What You are Going to Learn

In the last two lessons we introduced variables. Now we are going to put variables to work.

In this lesson we are going to show you how to use variables to remember what you have typed at the keyboard. We will introduce the pattern for going this.

Reading What You Type

We introduced you to the idea of a variables in the last two lessons. But while we showed you how to declare a variable and how to assign a value to it, we have not shown you what the variable can be used for. We’re going to show you that in this lesson.

Important

Make sure you have completed the last two lessons about variables before you try this lesson. You will need to understand variables first.

We are going to start by showing how you can use variables to make the program respond to the letters you type at the keyboard.

Input has Many Forms

The keyboard of your computer is just one form of data input that the computer can use. Another one is the mouse or trackpad or touchscreen. Your computer program can read the data sent from these inputs and use it within the program, for example to move the pointer on the screen.

Other forms of input are files, photos for example, saved on the computer’s hard disk. Or a network connection - your web browser reads web page data from the network connection. Or sensors, like a temperature sensor, connected to the computer, or a webcam.

Keyboard Input and Variables

We are going to show you how to read numbers and strings from the keyboard. We are going to change the hellome program too, so that you can type in your name and your age. We need to use variables to do this.

So far you have used the Println function from the fmt package to print strings and numbers to the screen. This is an example of the output from the program. Another example out output would be printing a document on paper using a printer. Or saving a new file to disk so that you can use it later.

Input and variables combined are the key to changing the program’s behaviour when it runs. Think about the hellome program for a minute. The program prints out your name when you run it. But what if you wanted to give the program to you friends? How would you get the program to print their name instead of yours?

You could change the program for each for your friends so that it printed their name. But, if you have lots of friends this will take a long time. You will to write a need a different program for each of your friends.

A better way would be to write just one program that asks your friends to type in their name when they run the program. To do this you need to use both variables and input.

How to Read What the User Types

Imagine you had a line like this one in your program

name = simpleio.ReadStringFromKeyboard()

Now it is your turn

What do you think this line might do?

The line is a variable assignment pattern. It assigns whatever the user types in to the variable name. The value for name comes from the ReadStringFromKeyboard function in the new simpleio package.

Lets break the line down to show you why. Look carefully at the line and you can see that it follows the pattern for variable assignment

variable-name = new-value

The variable name is easy to work out, that is just name. The value is a little harder, but not much.

Look carefully at these two lines

simpleio.ReadStringFromKeyboard()
fmt.Println("Please type in your name")

Can you see the two patterns in each line?

Both lines call a function. The first line calls the ReadStringFromKeyboard function and the second line calls the Println function.

The first pattern is at the start. It looks like this

package-name.name-of-function

In the first line the package name is simpleio and the function name is ReadStringFromKeyboard. In the second line the package name is fmt and the function name is Println

The second pattern is at the end. It looks like this.

name-of-function(...)

The ... could be anything, or nothing at all. But whatever it is we are telling the function that we want the function to process the something. In the case of the Println function the something is a string with the value Please type in your name. In the case of the ReadStringFromKeyboard function we don’t need to supply any extra information.

Now going back to the assignment of the name variable

name = simpleio.ReadStringFromKeyboard()

You can now see that there is a variable called name on the left of the = sign and a function from the simpleio package called ReadStringFromKeyboard on the right of the = sign.

But what is being assigned to the name variable? The answer is the result of the ReadStringFromKeyboard function. The result whatever the user typed in at the keyboard.

Notes

A function can have a result when it is executed. This result can be assigned to a variable. We will show you how to do this in a later lesson when we talk about functions.

Now it is your turn

Then this line runs in a program what happens if the user types in Bob or Alice? Can you work out what the type of the name variable should be?

If the user types in Bob the value of name is also Bob. If the user instead types Alice then the value of name is also Alice.

name must be a variable of type string. The clue to the type returned by the ReadStringFromKeyboard is in the name of the function.

The Keyboard Input Pattern

The pattern has two parts. First the simpleio package needs to be imported into the program. Second the part shows how to use the new functions to read the user input.

Every Go program has an import statement at the top of the program. The import keyword imports one or more packages into the program. A package is a collection of functions written by another programmer that can be used in another program.

So far the import line has included one package, fmt. Now it must include the simpleio package. The import block needs to change from

import "fmt"

to

import (
	"fmt"

	"github.com/gophercoders/simpleio"
)

The new lines includes the full path to the new simpleio package within the Go workspace.

Once the package has been imported into the program the functions within the simpleio package can be used. The package contains two functions ReadStringFromKeyboard and ReadNumberFromKeyboard.

The pattern to read a string is

variable-name = simpleio.ReadStringFromKeyboard()

the variable that is being assigned to, variable-name must have been declared as a string earlier in the program.

ReadStringFromKeyboard will read any keyboard input up to the Return key is pressed. The input is returned as a string. Any leading or trailing spaces will be removed. Remember that a string is anything that can by typed so this should be the expected behaviour. If the Return key is pressed before anything else is typed, this will result in an empty string.

