为什么 Go 中的 fmt.Scanf 不等待用户输入？

Question

我正在阅读 Caleb Doxsey 的围棋书，我有两个关于 fmt.Scanf http://www.golang-book.com/4 的问题

我想知道为什么程序在第二次 Scanf 之后没有停止并等待用户输入？以及如何测试用户是否输入了整数和/或没有留空？

package main

import (
"fmt"
//"math"
)


// compute square roots by using Newton's method

func main() {

var x float64           //number to take square root
var y float64           //this is the guess
var q float64           //this is the quotient
var a float64           //this is the average


// how do check if the user entered a number
fmt.Print("Enter a number to take its square root: ")
var inputSquare float64
fmt.Scanf("%f", &inputSquare)

// why doesn't program stop after 
// the Print statement and wait
// for user input?
fmt.Print("Enter first guess ")
var inputGuess float64
fmt.Scanf("%f", &inputGuess)

//x = 2
x = inputSquare
y = inputGuess

for i := 0; i < 10; i++ {   //set up the for loop for iterations
    q = x/y                 //compute the quotient; x and y are given
    a = (q + y) / x         //compute the average       
    y = a                   //set the guess to the average              
}                           //for the next loop


fmt.Println("y --> ", y)
//fmt.Println("Sqrt(2)", math.Sqrt(2))
}

Answer 1

更新：原为fixed almost a decade ago。 The docs for fmt 现在阅读

在所有扫描函数中，回车后紧跟换行符被视为普通换行符（\r\n 与 \n 含义相同）。

如果您仍然遇到扫描错误，mind that it isn't your IDE's fault。

---

它是 Issue 5391: fmt: Scanf 在 Windows 的行尾拒绝 \r\n。

作为一种解决方法并检查有效输入，写入，

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

和

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

解决方法是"%f\n" 格式字符串中的换行符。

包fmt

func Scanf

func Scanf(format string, a ...interface{}) (n int, err error)

Scanf 扫描从标准输入读取的文本，存储连续的 由空格分隔的值到由 格式。它返回成功扫描的项目数。

这是一个完整的工作程序：

package main

import (
    "fmt"
)

// compute square roots by using Newton's method
func main() {
    var x float64 //number to take square root
    var y float64 //this is the guess
    var q float64 //this is the quotient
    var a float64 //this is the average

    fmt.Print("Enter a number to take its square root: ")
    var inputSquare float64
    n, err := fmt.Scanf("%f\n", &inputSquare)
    if err != nil || n != 1 {
        // handle invalid input
        fmt.Println(n, err)
        return
    }

    fmt.Print("Enter first guess ")
    var inputGuess float64
    n, err = fmt.Scanf("%f\n", &inputGuess)
    if err != nil || n != 1 {
        // handle invalid input
        fmt.Println(n, err)
        return
    }

    x = inputSquare
    y = inputGuess
    for i := 0; i < 10; i++ {
        q = x / y       // compute the quotient; x and y are given
        a = (q + y) / x // compute the average
        y = a           // set the guess to the average
    }
    fmt.Printf("sqrt(%g) = %g\n", x, y)
}

输出：

Enter a number to take its square root: 2.0
Enter first guess 1.0
sqrt(2) = 1.414213562373095

我在 Windows 7 上使用 Go 1.1.1：

C:\>go version
go version go1.1.1 windows/amd64