Golang : Fix fmt.Scanf() on Windows will scan input twice problem

Tags : golang fmt-scanf-bug windows scan-input-twice

If you ever need to write a program that use Golang's fmt.Scanf() function on Windows operating system and test the program at the Command Prompt, chances are high that you will encounter weird output result such as a ghost that will hit the key on your behalf, adding 1+1=3 or unable to control your for-loop properly when processing the input parameter accepted by fmt.Scanf().

This is because fmt.Scanf() function will behave differently on Windows. Simply put, for Windows, the "Enter" button is "\r\n", but in fmt package, it only deal with "\n". See (https://github.com/golang/go/issues/5391)

To fix this issue, use fmt.Scan() (without f) instead of fmt.Scanf() function or if you insist, add "\n" after %v such as fmt.Scanf(%v\n, &variable)

The program below demonstrate how to replicate the fmt.Scanf() problem. You will have to run this in Windows Command Prompt.

 package main

 import (

 func main() {
 var number int
 fmt.Print("Enter a number : ")
 fmt.Scanf("%d", &number)

 fmt.Println("Number is : ", number)

Run the program at Windows command prompt and the output will look like this:

C:\Users\Sweet Logic>scanf

Enter a number : 4

Number is : 4

Enter a string : String is :

Notice that instead of waiting for you to enter the string input...it will just go ahead and accept the empty input. This ONLY happens on Windows Command Prompt. It will NOT happen on Linux, Unix or even Gogland Editor running on Windows.



C:/Go\bin\go.exe build -i -o "C:\Users\SWEETL~1\AppData\Local\Temp\Build main.go and rungo" C:/work/src/setReadOnly/main.go

"C:\Program Files\JetBrains\Gogland 171.3780.106\bin\runnerw.exe" "C:\Users\SWEETL~1\AppData\Local\Temp\Build main.go and rungo"

Enter a number : 12

Number is : 12

Enter a string : abasdasd

String is : abasdasd

Process finished with exit code 0

  See also : Golang : Scanf function weird error in Windows

Tags : golang fmt-scanf-bug windows scan-input-twice

By Adam Ng

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