dongshou1856 2019-04-11 21:58
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I need to read a CSV file and record the locations of lines with certain values into an array, then later go back and retrieve those lines in no particular order and with good performance, so random access.

My program uses csv.NewReader(file), but I see no way to get or set the file offset that it uses. I tried file.Seek(0,io.SeekCurrent) to return the file position, but it doesn't change between calls to reader.Read(). I also tried fmt.Println("+v +v ",reader,file) to see if anything stores the reader's file position, but I don't see it. I also don't know the best way to use the file position if I do find it.

Here's what I need to do:

file,_ = os.Open("stuff.csv")
reader = csv.NewReader(file)

//read file and record locations
for {
    line,_ = reader.Read()
    if wantToRememberLocation(line) {
         locations = append(locations, getLocation()) //need this function

//then revisit certain lines
for {
    reader.GoToLine(locations[random])  //need this function
    line,_ = reader.Read()

Is there even a way to do this with the csv library, or will I have to write my own using more primitive file io functions?

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  • doucan9079 2019-04-12 22:24

    Here's a solution using TeeReader. This example just saves all the positions and goes back and rereads some of them.

    //set up some vars and readers to record position and length of each line
    type Record struct {
        Pos int64
        Len int 
    records := make([]Record,1)
    var buf bytes.Buffer
    var pos int64
    file,_ := Open("stuff.csv")
    tr := io.TeeReader(file, &buf)
    cr := csv.NewReader(tr)
    //read first row and get things started
    data,_ := cr.Read()
    //length of current row determines position of next
    lineBytes,_ := buf.ReadBytes('
    length := len(lineBytes)
    pos += int64(length)
    records[0].Len = length
    records = append(records, Record{ Pos: pos })
    for i:=1;;i++ {
        //read csv data
        data,err = c.Read()
        if err != nil {break}
        //record length and position
        lineBytes,_ = buf.ReadBytes('
        lenth = len(lineBytes)
        pos += int64(length)
        records[i].Len = length
        records = append(records, Record{ Pos: pos })
    //prepare individual line reader
    line := make([]byte,1000)
    lineReader := bytes.NewReader(line)
    //read random lines from file
    for {
        i := someLineNumber()
        //use original file reader to fill byte slice with line
        file.ReadAt(line[:records[i].Len], records[i].Pos)
        //need new lineParser to start at beginning every time
        lineParser := csv.NewReader(lineReader)
        data,_ = lineParser.Read()
    本回答被题主选为最佳回答 , 对您是否有帮助呢?



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