Golang struct pointer reference lost when calling struct property method - pointers

I'm attempting to use a struct to manage accessing nodes on a tree. Whenever I access the method of the parent's child node, the parent reference on the subsequent call gets lost (i.e. parent.child.method(child) -> [parent becomes nil]-> parent(the previous child).child ... etc).
Here is the error snippet from my file.
type Node struct {
Left *Node
Right *Node
value int
}
func (parent *Node) determineSide(child *Node) (Node, Node) {
if child.Value < parent.Value {
if parent.hasLeftNode() {
return parent.Left.determineSide(child)
}
return parent.addLeftNode(child)
} else if child.Value > parent.Value {
if parent.hasRightNode() {
return parent.Right.determineSide(child)
}
return parent.addRightNode(child)
}
return *child, *parent
}
I attempted to solve this by trying to find a way to inform the method that the new reference should be parent.Left. Things like using *parent.Left and &parent.Left didn't seem to be correct.
A solution may might be to move this code outside of the struct and have another function handle the outcome for a quick fix, but I'd like to understand why this isn't working out of the box. Thought process here is influenced by using this.child.determineSide(child).
Full code is here.
Edit
Here is some output from the terminal that might give even further context. Looks like I'm having a check type issue leading to the problem.
parent &{<nil> <nil> 2}
parent.Left <nil>
parent.LeftNode true
child &{<nil> <nil> 1}
parent <nil>
child &{<nil> <nil> 1}

Okay, I know what u'r exactly asking finally.
New() methods returns a value, not a pointer, which means u can't see later change in caller. What the caller got is only a value copy of the Node. So the parent what u print will always be {Left:<nil> Right:<nil> Value:2}.
So the same with addLeftNode() and addRightNode().
Just use pointer, not value to achieve your goal.
See pointers_vs_values
I think it's just the Visit() method where the problem is.
It will never visit right child when u immediately return after visited left child.
The left and right child are not mutually exclusive, so the second if-clause should not use else if, which would be if.
The visiting order also has problem.
Before:
// Visit will automatically walk through the Child Nodes of the accessed Parent Node.
func (parent *Node) Visit() (Node, int) {
fmt.Println("Node value:", parent.Value)
if parent.hasLeftNode() {
return parent.Left.Visit()
} else if parent.hasRightNode() {
return parent.Right.Visit()
}
return *parent, parent.Value
}
Modified:
// Visit will automatically walk through the Child Nodes of the accessed Parent Node.
func (parent *Node) Visit() (Node, int) {
if parent.hasLeftNode() {
parent.Left.Visit()
}
fmt.Println("Node value:", parent.Value)
if parent.hasRightNode() {
parent.Right.Visit()
}
return *parent, parent.Value
}
Additionally, as to me, Visit() shouldn't return any values.

Problem originated from incorrect type checking. The function successfully handled the call, but the method I was using wasn't accurate in confirming whether a node was assigned.
// isNode checks whether the provided property is a Node.
func (parent *Node) isNode(property interface{}, typeset interface{}) bool {
fmt.Println(reflect.TypeOf(property) == reflect.TypeOf(typeset))
// this always referred to the address for the base Node struct or similar falsy.
return reflect.TypeOf(property) == reflect.TypeOf(typeset)
}
// hasLeftSide tests whether the Parent Node has a Node assigned to its left side.
func (parent *Node) hasLeftNode() bool {
return parent.Left != nil //parent.isNode(parent.Left, (*Node)(nil))
}
// hasRightSide tests whether the Parent Node has a Node assigned to its right side.
func (parent *Node) hasRightNode() bool {
return parent.Right != nil // parent.isNode(parent.Right, (*Node)(nil))
}

