golang populate struct field from file

Question

I'm pretty new to developing in golang, so if this is an elementary question I apologize. I didn't see a comparable question already asked; if there is one, please point me to it (thanks).

The full code (at the time of my asking this question, since it's not immutable) is at http://play.golang.org/p/idDp1E-vZo

I've declared a struct with four primitive fields, and I'm reading the values destined for Node.ipaddr from a file on the local filesystem (I'm getting the value of fileName as a flag at runtime; that code is trimmed out here but is in the link provided above.)

type Node struct {
    hostname string
    ipaddr   string
    pstatus  string
    ppid     int
}

file, err := os.Open(fileName)
if err != nil {
    panic(fmt.Sprintf("error opening %s: %v", fileName, err))
}

Because the file is line-delimited, I thought a bufio.Scanner would be ideal for reading the data from the file. Where I am struggling is finding an elegant way to actually pass the data into the struct elements.

I created an array of Node elements, and have considered a map, but I am not sure how I'd make practical use of it (yet).

var nodes []*Node
var nodemap = make(map[string]*Node) //do I even need this?

scanner := bufio.NewScanner(file)
for scanner.Scan() {
    if err := scanner.Err(); err != nil {
        fmt.Fprintln(os.Stderr, "error reading from file:", err)
        os.Exit(3)
    }

    //pass scanner.Text() into Node.ipaddr

}

Short of wrapping scanner.Scan() in an indexed for loop, I'm not at all sure how to proceed. If I do wrap scanner.Scan() in an indexed for loop, will the for loop be able to handle EOF elegantly -- I guess I'm not sure what the index limit/max comparison value should be in that case.

As always, thanks for any advice that you're willing to offer.

edit: The format of the input file is like this:

10.1.1.1
10.1.1.2
10.1.1.3

I anticipate approx 150 entries in the file, one IPv4 address per line.

Answer 1

The only thing you're missing is the append function to add a new node to the slice.

nodes = append(nodes, &Node{ipaddr: scanner.Text()})

Also, you can have an index on your for loop while still having the scanner.Scan() as the condition. This lets the EOF be handled elegantly while still allowing you to access that new Node if need be.

for i := 0; scanner.Scan(); i++ {
    if err := scanner.Err(); err != nil {
        fmt.Fprintln(os.Stderr, "error reading from file:", err)
        os.Exit(3)
    }

    nodes = append(nodes, &Node{ipaddr: scanner.Text()})
    fmt.Println(nodes[i])
}

Full code: http://play.golang.org/p/RvMQp-jgWF

Answer 2

You could use something along the lines of this :

func main() {
    file := strings.NewReader(IPS) //os.Open(fileName)

    var nodes []*Node

    scanner := bufio.NewScanner(file)
    for scanner.Scan() {
        if err := scanner.Err(); err != nil {
            fmt.Fprintln(os.Stderr, "error reading from file:", err)
            os.Exit(3)
        }
        ip := net.ParseIP(scanner.Text())
        if ip != nil {
            nodes = append(nodes, &Node{ipaddr: net.ParseIP(scanner.Text())})
        }

    }
    for _, n := range nodes {
        fmt.Printf("%s, %+v\n", n.ipaddr, n)
    }
}