What You are Going to Learn

In this lesson we are going to introduce variables. Computer programs use variables to remember things. Think of a word processor. The computer has to remember the words that you type. The word processor program uses variables for this.

We are going to show you the rules for variables and the patterns you need to use them in your programs.

How to Remember Things

So far you have learned how program in Go to do simple sums and print strings. But both of these programs have been fixed. Each program executed its instructions in sequence from the beginning to the end. You did not need to tell the program to remember anything from one line to the next.

To tell a program to remember something you need a variable - simply something that can vary. The variables are stored in the computers memory.

A variable in a program does not last forever. It only exists while your program is running. Once your program ends the variable does not exist in the computers memory anymore. So if you turn your computer off and on again the variable will not exist in the computer’s memory any more.

This is different from the programs, photos and music on your computer. They do exist if you turn your computer off and on again. These are all called files and they are stored on your computer’s hard disk or memory card or memory stick.

The difference is that the computer uses a different type of memory for variables and files. The type of memory used for variables is called random access memory or RAM for short.

Variables store the data in your program.

Different Memory Types

RAM is sometimes referred to as volatile memory. It is volatile, because when power is removed from this type of memory, the data is lost. This type of memory is very fast but is expensive to buy.

Non-volatile memory is slower, but remembers after the power is removed from it. This type of memory is used in devices like flash memory cards for digital cameras and USB memory sticks. These devices use non-volatile memory to store data, and have to be formatted or prepared by the computer before they can be used. A programmer would consider this memory as persistent storage, just like a hard disk.

Variables are an essential part of programming. It is almost impossible to write a program that does not use variables. Once you master variables you will be able to write more powerful programs.

Let’s see where the idea of a variable comes from.

Variables in Mathematics

Variables are also used in mathematics, especially in algebra. Lets take a simple example. If we look at the simple sum

3 + ? = 10

What number do you need to put where the ? is to make the sum correct. If you work this out, you find that ? should be the number 7 to give you

3 + 7 = 10

Which is correct.

Now we need to take this one step further. Let us look at this simple sum

x = 20 - 9

What value should x be replaced by? If you do the sum 20 - 9 you get the answer of 11. So x should have the value 11.

Remember that x has the value of 11, what if we tried this sum

y = x + 1

Now what is the value of y? We work this out you have to remember that x is really the number 11. So we can replace x in the sum with the value of 11. Like this

y = 11 + 1

Now can work out the value of y?

The answer is 12

Both x and y are variables. We are using a letter to represent a number. We can use one of the letters, x or y where we need the values 11 or 12 in the sums.

Variables in computer programs are very similar.

Variables in Programming

A variable allows you the programmer to use a letter, or name, to refer to a value that is stored in the computers memory. So we can say something like this

a = 8
name = "Bob"

We can now use a when we want to use the number 8. We can also use the word name to mean “Bob”. We could use them like this

fmt.Print("Hello. My name is ")
fmt.Println(name)
fmt.Print("I am ")
fmt.Print(a)
fmt.Println(" years old.")
Fig-1. Part of the hellobob program.

If we could run this (you cannot yet) what do you think would happen?

If you could run the code you would see this:

Hello. My name is Bob
I am 8 years old.

What do we have to do to make the hellobob program run? To that we need to show you how to create a variable.

Creating a Variable

To create a variable in Go you need three things

  • A keyword
  • A name for your variable
  • A type for your variable

Let’s look at the keyword first.

The Variable Keyword

You have to tell Go each time you want to create a variable. You do this by using a keyword. The keyword is

var

which is short for variable. A keyword is a word that has a special meaning in a Go program. You cannot use a keyword for anything else, so you cannot give a variable the same name as a keyword.

You have already used a couple of keywords before in Go. We did not explain their significance at the time. The keywords you have seen before are package, import and func. We will explain what all three of these keywords mean in later lessons.

