Programming Basics

Programming

Basics

Computer programmers use some standard techniques or building blocks to create their computer programs.

This Prezi will introduce you to some of these techniques

and will give you some Scratch examples that you can use in your own programs.

Graphics

Motion

If you are using sprites you need to be able to position them on the screen. This is done by using a system of screen coordinates.

The same system of coordinates can be used to move your sprites on the screen.

x is the horizontal position and y is the vertical position.

If you increase the x value the sprite will move to the right - if you decrease the x value the sprite will move to the left.

If you increase the y value the sprite will move up the screen - if you decrease the y value the sprite will move down the screen.

The red fish is at x: -100 y: 100

The yellow fish is at x: 100 y: 100

The blue fish is at x: -100 y: -100

The green fish is at x: 100 y: -100

Sequence

If a program is going to work it must run in the correct order or sequence.

You can program Scratch so that one sprite tells another sprite a joke. If the sprites don't speak in the right order the joke does not make sense.

Sprite 1 - "Doctor."

Sprite 2 - "Who's there?"

Sprite 1 - "How did you know?"

Sprite 1 - "Knock knock."

Sprite 2 - "Doctor who?"

Repeat

You will often want to do something more than once in your programs.

Computers are excellent at repeating instructions. They can do so over and over again, they never get tired and never make a mistake.

This is sometimes called looping or iteration.

The script on the left will make the bat flap its wings as it moves.

For an animation the bat needs to keep flapping its wings so we have used a repeat loop to make it flap itd wings ten times.

Variables

Sometimes you will want to store some data in your programs. A good example is a game that needs to keep a player's score.

We use something called a variable to keep the score. The varible will have to change each time a player scores some more points.

A good way to think of a variable is to imagine a little box into which a number can be placed. The variable must be given a name. We will call it n because it is going to store a number (variables can also store letters and words).

Suppose a player scores 4 goals - we must store 4 in the variable called n. If the player scores another point we will need to add 1 more to the variable. The variable needs to change to the variable plus 1. This can be written as n = n +1 (see the diagram below) so n= 4 + 1 = 5. The player's score is now 5.

We can do this with the script change n by 1.

Decisions

Programs can just follow a simple sequence of instructions but more often programs are written to respond to something that the user has done or to an event that has taken place.

In the game shown on the right the big green fish has to eat all the little red fish.

The program will hide the red fish when it has been eaten.

The program must make a decision - have the teeth of the large fish touched the red fish?

The movement of the red fish is placed in a loop that will make it repeat forever but if the red colour of the little fish touches the dark blue colour on the teeth of the large fish, the little fish will say 'Got me!' and will then be hidden to make it look as if it has been eaten.

This is also known as selection.

Many of the programs or apps that you use will be graphical.

They make use of colourful backgrounds and moving images or characters known as sprites.