How to flush Stdin after fmt.Scanf() in Go?

Question

Here's an issue that's bedeviling me at the moment. When getting input from the user, I want to employ a loop to ask the user to retry until they enter valid input:

// user_input.go
package main

import (
    "fmt"
)

func main() {
    fmt.Println("Please enter an integer: ")

    var userI int

    for {
        _, err := fmt.Scanf("%d", &userI)
        if err == nil {
            break
        }
        fmt.Println("Sorry, invalid input. Please enter an integer: ")
    }

    fmt.Println(userI)    
}

Running the above, if the user enters valid input, no problem:

Please enter an integer: 

3


3

exit code 0, process exited normally.


But try inputting a string instead?

Please enter an integer: 
what?
Sorry, invalid input. Please enter an integer:

Sorry, invalid input. Please enter an integer:

Sorry...

Etc, and it keeps looping character by character until the string is exhausted. Even inputting a single character loops twice, I assume as it parses the newline.

Anyways, there must be a way to flush Stdin in Go?

P.S. In the absence of such a feature, how would you work around it to provide equivalent functionality? I've failed even at that...

Answer 1

I would fix this by reading until the end of the line after each failure. This clears the rest of the text.

package main

import (
    "bufio"
    "fmt"
    "os"
)

func main() {
    stdin := bufio.NewReader(os.Stdin)

    fmt.Println("Please enter an integer: ")

    var userI int

    for {
        _, err := fmt.Fscan(stdin, &userI)
        if err == nil {
            break
        }

        stdin.ReadString('\n')
        fmt.Println("Sorry, invalid input. Please enter an integer: ")
    }

    fmt.Println(userI)
}

Answer 2

Is it bad to wake up an old question?

I prefer to use fmt.Scanln because A) it doesn't require importing another library (e.g. reader) and B) it doesn't involve an explicit for loop.

func someFunc() {
    fmt.Printf("Please enter an integer: ")

    // Read in an integer
    var i int
    _, err := fmt.Scanln(&i)
    if err != nil {
            fmt.Printf("Error: %s", err.Error())

            // If int read fails, read as string and forget
            var discard string
            fmt.Scanln(&discard)
            return
    }
    fmt.Printf("Input contained %d", i)
}

However, it seems like there ought to be a more elegant solution. Particularly in the case of fmt.Scanln it seems odd that the read stops after the first non-number byte rather than "scanning the line".

Answer 3

I ran into a similar problem for getting user input but solved it in a slightly different way. Adding to the thread in case someone else finds this useful:

package main

import (
    "bufio"
    "fmt"
    "os"
    "strings"
)

// Get first word from stdin
func getFirstWord() (string) {
    input := bufio.NewScanner(os.Stdin)
    input.Scan()
    ans := strings.Fields(input.Text())

    if len(ans) == 0 {
        return ""
    } else {
        return ans[0]
    }
}

func main() {
    fmt.Printf("Would you like to play a game?\n> ")
    ans := getFirstWord()
    fmt.Printf("Your answer: %s\n", ans)
}