Updating value in pointer reciever method but value not updating

Question

I am implementing a linked list in Go. I wrote a method to insert a node at the beginning of the linked list:

func (n *node) insertAtBegining(d int){
    nn := &node{d,n}
    n = nn
}

The method being a pointer receiver should update the node n, but it fails to do so.

For example,

head.printList()
head.insertAtBegining(30)
head.printList()

returns

42 -> 56 -> 89 -> nil
42 -> 56 -> 89 -> nil

You can see the full implementation at https://play.golang.org/p/rpI6lbAywOQ.

What am I doing wrong?

See possible duplicate: [Can the pointer in a struct pointer method be reassigned to another instance?](https://stackoverflow.com/questions/35421495/can-the-pointer-in-a-struct-pointer-method-be-reassigned-to-another-instance/35426997?r=SearchResults#35426997) — icza, Feb 01 '20 at 09:33

Brits · Accepted Answer · 2020-02-01T09:23:13.257

The root of the issue is that calling head.insertAtBegining(30) does not change the value of head (it still points at the node{42, nil}).

You have developed this on the asumption that the n = nn in insertAtBegining will update head but that is not the case. I think it's easier to understand if insertAtBegining is a standard function rather than a method:

insertAtBegining(head, 30)
...
func insertAtBegining(n *node, d int) *node{
    nn := &node{d,n}
    n = nn
}

As Go passes all parameters by value (you can pass a pointer but that is passed as a value) I think it's clear that this would not change head (if you wanted to do that you would need to define the function func insertAtBegining(n **node, d int) *node{ and pass it &head.

The issue is the same in your method.

The simplest fix for this is to redefine insertAtBegining as (playground):

func (n *node) insertAtBegining(d int) *node{
    nn := &node{d,n}
    return nn
}

It now returns the new head so you use it like: head = head.insertAtBegining(30).

Another approach is to encapuslate the list management within a struct; this is the approach taken in list.List. Doing this has the advantage that the user needs no understanding of how the list is stored.

Updating value in pointer reciever method but value not updating

1 Answers1