Just so you know, the complete list of Go’s keywords is

breakdefaultfuncinterfaceselect
casedefergomapstruct
chanelsegotopackageswitch
constfallthroughifrangetype
continueforimportreturnvar
Once you type var the next thing that Go expects, after a space, is the name you want to use for your variable.

The Name of the Variable

Once you have told Go that you want to create a variable the next thing you need to do is give your variable a name. You give your variable a name like this.

var age

Now we have a variable called age. This leads to an obvious question. How did we know that we wanted to call this variable age? We knew because we want the variable to store your age. We made sure we picked a variable name that had an obvious meaning when we read the program.

So what should you name your variables? You should name your variables after the use they have in your program. It is very important that this name is meaningful. Programs are read by programmers many times before they are run by a computer. The variable names you choose have to make sense to you and to someone who has never seen your program run.

But, you cannot call your variables anything. You are only allowed to use legal variable names. A variable name is only legal if it follows the rules for variable names. These rules are part of Go’s syntax rules.

The rules for variable names are

  • You cannot use a keyword as a variable name. You already know this rule.
  • You cannot use spaces in your variables names, or any other symbols like ().
  • You cannot use a package name for a variable name.
  • You cannot start a variable name with a number.
  • You cannot use the same name as another variable. Each variable name has to be unique.
  • You can only use the letters a to z or A to Z or the numbers 0 to 9 or the underscore character _.

If we show some examples this will be clear.

Variable NameLegalReason
ageYesThe name contains only lower case letters
AGEYesThe name contains only upper case letters
AgeYesThe name contains only upper and lower case letters
myAgeYesThe name contains only upper and lower case letters
myageYesThe name contains only lower case letters and an underscore
age1YesThe name contains only lower case letters and numbers
2ageNoThe name starts with a number
my ageNoThe name contains a space
my-ageNoThe name contains a minus sign, -
my+ageNoThe name contains a plus sign, +
fmtNoThe fmt is a package name
mapNomap is a keyword

Now we come to the last thing that you need for a variable its type.

Variable Type

The last thing you need to use a variable is the type of the variable you want to create.

But what is a type? To explain this think about the food you can buy in the supermarket. You can buy meat like chicken, pork, beef or fish. You can buy vegetables like carrots or potatoes. You can buy fruit like oranges, bananas or apples. These are all different types of food.

Go has types too. You have already seen two of them, integer numbers and strings.

So what does that have to do with programming? Well just like you cannot make carrot cake with bananas, you cannot do sums on a string. The type of a variable limits where and how you can use the variable.

This is a good! It means that Go can help you find mistakes. If you tried to subtract two strings for each other, Go will tell you that that does not make sense before your program runs.

The type to use for a number is int and the type for a string is string.

Putting it all Together

If we put all three parts together, the var keyword, the variable name and the variable type we can use variables in Go. Let’s try it.

    1 var age int
    2 var your_name string

You always read these lines from left to right. If we do that line 1 says “ This is a variable called age which is a number”. Line 2 says “This is a variable called your_name which is a string.”

This is called declaring a variable or just a _variable declaration.

Variable Type (again)

Now you know how to declare a variable, we can tell you what the computer does with the variable declaration.

When you declare a variable the program tells the computer to reserve, or allocate some memory for the variable. The amount of memory that the computer will allocate depends on the type of the variable. This is why the type is important. You are telling the computer how much memory you might need.

As the programmer you do not need to know exactly how the memory is allocated, or even how much memory was allocated. Go is a high level programming language. This means that (most) of the details of the computer’s memory and how it works are handled by Go.

This is an example of abstraction. You as a programmer can use variables and Go will take care of putting them in memory for you.

Setting the Value of a Variable

Now you have seen how to create a variable the last thing to show you is how to give a value to the variable. To do that you use the equals, = sign.

So if we want to set the value of age to 8 we would type

age = 8

If we want to set my_name to Bob you would type

my_name = "Bob"

There is a pattern you can use to remember how to set the value of a variable. The pattern is

name-of-variable = new-value

First you use the name of the variable that you want to set, then an equals, = sign and then the new value for the variable.

