fmt.Scanln Unexpected Newline Error: What\'s the Distinction Between fmt.Scan and fmt.Scanln?

fmt.Scanln Unexpected Newline Error: Delving into Go's Input Handling

Many Go developers encounter the "fmt.Scanln expected newline error" when interacting with user input. To understand this error, it's crucial to delve into the workings of fmt.Scan and fmt.Scanln.

fmt.Scan and Space-Separated Input
fmt.Scan reads space-separated inputs from standard input. Each argument provided to fmt.Scan corresponds to a single word or value separated by spaces. The presence of newlines is treated as spaces, allowing for multi-line inputs. This is exemplified by the following code:

<code class="go">import "fmt"

func main() {
  var i int
  _, err := fmt.Scan(&i)
  if err == nil {
    fmt.Println("read 1 integer: ", i)
  }
}</code>

Provided with input "123", this code will read and print "read 1 integer: 123".

fmt.Scanln and Newline-Delimited Input
In contrast, fmt.Scanln expects a newline character as a delimiter. After reading all input until the newline, it stops parsing. This is clearly stated in the Scanln documentation:

"Scanln is similar to Scan, but stops scanning at a newline and after the final item there must be a newline or EOF."

This behavior manifests itself in the following code:

<code class="go">import "fmt"

func main() {
  var s string
  _, err := fmt.Scanln(&s)
  if err == nil {
    fmt.Println("read 1 line: ", s)
  }
}</code>

With input "everybody loves ice cream", this code will read and print "read 1 line: everybody loves ice cream". The presence of "everybody" and "loves" separated by a space does not interrupt the parsing because fmt.Scanln is only delimited by the final newline.

Understanding the Error
In the initial example provided, Scanln encounters the error "expected newline" because the input string "everybody loves ice cream" does not end with a newline. To resolve this, a newline character must be present at the end of the input.

Alternative Methods for Reading Lines
If the goal is to read a line of input, using ReadString from the bufio.Reader package is a reliable alternative to fmt.Scanln:

<code class="go">import (
  "bufio"
  "os"
)

func main() {
  reader := bufio.NewReader(os.Stdin)
  line, err := reader.ReadString('\n')
  if err == nil {
    fmt.Println("read 1 line: ", line)
  }
}</code>

The above is the detailed content of fmt.Scanln Unexpected Newline Error: What\'s the Distinction Between fmt.Scan and fmt.Scanln?. For more information, please follow other related articles on the PHP Chinese website!