Reading input from stdin in golang

Question

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:

hello
bye

the output is:

hello
bye

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?

Answer 1

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 main

import (
    "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:

hello
bye

in.txt file:

hello
bye

I hope this helps.