The pattern to read a number is

variable-name = simpleio.ReadNumberFromKeyboard()

the variable that is being assigned to, variable-name must have been declared as an int earlier in the program.

ReadNumberFromKeyboard will read any integer (whole number) until the Return key is pressed. The input is returned as an int. Similar toReadStringFromKeyboard any leading or trailing spaces will be stripped. ReadNumberFromKeyboard will reject any input that is not solely made up of the characters 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. A valid number must be input by the user before program will continue.

The hellostranger Program

OK lets look at how you can put all of this to use in the hellostranger program. The hellostranger program will print out the name and age of the user. The user has to type their name and age in when the program runs.

Important

Before you can try this program you must first have the simpleio package installed.

Installing the package is easy. You just need to open a terminal window and type the following command

go get github.com/gophercoders/simpleio

Go will then download and install the package into your Go workspace.

Now let’s look at the hellostranger program. First type the program in to your text editor and run the program a few times. Remember to create a new directory for your program in your Go Workspace. Once you have typed in the program try running the program a few times with different names and ages.

Can you see what happens?

    1 package main
    2 
    3 import (
    4    "fmt"
    5 
    6    "github.com/gophercoders/simpleio"
    7 )
    8 
    9 func main() {
   10    var strangersName string
   11    var strangersAge int
   12 
   13    fmt.Println("The hello stranger program shows you how to use variables ")
   14    fmt.Println("to read input from the keyboard.")
   15    fmt.Println("")
   16 
   17    fmt.Println("Please type in your name.")
   18    strangersName = simpleio.ReadStringFromKeyboard()
   19    fmt.Println("Please tell me your age.")
   20    strangersAge = simpleio.ReadNumberFromKeyboard()
   21    fmt.Print("Hello ")
   22    fmt.Println(strangersName)
   23    fmt.Print("You are ")
   24    fmt.Print(strangersAge)
   25    fmt.Println(" years old.")
   26 }
Fig-1. The hellostranger code

The output from the program is run with a name of Rachel and an age of 10 is.

The hello stranger program shows you how to use variables
to read input from the keyboard.

Please type in your name.
Rachel
Please tell me your age.
10
Hello Rachel
You are 10 years old.

If you look carefully at the program you won’t be able to find the name Rachel anywhere. Rachel was the name we typed when we ran the program.

Lets look at the key points of the program.

Line 3-7, reproduced bellow, show the revised import statement showing how to include the new simpleio package.

import (
	"fmt"

	"github.com/gophercoders/simpleio"
)

Now lets look at how the user’s name is read from the keyboard. The relevant lines are lines 10 and 18.

var strangersName
...
strangersName = simpleio.ReadStringFromKeyboard()

Line 10 is a straight forward variable declaration. It declares a variable names strangersName of type string. Line 18 is the variable assignment. strangersName is assigned the result of the simpleio.ReadStringFromKeyboard() function. In other words whatever the user typed in as their name.

The line is printed to the terminal with line 22

fmt.Println(strangersName)

So whatever the user entered as their name is printed to the terminal window. The program does not care what this value of the strangersName is.

Lines 11, 20 and 24 are similar. The only difference is that the variable strangersAge is declared to be an int on line 11. strngersAge is assigned the result of the simpleio.ReadNumberFromKeyboard in line 20. Again the program does not know what value is assigned to strangersAge. Line 24 prints the value the user entered for strangersAge to the screen.

Now it is your turn

Run the program again and try these cases. See if you can work out what the program does.

  • What happens if you type in a number instead of your name?
  • What happens if you just type Return instead of typing your name?
  • Do you think the result is correct?
  • What happens if you try and type in your name when the program asks you for your age?
  • Can you guess how the program does this?

If you type a number for your name (or anything else) the program will display whatever you typed in. The program does not care what the value you typed in is.

If you pressed Return instead of typing your name the program will continue. In the output the program will not print your name. The program has nothing to print out! Remember that simpleio.ReadStringFromKeyboard() reads the user input until the Return key is pressed. If the user presses Return they will enter an empty string - a string with no runes.

These are not bugs. This is correct. Remember that a string is any sequence of runes i.e. characters that can be typed. It is not restricted to just the letters a to z.

If you tried to enter a string instead of a number for your age you will see something like this:

The hello stranger program shows you how to use variables
to read input from the keyboard.

Please type in your name.
Rachel
Please tell me you age.
ten
Sorry I don't think that was a number. Try again...
eight
Sorry I don't think that was a number. Try again...
8 or 9
Sorry I don't think that was a number. Try again...
10
Hello Rachel
You are 10 years old.

The program will continue to ask for an age until the user enters an age that is a number. This is the correct behaviour of simpleio.ReadNumberFromKeyboard(). The function is defined to have this behaviour.

The question is why? It is because something special is happening inside the simpleio.ReadNumberFromKeyboard() function.

If you look at the program you won’t see the string Sorry I don't think that was a number. Try again. anywhere in the program.

Inside simpleio.ReadNumberFromKeyboard() two things are happening. Firstly the letters that the user types are being tested to see if they are numbers. This uses a selection statement. Secondly the program keeps repeating until the user does type in a number. This uses repetition.

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