Keyboard Input putting variables to work

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.

We are going to start by showing how you can use variables to make the program respond to the words and numbers 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. Other examples of 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 your 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 would need 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()

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 starts after the .. 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 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 executing the ReadStringFromKeyboard function. The result is whatever the user typed in at the keyboard.

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 your programs.

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 until 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.

go get github.com/gophercoders/simpleio

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?

 1package main
 2
 3import (
 4	"fmt"
 5
 6	"github.com/gophercoders/simpleio"
 7)
 8
 9func 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}

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, show the revised import statement showing you 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 name that was typed in 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.

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.