Mathemagics!

So now we're going to do some basic maths and a bit more stuff with variables. Nothing very hard really. There's also quite a lot of shorthand stuff, which we'll do as we go along. First, let's add a value to x.

class Hello { public static void main(String[] args) { //System.out.println("Hello, world!"); int x = 2; x = x + 3; System.out.println("The value of x is " + x); } }

Compile and run this new program. It should say The value of x is 5. To be absolutely clear: we take the value of x and add 3. Then we put this new value (5) back into x using the = symbol (or operator). Remember that using = in this way is called assignment.

OK, cool. So let's use some other operators to do subtraction, addition and division. To multiply we use * and to divide we use /.

class Hello { public static void main(String[] args) { //System.out.println("Hello, world!"); int x = 2; x = x + 3; System.out.println("The value of x is " + x); x = x - 1; System.out.println("The value of x is " + x); x = x * 3; System.out.println("The value of x is " + x); x = x / 7; System.out.println("The value of x is " + x); } }

Let's try it:

[nurmes]btg: javac Hello.java [nurmes]btg: java Hello The value of x is 5 The value of x is 4 The value of x is 12 The value of x is 1 [nurmes]btg:

I hope this all makes sense. The last result should surprise you - twelve divided by seven is roughly 1.71428 on my calculator. However we said that x is an int, in other words a whole number. So why isn't it rounded up to two? Well, when using the / operator with whole numbers, the result is always rounded down. Got that? Good.

So for our next mission, we are going to learn some shorthand. Notice with something like x = x + 2, we are taking the value of x, doing something to it and putting it back where we found it. This isn't so bad for x, but if our variable had a longer name then we might be typing supercalifragilistic = supercalifragilistic + 2. That would suck. So we have a shorter way of doing these things. It's easier to demonstrate than explain:

class Hello { public static void main(String[] args) { //System.out.println("Hello, world!"); int x = 2; x += 3; System.out.println("The value of x is " + x); x -= 1; System.out.println("The value of x is " + x); x *= 3; System.out.println("The value of x is " + x); x /= 7; System.out.println("The value of x is " + x); } }

As you can see, the program does exactly the same thing:

[nurmes]btg: javac Hello.java [nurmes]btg: java Hello The value of x is 5 The value of x is 4 The value of x is 12 The value of x is 1 [nurmes]btg:

This is also covered in Lecture Note 7, Section 7.1. We're going to learn another special shorthand in Java, so let's remove some lines from our program and change some:

class Hello { public static void main(String[] args) { //System.out.println("Hello, world!"); int x = 2; x += 3; System.out.println("The value of x is " + x); System.out.println("The value of x is " + x++); System.out.println("The value of x is " + x); } }

Make sure you put x++ and not x in the second one. Now if we think of x as meaning "get the value of x", then x++ means "get the value of x, and then add one to x". Let's run the program and see what I mean:

[nurmes]btg: javac Hello.java [nurmes]btg: java Hello The value of x is 5 The value of x is 5 The value of x is 6 [nurmes]btg:

Duh? Where did x change from 5 to 6? Well, the line with the x++ added one to x. So how come that line printed out The value of x is 5? That's the whole point. I'll repeat what I said: x++ means "get the value of x, and then add one to x". We get the value of x (to print out - the value at this stage is 5, so we print a 5), and then add one to it. So nobody sees that we've added one until the third time, when we print out The value of x is 6.

If that was totally over your head, don't worry as I'll explain it again in a different way before the end of the lesson. Now let's try a very small change to the program:

class Hello { public static void main(String[] args) { //System.out.println("Hello, world!"); int x = 2; x += 3; System.out.println("The value of x is " + x); System.out.println("The value of x is " + ++x); System.out.println("The value of x is " + x); } }

Now the operator ++x is slightly different to x++. It means something like "add one to x, and then get the value of x". So we add one (making x equal to 6) and then get the value of x (which is now 6) and print it out. Look:

[nurmes]btg: javac Hello.java [nurmes]btg: java Hello The value of x is 5 The value of x is 6 The value of x is 6 [nurmes]btg:

Let's do two more operators from the same family. Here's the first one in action:

class Hello { public static void main(String[] args) { //System.out.println("Hello, world!"); int x = 2; x += 3; System.out.println("The value of x is " + x); System.out.println("The value of x is " + x--); System.out.println("The value of x is " + x); } }

[nurmes]btg: javac Hello.java [nurmes]btg: java Hello The value of x is 5 The value of x is 5 The value of x is 4 [nurmes]btg:

Can you see that x-- is very much like x++? Let's do the last one (you can probably guess what it is...):

class Hello { public static void main(String[] args) { //System.out.println("Hello, world!"); int x = 2; x += 3; System.out.println("The value of x is " + x); System.out.println("The value of x is " + --x); System.out.println("The value of x is " + x); } }

[nurmes]btg: javac Hello.java [nurmes]btg: java Hello The value of x is 5 The value of x is 4 The value of x is 4 [nurmes]btg:

OK, if my explanations suck then how about this for a way of understanding what these operators do:

Using "clever" operators	Equivalent
System.out.println("The value of x is " + x++);	System.out.println("The value of x is " + x); x = x + 1;
System.out.println("The value of x is " + x--);	System.out.println("The value of x is " + x); x = x - 1;
System.out.println("The value of x is " + ++x);	x = x + 1; System.out.println("The value of x is " + x);
System.out.println("The value of x is " + --x);	x = x - 1; System.out.println("The value of x is " + x);

The techincal jargon for these is incrementing (adding one) and decrementing (subtracting one). x++ and x-- are known as postfix operators (meaning that they come after x). ++x and --x are known as prefix operators (meaning that they come before x). These are covered in Lecture Note 6, Sections 6.3 and 6.4.1.

Well done, you've learned a whole big heap of operators including the last four which are quite hard. The next lesson is a really important thing about programming, and you won't get far without it, but it's not very difficult, so breathe easy, get a coffee and chill out. Then let's play Shooting Stars!.