Kth from the last element in a list using golang - list

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