使用golang从列表中的最后一个元素开始的Kth

Question

I have the code to find the kth from the last element in a list in golang. I wrote a recursive function. When it reaches the end of the list, it will return the count as 1 and increments in further returns. When the count == k then return the node value. But I am getting 'nil pointer dereference' error. Could anyone help me in this?

package main    
import (
    "container/list"
    "fmt"
)

var sMap map[int]bool

func main() {
    l := list.New()
    for i := 1; i < 100; i++ {
        l.PushBack(i)
    }    
    kFromLastElemRec := findKFromLastRecr(l.Front(), 3, WrapObj{0})
    fmt.Println(kFromLastElemRec.Value.(int))    
}
//Object to store the count 
type WrapObj struct {
    count int
}
//ERROR
//recursive function to find the kth from last element
func findKFromLastRecr(head *list.Element, k int, wrapper WrapObj) *list.Element {
    if head == nil {
        return nil
    }

    resNode := findKFromLastRecr(head.Next(), k, wrapper)
    wrapper.count = (wrapper.count) + 1
    if wrapper.count == k {
        return head
    }
    return resNode
}

Answer 1

You need to pass a pointer to WrapObj to the findKFromLastRecr() function.

Similar to the languages in the C family, everything in Go is passed by value. That is, a function always gets a copy of the thing being passed, as if there were an assignment statement assigning the value to the parameter.

For instance, passing a WrapObj value to a function makes a copy of the WrapObj, but not the WrapObj it points to.

Therefore, without a pointer to WrapObj, each findKFromLastRecr() function gets a copy of WrapObj and the increase can not be shared by the outer findKFromLastRecr() functions.

It could be useful to check the pointers section from golang-book.

package main    
import (
    "container/list"
    "fmt"
)

var sMap map[int]bool

func main() {
    l := list.New()
    for i := 1; i < 100; i++ {
        l.PushBack(i)
    }    
    kFromLastElemRec := findKFromLastRecr(l.Front(), 3, &WrapObj{0})
    fmt.Println(kFromLastElemRec.Value.(int))    
}
//Object to store the count 
type WrapObj struct {
    count int
}
//ERROR
//recursive function to find the kth from last element
func findKFromLastRecr(head *list.Element, k int, wrapper *WrapObj) *list.Element {
    if head == nil {
        return nil
    }

    resNode := findKFromLastRecr(head.Next(), k, wrapper)
    wrapper.count = (wrapper.count) + 1
    if wrapper.count == k {
        return head
    }
    return resNode
}

Output

97