I know Windows has a Calculator, but we are going to build our own. This will give you an idea of how real Calculators actually work.

The Calculator should be able to:

1. Add, Subtract, Multiply and Divide 2 numbers.

2. Be able to work with decimal points (2.5 for example).

3. Display the sum and the result on the screen.

4. Have a “clear” button to reset everything ready for the next calculation.

Let’s look at the steps that you need for a calculation

1. Clear the variables.

2. Click on the numbers for the first number.

3. Click on the operator you want to use.

4. Click on the numbers for the second number.

5. Click on the equals button.

6. Display the result.

Hint: When you click on the Operator, this indicates we have got the first number and need to start storing the second number. See variable 6.

It sounds like there is a lot to do, but if we are smart with the way we work, there is a lot of code that is very similar and we can copy and paste it and then modify it slightly.

You will need 6 variables (for all Sprites) to share:

1. The first number in the calculation (NumberOne), not visible.

2. The second number in the calcualtion (NumberTwo), not visible.

3. The complete sum (Question), visible.

4. The operator (+,-,x,/) for the calculation (Operator), not visible.

5. The result of the calculation (Result), visible.

6. A variable to indicate what number we are working with (SecondNumber). not visible.

Hint: When you press clear, all the variables should be set to blank, except the SecondNumber which should be 0

You will need 17 Sprites, don’t panic! Remember copy and paste is your friend:

11 Number Sprites 0,1,2,3,4,5,6,7,8,9 and the decimal point.

4 Operator Sprite +,-,x and /.

1 Equals (=) Sprite.

1 Clear Sprite.

Scratch code blocks that will be used:

Operators, we will use the join operator a lot as well as the +,-,x and /.

If Else block with variable 6 to decide what number to update.

set variable blocks – lots of these!

Hint:

Start by creating the variables.

Start with just 4 Sprites, 1, +, clear and Equals

Once you can get 1 + 1 working, the rest is quite easy.

The answer is 2 (not 11) by the way, just in case.

Coding Tips:

Here is some of the code that we will be using:

This is the code when you click on a number Sprite, this example is for the number 1, can you see what needs to change if this was the number 2 sprite?

This is the code for the + Sprite

And this is the code in the = Sprite

And finally, the CLR sprite

This weeks challenge is to build a working clock.

This is not just an ordinary clock though, we are going to build a clock that works in the same way as the Railway clocks works in Switzerland.

These clocks work in the following way:

• The Clock waits for a signal from the master clock
• When it gets the signal, it moves the minute hand on by one minute (and the hour hand if necessary)
• then the second hand moves round once in about 58 seconds
• It then waits for the signal again from the master clock.

This means that all the clocks on all the Stations are synchronised exactly.

In Computer speak this is called Event Driven Programming, where the code simply waits for an Event before doing anything.

So what’s the plan?

We need 4 Sprites

1. For the Clock Face
2. For the Hour Hand
3. For the Minute Hand
4. For the Second Hand

You can create them however you want but this is what the Swiss railway clocks look like and I will try and create something similar.

Remember one thing, when creating the Sprites make sure they are centred correctly.

So, the clock face should be perfectly centred and then the hands should be centred near one end something like this, I have highlighted where the centre is, with the green circle.

The code

Remember this will be event driven, so we will be using Broadcasts a lot. We will also have to calculate how far to move each hand, this means we will be using the number 360 a lot as that is one complete circle.

So just to let you know how far each hand should move:

• The Second hand will move 360 degrees for each event.
• The Minute hand will move 6 degrees for each event (360/60 minutes)
• The Hour hand will move 0.5 degree for each event (360/60 minutes/ 12 hours)

The first event will be broadcast from the Stage:

The Minute hand will receive this broadcast and move 6 degrees it will then broadcast another 2 events, one to the Hour hand and one to the second hand so they can move.

This is the code for the Minute hand which should be enough to work out the rest of the code as well.

The extra code above is to make sure the Minute hand starts in the correct position.

And one final thing which might be a little tricky, but I’ll leave it as a challenge for you, the Second hand should only take 58 seconds to go all the way around.