Why Doesn't Go's `fmt.Scanf` Always Wait for User Input?

Why Fmt.Scanf in Go Does Not Wait for User Input

In Caleb Doxsey's Go programming book, a question arises regarding the use of fmt.Scanf. The issue stems from the fact that the program does not pause after the second Scanf, expecting user input. Additionally, the task of validating user input, specifically integers and non-empty values, needs to be addressed.

To understand this behavior, we need to delve into the inner workings of fmt.Scanf. In earlier versions of Go, fmt.Scanf had a limitation: it considered a carriage return (r) followed by a newline (n) as a valid newline character. However, this behavior clashed with the Windows convention of using just r as a newline character. As a result, fmt.Scanf would immediately proceed to the next Scanf without waiting for user input.

To resolve this issue and improve the input handling, consider the following modified code that includes checks for valid input:

import (
    "fmt"
)

func main() {
    var inputSquare float64
    n, err := fmt.Scanf("%f\n", &inputSquare)
    if err != nil || n != 1 {
        fmt.Println(n, err)
    }

    var inputGuess float64
    n, err = fmt.Scanf("%f\n", &inputGuess)
    if err != nil || n != 1 {
        fmt.Println(n, err)
    }
}

The inclusion of "n" in the format string ensures that fmt.Scanf only proceeds once the user has entered a new line, eliminating the issue of it moving forward without waiting for input. Moreover, we now have input validation using the n and err variables returned by Scanf. If n is not equal to 1, it indicates that there was an error or that the user did not enter any valid input. err provides additional context about the error that occurred.

In conclusion, fmt.Scanf behaved unexpectedly in the past due to a compatibility issue on Windows. However, this issue has since been resolved, and using a newline check in the format string ensures that it now waits for user input. Additionally, the inclusion of input validation allows us to verify whether the user has entered valid values, such as integers or non-empty responses.

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