Computer Programming is to let the computer know what we want it to execute.

Computer understands only a special language. When you want to communicate with your friends, you use human languages like english, french or spanish. When you want to communicate with a computer you use computer languages like ruby, python or clojure. All of them are examples of computer programming languages.


Enough introduction, let’s start to do some real programming.

The 3 steps of an expression

Let’s imagine you don’t remember your table of multiplications and you want the computer to calculate 7*8 for you. How are you going to do that?

For that purpose, you will have to master the 3 steps of an expression:

  1. First, you need to tell the computer that you want him to execute something. For that you use the parenthesis: (). The computer will execute for you the content of the parenthesis.

  2. Then, you need to tell him what operation you want him to execute: in our case, the operation is the multiplication. The symbol for multiplication is: *.

  3. Finally, you need to tell him what are the details of the operation. We call them the operands. In our case, the operands are 7 and 8. The operands must be separated by one or more white spaces one from the other and one from the operation.

Combining all of that, we get:

(* 7 8)

Now, modify the operands above and try to replace 7 and 8 by 4 and 5.

What happened?

Did you get 20?

Now, try to add more operands: for instance you could type (* 2 3 4 6 8 2 3).

Try to add more white spaces between the operands, or between an operand and a parenthesis.

What results do you get?

When you talk to a friend you use sentences. When you talk to a computer, you use expressions.


If you are having difficulties with one exercise, read again the details of the 3 steps of an expression.

A. Write a program that calculates 7*8


Did you get 56?

B. Write a program that calculates 2*3*4*5


Did you get 120?

C. Write a program that calculates 2+3+4+5


Did you get 14?

Here are the solutions in video.

Send us a screenshot with your programs to

Now, you are ready for chapter 2.