variables - 去编译错误: undefined variables

Question

编程新手/甚至更新。一个小的 go 程序有问题 - 不会编译带有 undefined variable 错误。代码:

package main

import (
  "fmt"
  "io"
  "os"
)

const file = "readfile.txt" 
var s string

func lookup(string) (string, string, string) {
    artist := s
    album := s
    year := s

    return artist, album, year
}

func enterdisk() (string, string, string) {
  var artist string
    var album string
    var year string

    println("enter artist:")
  fmt.Scanf("%s", &artist)

    println("enter album:")
  fmt.Scanf("%s", &album)

    println("enter year:")
  fmt.Scanf("%s", &year)

    return artist, album, year
}

func main() {

  println("enter UPC or [manual] to enter information manually:")
  fmt.Scanf("%s", &s)

    s := s
  switch s { 
        case "manual\n": artist, album, year := enterdisk()
        default: artist, album, year := lookup(s)
  }

    f,_ := os.OpenFile(file, os.O_APPEND|os.O_RDWR, 0666) 
  io.WriteString(f, (artist + ", \"" + album + "\" - " + year + "\n")) 

    f.Close()
    println("wrote data to file")
}

和错误:

catalog.go:49: undefined: artist
catalog.go:49: undefined: album
catalog.go:49: undefined: year

但是，这些变量在代码运行之前不会被定义。此外，“查找”函数尚未编写，它只返回传递的内容。我知道 lookup 和 enterdisk 函数是独立工作的，但我正在尝试测试 switch 语句。

我已经尝试在 main 中声明变量，但是我得到了这个错误:

catalog.go:49: artist declared and not used
catalog.go:49: album declared and not used
catalog.go:49: year declared and not used

附注我读过http://tip.goneat.org/doc/go_faq.html#unused_variables_and_imports ，我同意，如果这只是语义，我仍然想修复它。我只是不知道怎么办!

Answer 1

了解 blocks和 declarations and scope在 Go .

Each clause in a switch or select statement acts as an implicit block.

Blocks nest and influence scoping.

The scope of a declared identifier is the extent of source text in which the identifier denotes the specified constant, type, variable, function, or package.

The scope of a constant or variable identifier declared inside a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl for short variable declarations) and ends at the end of the innermost containing block.

switch s { 
    case "manual\n": artist, album, year := enterdisk()
    default: artist, album, year := lookup(s)
}
. . .
io.WriteString(f, (artist + ", \"" + album + "\" - " + year + "\n")) 

short variable declarations的范围switch case 和 中的 artist、album 和 year 变量code>default case 子句在每个子句(最里面的包含 block )内开始和结束。 artist、album 和 year 变量不再存在，并且对 WriteString() 语句不可见。

相反，写:

var artist, album, year string
switch s {
case "manual\n":
    artist, album, year = enterdisk()
default:
    artist, album, year = lookup(s)
}
. . .
io.WriteString(f, (artist + ", \"" + album + "\" - " + year + "\n"))

Unlike regular variable declarations, a short variable declaration may redeclare variables provided they were originally declared in the same block with the same type, and at least one of the non-blank variables is new. As a consequence, redeclaration can only appear in a multi-variable short declaration.

因此，artist、album 和 year 变量不再使用 short variable declarations 声明(和分配)在 switch case 子句中因为这会隐藏外部 block 中的变量声明，所以它们只是被赋值。