Is an If-Clause Necessary to Break Out of bufio.Scanner's Input Loop?

Breaking Out of input.Scan() Without an If-Clause

Input processing is crucial for interacting with users in Go programs. The bufio.Scanner package provides a convenient way to read input from the console. However, default Scanner's split function, ScanLines, may behave differently than expected, leaving some wondering if an if-clause is necessary to break out of the input loop.

The provided code snippet demonstrates a common approach for breaking out of the input loop using an if-clause:

input := bufio.NewScanner(os.Stdin)
for input.Scan() {
    if input.Text() == "end" { break }
    fmt.Println(input.Text())
}

However, the documentation states that ScanLines will return false when it reaches the end of the input or encounters an error. The following passage from the documentation suggests that an if-clause may not be required:

Scan advances the Scanner to the next token, which will then be available through the Bytes or Text method. It returns false when the scan stops, either by reaching the end of the input or an error. After Scan returns false, the Err method will return any error that occurred during scanning, except that if it was io.EOF, Err will return nil.

Upon closer examination, it becomes apparent that this assumption is incorrect. ScanLines is actually Scanned functions' pre-defined default split function by default. The documentation explicitly states that ScanLines returns each line of text, stripped of any trailing end-of-line marker. This means it will return empty lines, and the last non-empty line of input will be returned even if it has no newline.

As such, an empty line does not signal the end of the input stream, and it becomes critical to use an if-clause or alternative method to handle early exit scenarios. The following snippet demonstrates an alternative approach:

input := bufio.NewScanner(os.Stdin)
for {
    if !input.Scan() {
        break
    }
    if input.Text() == "end" { break }
    fmt.Println(input.Text())
}

In conclusion, although ScanLines will return false when it reaches the end of the input, the absence of an end-of-line marker in the last line and the return of empty lines make it necessary to employ an if-clause or alternative approach to break out of the input loop gracefully.

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