What Causes \"fmt.Scanln Expected Newline\" Error and How to Resolve It?

fmt.Scanln Expectation

When using fmt.Scanln in Go, one may encounter the error: "fmt.Scanln expected newline." Understanding this error and its implications is crucial for debugging and writing robust code.

Behavior of fmt.Scan vs fmt.Scanln

fmt.Scan reads space-separated values from standard input, treating newlines as space characters. This means it will continue parsing input until either all arguments are filled or an error occurs. On the other hand, fmt.Scanln also reads from standard input but expects a newline character as the termination point. Any input after that won't be parsed.

Example

In the provided Go code, the issue arises when using fmt.Scanln to read a string s. The input includes a newline character, which triggers the end of input for fmt.Scanln. However, the subsequent input characters "everybody loves ice cream" are still present and are consumed by the subsequent fmt.Scan. This causes the error, as fmt.Scan expects a newline at the end of input.

Alternatives

To read a line of text including spaces and ending with a newline, consider using bufio.Reader's ReadString method:

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

func main() {
    reader := bufio.NewReader(os.Stdin)
    line, err := reader.ReadString('\n')
}</code>

Conclusion

Understanding the distinct behaviors of fmt.Scan and fmt.Scanln is important to prevent errors and write code that aligns with expectations. When reading a line of text, alternatives like bufio.Reader and ReadString provide more explicit and efficient solutions.

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