What You are Going to Learn?

Computers are used to process data. All data is made up of numbers. Yes, really! Everything is just a bunch of numbers to a computer. These are the only things they understand.

We are going to explain how numbers are used in Go programs. Then we are going to show you how to do type sums in Go.

Before you begin

Before we begin we are going to assume that you have Go installed and it is working on our computer. If you have not installed Go yet then you need to read our Go install guide. We are also assuming that you have installed either Atom or LiteIDE on your computer. Again if you have not done this yet you need to go and read our editor install guide.

Once you have Go installed it is time to start.

Everything is a Number

Computers process data, that’s their job. But they only process one type of data - numbers. Everything, and we mean everything is just a bunch of numbers to a computer. You might think a picture is a picture or a word is a word, but not to a computer. They are all numbers.

Now it is your turn

Rainbows are made up of seven colours

  • Red
  • Orange
  • Yellow
  • Green
  • Blue
  • Indigo
  • Violet

How could you represent these as numbers?

You have to pick a number to each colour. For example

  • Red = 1
  • Yellow = 2
  • Orange = 3
  • Green = 4
  • Blue = 5
  • Indigo = 6
  • Violet = 7

Now the sequence

7, 6, 5, 4, 1, 2, 3 means

Violet, Indigo, Blue, Green, Red, Yellow, Orange

This is similar to what the computer does with colours. The computer just uses much bigger numbers to represent lots more colours.

Starting small

Go programs are programmed by typing. You type Go commands into your text editor and save the file as a Go source code file. That’s a file with a .go file extension, like the helloworld.go that we saw when you tested your Go installation.

This is a different process compared to programming in Scratch. You program in Scratch by joining coloured blocks together instead of typing commands. But this is not what professional programmers do, they type instructions.

We are going to use Go to teach you what is inside the coloured blocks that Scratch uses.

Starting with numbers.

Numbers in Go

We are going to start with whole numbers. These are numbers you use when you count like 1, 2, 3, 10, 50 or 100 and 0. Go calls these numbers integer numbers, or int for short. You can use numbers in Go the same way you use them at school. You can add them, subtract them, multiply them and divide them.

Sums in Go

Typing sums in Go is easy. It is almost the same as you are used to.

Addition is typed with a + typed Shift+= like this 1 + 2

Subtraction is typed with a - like this 6 - 3

Multiplication is a little harder. You cannot type a ×, instead you have to type a * typed Shift+8 for multiplication like this 4 * 3

Division is a similar to multiplication. You cannot type a ÷, instead you have to type a / like this 10 / 2

Now it is your turn

Can you work out the answers to these sums?

4 + 5 = ??
7 - 3 = ??
4 * 8 = ??
16 / 4 = ??

The answers are

4 + 5 = 9
7 - 3 = 4
4 * 8 = 32
16 / 4 = 4

The Numbers Program

Let’s write a Go program to show you this.

Open your terminal or command prompt. We are going to put each Go program in its own directory. This is the recommended practice for Go programs. In your terminal you need to change to the location of your Go Workspace. To do this type

On Linux, Raspberry Pi and Mac OS X

cd $GOPATH/src/

On Windows

cd %GOPATH%\src\

Now you need to make a new directory. We need to call this numbers after the program we will write. Then we need to change directory into the new new numbers directory.

mkdir numbers
cd numbers

Now you need to start you editor, either Atom or LiteIDE

On Linux, Windows and MacOS X

atom numbers.go

On Raspberry Pi

liteide numbers.go

The numbers.go tells Atom or liteIDE start with the file number.go open in the editor. If the file does not exist the editor will create it for you.

Once you editor starts you can type in the numbers.go program.

Type carefully

Remember to type in numbers.go exactly as we have it here.

    1 package main
    2 
    3 import "fmt"
    4 
    5 func main() {
    6    fmt.Println("The numbers program shows you how to add, subtract")
    7    fmt.Println("multiple and divide integer numbers.")
    8    fmt.Println("One plus one is typed: 1+1")
    9    fmt.Print("1+1=")
   10    fmt.Println(1 + 1)
   11 
   12    fmt.Println("Ten subtract three is typed: 10-3")
   13    fmt.Print("10-3=")
   14    fmt.Println(10 - 3)
   15 
   16    fmt.Println("Three multiplied by four is typed: 3*4")
   17    fmt.Print("3*4=")
   18    fmt.Println(3 * 4)
   19 
   20    fmt.Println("Six divided by two is typed: 6/2")
   21    fmt.Print("6/2=")
   22    fmt.Println(6 / 2)
   23 }
Fig-1. The numbers.go code

Once you have typed the program in, you need to save it. Once you have saved it you need to run it with:

go run numbers.go

If you typed the program correctly you should see

The numbers program shows you how to add, subtract
multiple and divide integer numbers.
One plus one is typed: 1+1
1+1=2
Ten subtract three is typed: 10-3
10-3=7
Three multiplied by four is typed: 3*4
3*4=12
Six divided by two is typed: 6/2
6/2=3

The important parts of the program are lines 9 and 10, and the similar lines 13 and 14, 17 and 18, and 21 and 22.

Lines 6, 7 and 8 use the Println function from the fmt (short for format) package to print what is between the “’s. You have used this function before when you wrote Hello World to test your Go install.

Line 9 is very similar to the Println function used in line 8. The Print function from the fmt also prints what is between the “’s but does not take a new line at the end.

Line 10 is the key line. Let us look at it more closely.

fmt.Println(1 + 1)
Fig-2. Line 10 from numbers.go

The first part of line 10 used the Println function from the fmt package. The question you now need to ask is what will this print? The answer is as simple as 2.

The next question to ask is why is it 2? OK, well it is 2 because that is the answer you would expect if you added one and one together.

But why does it not print 1+1? Well, in simple terms it is because the 1+1 is not inside “’s. We will see why this is important when we talk about words.

So what happens then? When you run the program the computer sees the fmt.Println and knows that you want to print something to the terminal window. But at this point it does not know what you want to print. To work that out it looks at what is inside the brackets, the (...).

Inside the brackets it sees 1+1. The computer has not seen any “’s so it knows that it cannot print this. But because there are no “’s the computer knows that it has to work out the answer to 1+1 - the calculation - and then print the answer.

Which is why you see a 2.

If we look at the output

1          +            1     =            2
|<---- line 9 prints this ---->|<---- line 10 prints this ---->|
Fig-3. The output of lines 9 and 10 from numbers.go

line 9 prints the first part, which is everything to up to and including the equals sign. Then line 10 prints the answer, which is 2, on the end and then takes a new line.

Now you know how the program works we can explain the bit at the start of the program. These are lines 1 and line 5

package main
....
func main() {

These come as a pair. Line 1 declares that this file is part of the main package. The main package on line 1 must contain the main function on line 5.

The main function on line 5 is where the program begins execution - not line 1. Program execution then proceeds in sequence from this point onwards i.e. down the screen as you look at it. There must be exactly one main function in any program. There must also be exactly one main package in any program. The package main line must also be the first line in the source code file. No program code can appear before this line.

The next funny looking line is line 3.

import "fmt"

This is a package import line. Before Go can use a package, it must first import it. Packages contain lots of useful stuff written by other programmers that you can use. This useful stuff is called functions.

The name of the package that is to be imported must be placed inside inverted commas. In this case the program is importing the fmt package, short for “Format”, which contains the functions such as Println that prints text to the terminal window.

Don’t panic if you don’t understand this yet. You will see these lines in almost every Go program you write.

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