Golang pointer not updated when passed to method as nil

Question

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()
}

Burak Serdar · Accepted Answer · 2020-05-25 20:04:34Z

2

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
}

answered May 25, 2020 at 20:04

Burak Serdar

52.5k4 gold badges54 silver badges74 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Teddy Radenkov Over a year ago

That solution work perfectly but is it possible to do it without return the node? Can L.head be passed by refrence? I thought it was, given that it's a pointer.

Burak Serdar Over a year ago

You can pass &L.head, and set the head in the function using *node=&Node{}. It is a little harder to look at, but it will work. When you pass a pointer to a function, that function sets the value pointed to by that pointer. If you want to set L.head you have to send &L.head, not L.head.

Collectives™ on Stack Overflow

Golang pointer not updated when passed to method as nil

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related