You can only set a variable to a value that makes sense according to the type of the variable. If you tried to set the variable age to the string "eight" like this

age = "eight"

Your program will not run. Go will tell you that you are trying to set a variable which is a number to a value that is a string.

Setting the value of a variable is called assignment. And used in this context, the equals sign is in fact called the assignment operator.

The Variable Program

You can now put this altogether in the hellobob program.

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 hellobob after the program we will write. Then we need to change directory into the new new hellobob directory.

mkdir hellobob
cd hellobob

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

On Linux, Windows and MacOS X

atom hellobob.go

On Raspberry Pi

liteide hellobob.go

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

From now on

From now on when we want you to write a program we will just tell you to open your text editor and type in the program. You have to remember to open a terminal, create the directory and start the editor with the new file, ready for you to type.

Once your editor starts type in the hellobob.go program exactly as you see it.

    1 package main
    2 
    3 import (
    4    "fmt"
    5 )
    6 
    7 func main() {
    8    var name string
    9    var age int
   10 
   11    name = "Bob"
   12    age = 8
   13 
   14    fmt.Println("The hellobob program shows you how to use variables.")
   15    fmt.Println("")
   16 
   17    fmt.Print("Hello, my name is ")
   18    fmt.Print(name)
   19    fmt.Println(".")
   20    fmt.Print("I am ")
   21    fmt.Print(age)
   22    fmt.Println(" years old.")
   23 }
Fig-1. The hellobob.go code

Once you have typed the program in, you need to save it. Once you have saved it you need go to your Terminal or Command Prompt window and type run it with:

go run hellobob.go

From now on

From now on when we want you to run your program we will just tell you to run it. You have to remember to use your terminal or command line window and type

go run
followed by the name of the .go file you want to run. Just like the you have done with hellobob.go

If you typed the program correctly you should see

The hellobob program shows you how to use variables.

Hello, my name is Bob.
I am 8 years old.

The important parts of the hellobobprogrm are lines 8 and 9, 11 and 12 and lines 18 and 21. The other lines are mostly Println lines that print out the strings to the terminal. These are the same as the Println and Print lines that you used in both the stringfun and numbers programs.

The Variable Declarations

Lines 8 and 9 and where you declare the variables that you used in the program. Here they are again.

var name string
var age int

In both lines you can see the three things you need to declare a variable. There is a var keyword, a name for your variable, and then a type for the variable.

The first variable that you declared is called name. The name variable is a type of string. You can only remember words in this variable.

The second variable that you declared is called age. The age variable is of type int, short for integer, which is a number. You can only store numbers in age.

At this point in the program, lines 8 and 9, you have not assigned any values to the variables.

The Variable Assignments

Lines 11 and 12 are where you assigned values to the variables. Here they are again.

name = "Bob"
age = 8

Both lines follow the same pattern. To the left of the equals, =, sign you have typed the name of the variable you want to change. To the right of the equals sign you have typed the new value for the variable.

The first line assigns, or sets, the new value of the name variable to the string Bob. You have declared that the name variable is a string so you can only change its value to another string.

The second line sets the value of age to the number 8. Remember that you had declared age to be an int (a number) type, so you can only change its value to another number.

Using the Variable

Lines 18 and 21 are where you used the variables. Here they are again.

fmt.Print(name)
...
fmt.Print(age)

Notes

The ... means that we have removed one or more lines.

These lines show you how to use your variables. As you can see it is really easy to use a variable. You just type the name of the variable when you want to use it. When the program reaches an instruction with the variable, its current value will be used in place of the variable name.

In this case the values are used by the Print function in the fmt package to print the values to the terminal.

You can see this in the output of the program

The hellobob program shows you how to use variables.

Hello, my name is Bob.
I am 8 years old.

Where you have used the variable name name the program has printed Bob. Where you used the variable name age the program has printed 8.

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