How to Read a File Starting from a Specific Line Number in Go using a Scanner?-Golang-php.cn

How to Read a File Starting from a Specific Line Number in Go using a Scanner?

Barbara Streisand

Release： 2024-11-08 10:01:02

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How to Read a File Starting from a Specific Line Number in Go using a Scanner?

How to Read a File Starting from a Specific Line Number Using Scanner?

Initially, you may encounter challenges in reading a file line by line and saving the progress in Go. Scanner from the bufio package doesn't provide methods for line numbers. However, here are potential solutions:

1. Extending Scanner with a Split Function

bufio.Scanner can be extended to maintain positions. Implement a split function that splits the input into tokens (lines) and track the read bytes. The example below uses the builtin bufio.ScanLines() as a base and maintains the position (pos) using the advance return value:

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

    pos := start
    scanLines := func(data []byte, atEOF bool) (advance int, token []byte, err error) {
        advance, token, err = bufio.ScanLines(data, atEOF)
        pos += int64(advance)
        return
    }
    scanner.Split(scanLines)

    for scanner.Scan() {
        fmt.Printf("Pos: %d, Scanned: %s\n", pos, scanner.Text())
    }
    return scanner.Err()
}

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2. Using bufio.Reader

bufio.Reader allows you to read a line with ReadBytes() using 'n' as the delimeter. The example below handles multiple line terminator sequences (rn) and strips them from the read line:

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

    r := bufio.NewReader(input)
    pos := start
    for {
        data, err := r.ReadBytes('\n')
        pos += int64(len(data))
        if err == nil || err == io.EOF {
            if len(data) > 0 && data[len(data)-1] == '\n' {
                data = data[:len(data)-1]
            }
            if len(data) > 0 && data[len(data)-1] == '\r' {
                data = data[:len(data)-1]
            }
            fmt.Printf("Pos: %d, Read: %s\n", pos, data)
        }
        if err != nil {
            if err != io.EOF {
                return err
            }
            break
        }
    }
    return nil
}

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Testing the Solutions

To test the solutions, you can use the content "firstrnsecondnthirdnfourth" as input with both a clean start (start = 0) and a resume position (start = 14). The output will display the positions and read lines:

--SCANNER, start: 0
Pos: 7, Scanned: first
Pos: 14, Scanned: second
Pos: 20, Scanned: third
Pos: 26, Scanned: fourth
--READER, start: 0
Pos: 7, Read: first
Pos: 14, Read: second
Pos: 20, Read: third
Pos: 26, Read: fourth
--SCANNER, start: 14
Pos: 20, Scanned: third
Pos: 26, Scanned: fourth
--READER, start: 14
Pos: 20, Read: third
Pos: 26, Read: fourth

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