Go中字符串的顺序输出和输入

Question

There is a program that reads two inputs from command line: username and password. The string "Password: " should be printed in console only after entering the username, but in my program it prints right away along with the first string "Username: ". How to fix this?

package main

import (
    "bufio"
    "os"
    "fmt"
)
// A simple program that verifies the user by username and password.

var loginstatus bool
var username, password string
func login(username, password string) bool {
    if username == "user123" && password == "pass123" {
        return true
    } else {
        return false
    }
}

func main() {
    fmt.Print("Username: ")
    user := bufio.NewScanner(os.Stdin)
    fmt.Print("Password: ")
    pass := bufio.NewScanner(os.Stdin)


    for user.Scan() && pass.Scan() {
        if login(user.Text(), pass.Text()) {
            fmt.Println("Signed in.")
            os.Exit(3)
        } else {
            fmt.Println("Incorrect username or password, please try again:")
        }
    }
}

Answer 1

The line bufio.NewScanner(os.Stdin) just creates a new scanner for stdin it does not actually do the scanning and will not block waiting for user input. It will not actually read from stdin until you call Scan by which point "Username:" and "Password:" have already been printed.

You probably don't need two scanners for username and password. One scanner should be enough and you probably want to move the printing of "Username:" and "Password:" into the for loop if you want to be able to try again so it would look something like:

func main() {
    scanner := bufio.NewScanner(os.Stdin)

    for {
        fmt.Print("Username: ")
        if !scanner.Scan() {
            break
        }
        user := scanner.Text()
        fmt.Print("Password: ")
        if !scanner.Scan() {
            break
        }
        pass := scanner.Text()
        if login(user, pass) {
            fmt.Println("Signed in.")
            os.Exit(3)
        } else {
            fmt.Println("Incorrect username or password, please try again:")
        }
    }
}