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27
4
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:
gói chính
nhập khẩu (
"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
}
hàm 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!
câu trả lời hay nhất
了解 blocksVà declarations and scopehiện hữu Đi .
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công tắc trường hợp Và TRONG case 子句在每个子句(最里面的包含 block )内开始和结束。 artist、album Và năm 变量code>defaultartist、album Và năm 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()
mặc định:
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 Và năm 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。
关于variables - 去编译错误: undefined variables，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/7329389/
27
4
0
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用PHP 7.2编写套接字服务器。根据Firefox 60中的“网络”选项卡，服务器的一些HTTP响应的第一行随机变为undefined undefined undefined。因此，我尝试记录套接字
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