Introduction to Scratch

Familiarising yourself with ‘Scratch’

Instructions and Controls

Scripts Area

Sequence of Instructions

SpritesThe Stage

If a computer is a box…

…think of a program as a man inside the box!

…and programming is all about instructing the man to do what

you want!

“Tell me what to do and I will do it!!”

The man in the computer (program)

thinks just like a human.

If we taught the man how to have a conversation with the

computer user, what would be the first thing we would tell him to

do?

Inputs → Storage → Process → Output• Conversations start with a question.• In other words the man in the computer might

ask the user for some information (INPUT).

• Example:“What is your name?”

The user would then type IN their name!

INPUT “What is your name?”

Inputs → Storage → Process → Output

• Then the man in the computer will store the answer in its brain (STORAGE) and think about how to respond (PROCESS).

1. Remember the answer (STORE IT)

2. Think about how to respond (Process)

Inputs → Storage → Process → Output• The man in the computer would finally

respond (OUTPUT).

• Example:“So, your name is BOB! What a great name!”

The computer would display this on the screen (OUTPUT).

OUTPUT

Inputs → Storage → Process → Output• All programs work in this way…• Think of a word processor:

INPUT

STORAGE/PROCESS

OUTPUT

1. Users can press a letter on a keyboard (input)

2. The computer stores this event and decide how to respond (storage/process)

3. And then display the letter on the screen (output)

• The ASK script asks the user to enter (input) a value into scratch?

Inputs → Storage → Process → Output

What do you get when you ask a question?

ANSWER!

Once you have inputted a value into scratch it is stored in a ‘variable’ called answer.

This is what the script looks like

Inputs → Storage → Process → Output

What is a variable?• In programming, a variable can be thought of

as a storage box.• The box may be given a name, and it may

hold various different things.

• In scratch, is the name of the variable, and it will store whatever you type into scratch.

Variable called ‘ANSWER’ Contents is anything we type in

Storage

Inputs → Storage → Outputs• Which of the following scripts do you

think will help us output a value onto the screen?

The Join Script

• We can also join words and the ‘ANSWER’ together to create sentences:

Moving your Sprite• In programming, things always happen in a

set order – a sequence.• Sequences are first written as a flow chart

and then programmed in a game.• If we want to move a sprite left the following

sequence would have to occur:

Press right arrow key

Sprite faces to the right

Sprite moves a step to the right

In Scratch, we program the sprite like this:

Inputs, Outputs, Variables and Decisions

Inputs → Storage → Process → Output• In Scratch, which of the following scripts

will help us program the man (inside the computer) to demand an input from a user?

• The ASK script asks the user to enter (input) a value into scratch?

Inputs → Storage → Process → Output

What do you get when you ask a questions?

ANSWER!

Once you have inputted a value into scratch it is stored in a ‘variable’ called answer.

This is what the script looks like

Inputs → Storage → Process → Output

What is a variable?• In programming, a variable can be thought of

as a storage box.• The box may be given a name, and it may

hold various different things.

• In scratch, is the name of the variable, and it will store what ever you type into scratch.

Variable called ‘ANSWER’ Contents is anything we type in

Storage

• Which of the following scripts do you think will help us program the man (inside the computer) to output a value onto the screen?

Inputs → Storage → Process → Output

Summary

INPUT SCRIPTS:

STORAGE SCRIPTS:

OUTPUT SCRIPTS:

The “join” script is a nice extra to combine set text with the user’s text to form sentences when outputting to the screen

IF statements are how programs make decisions (process data).

IF this occurs, I will do thisELSE, I will do that

IF – ELSE statements allow programs to make decisions based on certain conditions occurring.

Inputs → Storage → Process → Output

What’s going on here?Program asks the user to input a number from 1 to 3 Program stores their input

in the variable called ‘answer’

IF answer contains 1, the program will display a message

IF answer contains 2, the program will display a different message

IF answer contains 3, the program will display a different message

Inputs, Outputs, Variables and Decisions

You are all going to be real computer programmers!!!

Today• We will create a simple calculator in

scratch which will add two numbers together and display the result

2 5

So how can we get our sprite to do the following?

Ask for a number?

Store the number?

Ask for another number?

Store the number?

Display their

‘Sum’

First issue:

Scratch only has one variable (storage box) “answer”.

Scratch can therefore only store one number!?

Number 1 Number 2We need 2!!

What is a variable?• In programming, a variable can be thought of

as a storage box.• The box may be given a name, and it may

hold various different things.

• In scratch, is the name of a ready made variable, and it will store what ever you type into scratch.

Variable called ‘ANSWER’ Contents is anything we type in

Storage

So how can we get our sprite to do the following?

Ask for a number?

Store the number?

Ask for another number?

Store the number?

Display their

‘Sum’