Reading user input

You will learn how to read user input, both a simpler technique and a more advanced one using formatters.

Introduction

This chapter will cover:

• The Scan() method to read user input.
• Reading one input.
• Reading multiple inputs.
• Working with formattters.

User input

It’s an important thing to be able to read user input from the console. It gives the user a chance to interact with the program. Things to consider are how you are asking the user to input, is it one word, several inputs. Will the user separate input by space or newline? Regardless of what approach you go with, try to communicate the chosen approach to the user.

Managing user input with fmt

So far, you’ve seen how the fmt package can be used to print to the console. It can also be used to read user input.

The fmt library has a built-in Scan() method that we will use to capture the user input.

Reading one input

You might start out wanting to read one input from the user. That’s what the Scan() method does by default.

Here’s some code showing how to collect user input:

package main

import "fmt"

func main() {
    var response string
    fmt.Scan(&response)
    fmt.Println("User typed: ", response)
}

Note how you send in the string response as a reference, using the & operator. It’s done this way as the Scan() method will modify the variable you send in.

When you run this code, you will see the below output:

hello
User typed: hello

Reading multiple inputs

You can provide several arguments to the Scan() method. By providing several arguments, you can collect more than one user input and separate each input, in the Scan() function, with a comma. Here’s how to apply this technique:

var a1, a2 string
// multiple input
fmt.Scan(&a1, &a2)

// formatted string to print out the user input
str := fmt.Sprintf("a1: %s a2: %s", a1, a2)

Note how a1 and a2 is sent into Scan() and they are comma-separated. So how will those codes run?

When you run this code, there are two ways for the user to input. The user can either separate the values by space like so:

hi you
a1: hi a2: you

or by newline:

hi 
you
a1: hi a2: you

Scanf(), scan the input with formatters

So far, you’ve seen how you can collect input with spaces and endlines as separators. You can also be specific in how you collect input. Imagine that the user wants to type in an invoice number like so “INV200” or “INV114” and what you are interested in is the number part, or you are interested in the prefix as well?

The answer to solving this is in the Scanf() function. It takes formatters. With formatters, you’re able to pick the part of the user input you are interested in and place that into the variable you want.

In the above-mentioned case, you can construct code looking like so:

var prefix string
var no int
// inv110
fmt.Scanf("%3s%d", &prefix, &no)
fmt.Printf("prefix: %s, invoice no: %d", prefix, no)

The interesting part lies in the first argument of Scanf(), namely 3s%d. What this means is, take the first 3 characters, %3s and interpret them as a string. Then interpret and place the remaining as a number, %d.

Running this program, you will get an output like so:

inv200
prefix: inv, invoice no: 200

“inv” is placed in prefix and 200 in no variable.

Learn more

To learn more about this area, check out this link https://pkg.go.dev/fmt#Scanf

Assignment - read user input

In this assignment, you will read user input for a card game. The objective is to capture the names of the players.

1. Create a file main.go with the following content:

   package main
   
   import "fmt"
   
   func main() {
   
   }
   

2. Add the following code to the main() method:

   fmt.Println("Enter player names (separated by space or newline):")
   var player1, player2 string
   fmt.Scan(&player1, &player2)
   fmt.Println("Players are: ", player1, player2)
   

3. Run the program using go run main.go:

   go run main.go
   

   You should see the following output:

   Enter player names (separated by space or newline):
   Alice Bob
   Players are:  Alice Bob
   

🚀 Challenge

Try modifying the program so it takes several players first and then ensures all players get a name. Here’s an example output of such a program:

Enter number of players: 
3
Enter Player 1 name: 
Alice
Enter Player 2 name:
Bob
Enter Player 3 name:
Jean
Players are: Alice Bob Jean

Solution

package main

import "fmt"

func main() {
 fmt.Println("Enter player names (separated by space):")
 var player1, player2 string
 fmt.Scan(&player1, &player2)
 fmt.Println("Players are: ", player1, player2)
}