Go User Input

The Scan function

As promised, we will now see how to ask the user for input.

In Go, this is done using the Scan function from the fmt package. A simple example is the following:

package main

import "fmt"

func main() {
	var name string
	fmt.Print("What is your name? ")
	fmt.Scan(&name)
	fmt.Printf("Hello, %s!\n", name)
}

Output

What is your name? Chandler Bing
Hello, Chandler!

1. You declare a variable to store the input.
2. You print a message to ask the user for input.
3. You call the Scan function with the address of the variable as an argument.

An address is a value that uniquely identifies a location in memory. You can have two variables that share the same address if they are stored in the same location in memory.

Notice - We have provided the input as Chandler Bing to the Scan() function in our program. However we are only getting Chandler as the output. This is beacuse the Scan() method takes input until it finds the space in input string. So in our example after Chandler there is a space and hence the last name was not taken by the Scan() method

How to Read Multiple Inputs Using Scan() Method

The Scan() function can take multiple inputs from the user, let us take an example to demonstrate how this works.

package main

import "fmt"

func main() {
	var fname string
	var lname string
	fmt.Println("Please enter your First Name and Last Name: ")
	fmt.Scan(&fname, &lname)
	fmt.Printf("Hello, %s %s!\n", fname, lname)
}

Output

Please enter your First Name and Last Name: 
Chandler
Bing
Hello, Chandler Bing!

Notice -

• We are using a single Scan() function to read both fname and lname
• We are providing two inputs values first name and last name to the Scan()
• We have provided each input values in two different lines/newlines.
• The Scan() function reads the input from two different lines and assigns to respective variables

The another way to pass values to the Scan() method is by provding both the values in a single line with space as a separator.

Please enter your First Name and Last Name: 
Chandler Bing
Hello, Chandler Bing!

Here the first value before space is assigned to fname variable and the value after space is assigned to lname variable.

Rewriting the conversion program

Let us rewrite the conversion program from the previous chapter to use the Scan function instead of hard-coding the temperature:

package main

import "fmt"

func main() {
	var celcius float64
	fmt.Print("Enter a temperature in Celsius: ")
	fmt.Scan(&celcius)
	fahrenheit := celcius*9/5 + 32
	fmt.Printf("%.2f°C = %.2f°F\n", celcius, fahrenheit)
}

Output

Enter a temperature in Celsius: 37
37.00°C = 98.60°F

Note: fahrenheit is no longer a constant because its value is not known at compile time, it depends on the user input.

The Scanln function

The Scan function only reads a single word, delimited by a space or a newline. If you want to read an entire line, you can use Scanln instead (from the same package):

package main

import "fmt"

func main() {
	var fname string
	var lname string
	fmt.Println("Please enter your First Name and Last Name: ")
	fmt.Scanln(&fname, &lname)
	fmt.Printf("Hello, %s %s!\n", fname, lname)
}

Output

Please enter your First Name and Last Name: 
Chandler Bing
Hello, Chandler Bing!

Please enter your First Name and Last Name: 
Chandler
Hello, Chandler !

Note- Unlike Scan() function the Scanln() method cannot take multiple inputs in new lines. If you want to read multiple inputs in Scanln() method we need to seperate it by space. We cannot pass the inputs in two different lines, the moment Scanln() encouters enter it stops taking further input and the statement gets terminated.

The Scanf function

Finally, you can use the Scanf() function to ask for input in a specific format. Below are the format specifiers for various data types.

• %d for integers,
• %f for floating-point numbers (you can specify the number of digits after the decimal point, e.g. %.2f for two digits),
• %s for strings,
• %t for booleans.

For example, if you want to read a number followed by a unit, you can use the following code:

package main

import "fmt"

func main() {
	var value float64
	var unit string
	fmt.Print("Enter a value and a unit (e.g. 12.3kg): ")
	fmt.Scanf("%f%s", &value, &unit)
	fmt.Printf("You entered: %.2f%s\n", value, unit)
}

Output

Enter a value and a unit (e.g. 12.3kg): 76.4 pounds
You entered: 76.40pounds

It is less commonly used than the other two functions, but it still has its uses.

Note- Unlike Scan() function the Scanf() method cannot take multiple inputs in new lines. If you want to read multiple inputs in Scanf() method we need to seperate it by space. We cannot pass the inputs in two different lines, the moment Scanf() encouters enter it stops taking further input and the statement gets terminated.