内存记事本Golang开发第三阶段：重复打印提示问题求助

我正在开发Golang版内存记事本的第三阶段，遇到一个问题：Enter a command and data: 这个提示会重复打印两次而非一次，找不到原因。

预期流程：CLI打印Enter the maximum number of notes: ，获取并保存用户输入后，打印一次Enter a command and data: 。但实际该提示会在同一行打印两次。

相关代码如下：

package main

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

func readFromStdin() string {
    fmt.Print("Enter a command and data: ")
    reader := bufio.NewReader(os.Stdin)
    line, _ := reader.ReadString('\n')
    //line = line[:len(line)-2]
    line = strings.TrimSpace(line)

    return line
}

func main() {
    var notes []string
    var maximumNotes int

    for {
        if maximumNotes <= 0 {
            fmt.Print("Enter the maximum number of notes: ")
            _, _ = fmt.Scan(&maximumNotes)
        }
        line := readFromStdin()
        var joinedNote string
        var note []string

        splittedString := strings.Split(line, " ")
        if splittedString[0] == "create" && len(notes) >= maximumNotes {
            fmt.Println("[Error] Notepad is full")
        }
        if splittedString[0] == "create" && len(splittedString) <= 1 {
            fmt.Println("[Error] Missing note argument")
        }
        if splittedString[0] == "create" && len(splittedString) > 1 && len(notes) < maximumNotes {
            i := 1
            for ; i < len(splittedString); i++ {
                note = append(note, splittedString[i])
            }
            joinedNote = strings.Join(note, " ")
            notes = append(notes, joinedNote)
            fmt.Println("[OK] The note was successfully created")
        }
        if splittedString[0] == "list" && len(notes) <= 0 {
            fmt.Println("[Info] Notepad is empty")
        }
        if splittedString[0] == "list" {
            for i, noteList := range notes {
                //newNote := strings.TrimSpace(noteList)
                fmt.Printf("[Info] %d: %s\n", i+1, noteList)
            }
        }
        if splittedString[0] == "clear" {
            notes = nil
            fmt.Println("[OK] All notes were successfully deleted")
        }

        if splittedString[0] == "exit" {
            fmt.Println("[Info] Bye!")
            os.Exit(0)
        }
    }
}

问题原因 #

问题出在fmt.Scan(&maximumNotes)的行为上：fmt.Scan只会读取输入中的数字部分，当用户输入数字后按下回车键，这个换行符会残留在标准输入缓冲区中。后续调用readFromStdin时，bufio.NewReader(os.Stdin).ReadString('\n')会直接读取到这个残留的换行符，导致readFromStdin函数立即返回一个空字符串（经过TrimSpace处理后），主循环随即再次调用readFromStdin，于是提示被打印了第二次。

解决方案 #

方案一：清理fmt.Scan后的残留换行符

在读取完maximumNotes后，手动读取并丢弃缓冲区里的剩余字符，包括换行符：

if maximumNotes <= 0 {
    fmt.Print("Enter the maximum number of notes: ")
    _, _ = fmt.Scan(&maximumNotes)
    // 丢弃缓冲区中剩余的换行符
    bufio.NewReader(os.Stdin).ReadString('\n')
}

方案二：统一使用bufio处理所有输入

避免混合使用fmt.Scan和bufio，把读取最大笔记数的逻辑也改用bufio实现，再转换为整数：

package main

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

func readFromStdin(prompt string) string {
    fmt.Print(prompt)
    reader := bufio.NewReader(os.Stdin)
    line, _ := reader.ReadString('\n')
    line = strings.TrimSpace(line)
    return line
}

func readIntFromStdin(prompt string) int {
    line := readFromStdin(prompt)
    num, _ := strconv.Atoi(line)
    return num
}

func main() {
    var notes []string
    var maximumNotes int

    for {
        if maximumNotes <= 0 {
            maximumNotes = readIntFromStdin("Enter the maximum number of notes: ")
        }
        line := readFromStdin("Enter a command and data: ")
        // 后续逻辑保持不变
        var joinedNote string
        var note []string

        splittedString := strings.Split(line, " ")
        if splittedString[0] == "create" && len(notes) >= maximumNotes {
            fmt.Println("[Error] Notepad is full")
        }
        if splittedString[0] == "create" && len(splittedString) <= 1 {
            fmt.Println("[Error] Missing note argument")
        }
        if splittedString[0] == "create" && len(splittedString) > 1 && len(notes) < maximumNotes {
            i := 1
            for ; i < len(splittedString); i++ {
                note = append(note, splittedString[i])
            }
            joinedNote = strings.Join(note, " ")
            notes = append(notes, joinedNote)
            fmt.Println("[OK] The note was successfully created")
        }
        if splittedString[0] == "list" && len(notes) <= 0 {
            fmt.Println("[Info] Notepad is empty")
        }
        if splittedString[0] == "list" {
            for i, noteList := range notes {
                fmt.Printf("[Info] %d: %s\n", i+1, noteList)
            }
        }
        if splittedString[0] == "clear" {
            notes = nil
            fmt.Println("[OK] All notes were successfully deleted")
        }

        if splittedString[0] == "exit" {
            fmt.Println("[Info] Bye!")
            os.Exit(0)
        }
    }
}

内容的提问来源于stack exchange，提问作者Jotham Ntekim