CODEWITHSUNDEEP
pointersgorecursion
Teddy Radenkov
Teddy Radenkov

Reputation: 91

Golang pointer not updated when passed to method as nil

I'm trying to create a linear linked list in go with recursive functions but for some reason I am not getting the correct result.

When I add a number to the list I noticed that L.head is never updated after the return of the recursiveAdd method. Shouldn't it be updated given that it is a pointer?

Expected result after list.Display(): 1 2 3

Actual result: empty string

package main

import "fmt"

type Node struct {
    num int
    next *Node
}

type List struct {
    head *Node
}

func (L *List) Add(n int) {
    L.recursiveAdd(L.head, n)
}

func recursiveAdd(node *Node, n int){
    if node == nil {
        node = &Node{n, nil}
        return
    }
    L.recursiveAdd(node.next, n)
}

func (L *List) Display() {
    recursiveDisplay(L.head)
}

func recursiveDisplay(n *Node){
    if n == nil {
        return
    }
    fmt.Println(n.num)
    recursiveDisplay(n.next)
}

func main(){
    list := List{}

    list.Add(1)
    list.Add(2)
    list.Add(3)

    list.Display()
}

Upvotes: 1

Views: 533

Answers (1)

Burak Serdar
Burak Serdar

Reputation: 51587

List.Add does not update L.head. You pass L.head to recursiveAdd, but what you're passing is a copy of L.head, which is nil, and when you assign a value to that you only update the copy of the pointer on stack, not L.head. You can do:

func (L *List) Add(n int) {
    l.head=L.recursiveAdd(L.head, n)
}

func (L *list) recursiveAdd(node *Node, n int) *Node{
    if node == nil {
        node = &Node{n, nil}
        return node
    }
    node.next=L.recursiveAdd(node.next,n)
    return node
}

Upvotes: 2

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