英文:
Reading input from stdin in golang
问题
我有这段 Go 代码：
func readTwoLines() {reader := bufio.NewReader(os.Stdin)line, _ := reader.ReadString('\n')fmt.Println(line)line, _ = reader.ReadString('\n')fmt.Println(line)}
对于输入：
hellobye
输出是：
hellobye
一切正常。但是，如果我为每一行创建一个读取器：
func readTwoLines() {line, _ := bufio.NewReader(os.Stdin).ReadString('\n')fmt.Println(line)line, err := bufio.NewReader(os.Stdin).ReadString('\n')if err != nil {fmt.Println(err)}fmt.Println(line)}
在第二行读取时会出现 EOF 错误。
为什么会发生这种情况？
英文:
I have this go code:
func readTwoLines() {reader := bufio.NewReader(os.Stdin)line, _ := reader.ReadString('\n')fmt.Println(line)line, _ = reader.ReadString('\n')fmt.Println(line)}
For the input:
hellobye
the output is:
hellobye
Everything ok. But now, if I create one reader per line:
func readTwoLines() {line, _ := bufio.NewReader(os.Stdin).ReadString('\n')fmt.Println(line)line, err := bufio.NewReader(os.Stdin).ReadString('\n')if err != nil {fmt.Println(err)}fmt.Println(line)}
there is an EOF error, in the second line reading.
Why is it happening?
答案1
得分: 21
对于简单的用途，使用Scanner可能更方便。你不应该使用两个读取器，首先读取，缓冲区大小为4096字节：
// NewReader返回一个新的Reader，其缓冲区具有默认大小。func NewReader(rd io.Reader) *Reader {return NewReaderSize(rd, defaultBufSize)}
其中defaultBufSize = 4096。
即使你的输入包含4000字节，第二次读取仍然没有内容可读。但是，如果你输入的内容超过4096字节，它将起作用。
如果在找到分隔符之前，ReadString遇到错误，它将返回错误之前读取的数据和错误本身（通常是io.EOF）。
这是设计如此，参见文档：
// ReadString读取输入中第一个分隔符之前的内容，返回包含数据和分隔符的字符串。// 如果在找到分隔符之前，ReadString遇到错误，它将返回错误之前读取的数据和错误本身（通常是io.EOF）。// 当且仅当返回的数据不以分隔符结尾时，ReadString返回err != nil。// 对于简单的用途，使用Scanner可能更方便。func (b *Reader) ReadString(delim byte) (string, error) {bytes, err := b.ReadBytes(delim)return string(bytes), err}
尝试这个：
package mainimport ("bufio""fmt""os")func main() {scanner := bufio.NewScanner(os.Stdin)for scanner.Scan() {fmt.Println(scanner.Text()) // Println会添加最后的'\n'}if err := scanner.Err(); err != nil {fmt.Fprintln(os.Stderr, "reading standard input:", err)}}
运行：
go run m.go < in.txt
输出：
hellobye
in.txt文件：
hellobye
希望这可以帮到你。
英文:
For simple uses, a Scanner may be more convenient.
You should not use two readers, first read, buffers 4096 bytes of input:
> // NewReader returns a new Reader whose buffer has the default size.
> func NewReader(rd io.Reader) *Reader {
> return NewReaderSize(rd, defaultBufSize)
> }
and defaultBufSize = 4096
and even your input contains 4000 bytes, still second read got nothing to read.
but if you enter input more than 4096 bytes it will work.
> If ReadString encounters an error before finding a delimiter, it
> returns the data read before the error and the error itself (often
> io.EOF).
it is by design, see doc:
// ReadString reads until the first occurrence of delim in the input,// returning a string containing the data up to and including the delimiter.// If ReadString encounters an error before finding a delimiter,// it returns the data read before the error and the error itself (often io.EOF).// ReadString returns err != nil if and only if the returned data does not end in// delim.// For simple uses, a Scanner may be more convenient.func (b *Reader) ReadString(delim byte) (string, error) {bytes, err := b.ReadBytes(delim)return string(bytes), err}
try this:
package mainimport ("bufio""fmt""os")func main() {scanner := bufio.NewScanner(os.Stdin)for scanner.Scan() {fmt.Println(scanner.Text()) // Println will add back the final '\n'}if err := scanner.Err(); err != nil {fmt.Fprintln(os.Stderr, "reading standard input:", err)}}
run:
go run m.go < in.txt
output:
hellobye
in.txt file:
hellobye
I hope this helps.
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