How to Read a File Starting from a Specific Line Number Using bufio.Scanner in Go?

Reading a File Starting from a Specific Line Number using Scanner

In Go, you can use a bufio.Scanner to read a file line by line. However, bufio.Scanner does not provide a built-in option to start reading from a specific line number.

Here's a way to extend bufio.Scanner with a custom SplitFunc that allows you to skip lines and start reading from a desired line number:

func scannerWithStartLine(input io.ReadSeeker, start int64) (*bufio.Scanner, error) {
    if _, err := input.Seek(start, 0); err != nil {
        return nil, err
    }

    scanner := bufio.NewScanner(input)

    scanner.Split(func(data []byte, atEOF bool) (n int, err error) {
        if atEOF && len(data) == 0 {
            return 0, io.EOF
        }
        for i := 0; i < len(data); i++ {
            if data[i] == '\n' {
                return i + 1, nil
            }
        }
        return 0, nil
    })

    return scanner, nil
}

This function takes an io.ReadSeeker (such as a file) and a starting line number, and returns a bufio.Scanner that skips lines until the starting line number is reached.

To use this extended bufio.Scanner, you can follow these steps:

1. Open the file and set the starting line number.
2. Create a bufio.Scanner using the scannerWithStartLine function.
3. Call scanner.Scan() to read lines from the file, starting from the specified line number.

Here's an example:

file, err := os.Open("input.txt")
if err != nil {
    // Handle error
}

startLine := 5

scanner, err := scannerWithStartLine(file, startLine)
if err != nil {
    // Handle error
}

for scanner.Scan() {
    fmt.Println(scanner.Text())
}

This code will read the lines of the file "input.txt" starting from line 5.

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