Golang柜台HTML标记的地图无法正常工作

Question

I got a problem. I need to write a function to populate a mapping from element names—p, div, span, and so on — to the number of elements with that name in an HTML document tree. I made function outline2, it's not working, here is the eroor log:

html
panic: assignment to entry in nil map

    goroutine 1 [running]:
    panic(0x4c3b40, 0xc042010a90)
            F:/Go/src/runtime/panic.go:500 +0x1af
    main.outline2(0x0, 0xc0420320e0)
            F:/Go_Stuff/Books/Golang_stuff/exercises/src/gopl.io/ch5/outline/main.go:29 +0x1ae
    main.outline2(0x0, 0xc042032070)
            F:/Go_Stuff/Books/Golang_stuff/exercises/src/gopl.io/ch5/outline/main.go:34 +0xe3
    main.main()
            F:/Go_Stuff/Books/Golang_stuff/exercises/src/gopl.io/ch5/outline/main.go:23 +0x77

Here is the code:

func main() {
    doc, err := html.Parse(os.Stdin)
    if err != nil {
        fmt.Fprintf(os.Stderr, "outline: %v
", err)
        os.Exit(1)
    }
    outline2(nil, doc)
}
func outline2(tags map[string]int, n *html.Node) {
    fmt.Println(n.Data)
    if n.Type == html.ElementNode {
        fmt.Println(n.Data)
        tags[n.Data] += 1 // push tag
        fmt.Println(n.Data)

    }
    for c := n.FirstChild; c != nil; c = c.NextSibling {
        outline2(tags, c)


    }
}

Please, point out my mistakes. I don't know what to do =(

Answer 1

Error is exact. You can't assign in nil map. You must first allocate. Also you need something to work, you func return nothing. So

outline := make(map[string]int) //allocate and name your map
outline2(outline, doc)
fmt.Println(outline) //do something with it

should work