocks to create a stack . Each block has the action it performs written on the brick. Each of these particular blocks has a notch at the top and bottom, which means that the brick can come anywhere in a stack. Hat Blocks The other type of connectable block is called a hat block which only occurs at the beginning of a stack. It usually specifies when the stack will be executed. Grouping Blocks Grouping blocks surround a stack of other blocks (they are also called crocodile mouths because of the oral resemblance). A grouping block can be a looping block, in which case, the inner stack is executed more than one time (as in the example below), or it could be conditional, in which case, it may not be executed at all. Value Blocks Embeddable blocks come in two flavors: value blocks and conditional blocks. The value blocks contain some value that can be placed into another block to affect how it behaves. For instance, in the following stack, when the "Kitty Cat" sprite is clicked, it changes its size. The size is determined by the value block, `pick random` which will choose a random number and give this random number to the `set size to` block. Conditional Blocks Conditional blocks evaluate something and return either true or false . These blocks are distinguishable from value blocks by having pointy sides. In the following example, the `if` block has a conditional block that compares if the sprite is touching a particular color, and if so, it executes the commands inside. In this case, it hides the sprite. Conclusion In this chapter, we were introduced to the concepts and ideas of Scratch. You can't stop now, since we just scratched the surface. Now that you have the concepts in mind, jump to the next chapter and let's start building your first project. Continue to the next chapter on building your first Scratch project, or return to the previous chapter. Tell others about this article:
	2009, Howard Abrams Some rights reserved	Originally written on 30 November 2008. Updated on 8 January 2009.