Related

Unable to change pathBuff/path variable in async function

I was unsure if I should post this here or in code review.
Code review seems to have only functioning code.
So I've a multitude of problems I don't really understand.
(I’m a noob) full code can be found here: https://github.com/NicTanghe/winder/blob/main/src/main.rs
main problem is here:
let temp = location_loc1.parent().unwrap();
location_loc1.push(&temp);
I’ve tried various things to get around problems with borrowing as mutable or as reference,
and I can’t seem to get it to work.
I just get a different set of errors with everything I try.
Furthermore, I'm sorry if this is a duplicate, but looking for separate solutions to the errors just gave me a different error. In a circle.
Full function
async fn print_events(mut selector_loc1:i8, location_loc1: PathBuf) {
let mut reader = EventStream::new();
loop {
//let delay = Delay::new(Duration::from_millis(1_000)).fuse();
let mut event = reader.next().fuse();
select! {
// _ = delay => {
// print!("{esc}[2J{esc}[1;1H{}", esc = 27 as char,);
// },
maybe_event = event => {
match maybe_event {
Some(Ok(event)) => {
//println!("Event::{:?}\r", event);
// if event == Event::Mouse(MouseEvent::Up("Left").into()) {
// println!("Cursor position: {:?}\r", position());
// }
print!("{esc}[2J{esc}[1;1H{}", esc = 27 as char,);
if event == Event::Key(KeyCode::Char('k').into()) {
if selector_loc1 > 0 {
selector_loc1 -= 1;
};
//println!("go down");
//println!("{}",selected)
} else if event == Event::Key(KeyCode::Char('j').into()) {
selector_loc1 += 1;
//println!("go up");
//println!("{}",selected)
} else if event == Event::Key(KeyCode::Char('h').into()) {
//-----------------------------------------
//-------------BackLogic-------------------
//-----------------------------------------
let temp = location_loc1.parent().unwrap();
location_loc1.push(&temp);
//------------------------------------------
//------------------------------------------
} else if event == Event::Key(KeyCode::Char('l').into()) {
//go to next dir
} if event == Event::Key(KeyCode::Esc.into()) {
break;
}
printtype(location_loc1,selector_loc1);
}
Some(Err(e)) => println!("Error: {:?}\r", e),
None => break,
}
}
};
}
}
also, it seems using
use async_std::path::{Path, PathBuf};
makes the rust not recognize unwrap() function → how would I use using ?
There are two problems with your code.
Your PathBuf is immutable. It's not possible to modify immutable objects, unless they support interior mutability. PathBuf does not. Therefore you have to make your variable mutable. You can either add mut in front of it like that:
async fn print_events(mut selector_loc1:i8, mut location_loc1: PathBuf) {
Or you can re-bind it:
let mut location_loc1 = location_loc1;
You cannot have borrow it both mutable and immutably - the mutable borrows are exclusive! Given that the method .parent() borrows the buffer, you have to create a temporary owned value:
// the PathBuf instance
let mut path = PathBuf::from("root/parent/child");
// notice the .map(|p| p.to_owned()) method - it helps us avoid the immutable borrow
let parent = path.parent().map(|p| p.to_owned()).unwrap();
// now it's fine to modify it, as it's not borrowed
path.push(parent);
Your second question:
also, it seems using use async_std::path::{Path, PathBuf}; makes the rust not recognize unwrap() function → how would I use using ?
The async-std version is just a wrapper over std's PathBuf. It just delegates to the standard implementation, so it should not behave differently
// copied from async-std's PathBuf implementation
pub struct PathBuf {
inner: std::path::PathBuf,
}

Recursion function on a struct in Golang

I have the following code to organise *Widget structs into a hierarchy. Parent() returns the widget with an ID of the caller's parentID. The hierarchy can be up to 4 or 5 levels deep.
type Widget struct {
ID int64
ParentID int64
}
func (w *Widget) Parent() *Widget {
// Returns the widget with an ID of w.ParentID
}
What I want to achieve is a function which aggregates the parent, as well as the parent's parent etc. to the top of the hierarchy, and returns a slice of all of the parent IDs. As I don't know the depth of the hierarchy, I think I need some sort of programmatic recursion to get the parent of each parent, which would do something like the following:
func (w *Widget) AllParents() []*Widget {
var parentWidgets []*Widget
x := w.Parent()
parentWidgets = append(parentWidgets, x)
y := x.Parent()
parentWidgets = append(parentWidgets, y)
...
return parentWidgets
}
Is there a more idiomatic way of achieving this?
You just need to step higher and higher in the hierarchy until you reach the root. Assuming Widget.Parent() returns nil if the Widget is the "root" (meaning it has no parent):
Iterative solution
Here is the solution with a simple for loop:
func (w *Widget) AllParents() []*Widget {
var ws []*Widget
for parent := w.Parent(); parent != nil; parent = parent.Parent() {
ws = append(ws, parent)
}
return ws
}
This method will return nil if called on the "root" (zero value for all slice types). In other cases, it will return the "path" from the direct parent leading to the "root".
Here's a little test program. It creates a root widget with ID = 0, a child with ID = 1 and a "grandchild" with ID = 2, and prints AllParents() called on each. To implement Widget.Parent(), I used a simple "widget registry" map.
Result as expected:
Parents of 0:
Parents of 1: 0
Parents of 2: 1 0
Try it on the Go Playground.
var wreg = map[int64]*Widget{}
func (w *Widget) Parent() *Widget {
// Returns the widget with an ID of w.ParentID
return wreg[w.ParentID]
}
func main() {
w := &Widget{0, -1}
wreg[w.ID] = w
w2 := &Widget{1, 0}
wreg[w2.ID] = w2
w3 := &Widget{2, 1}
wreg[w3.ID] = w3
printParents(w)
printParents(w2)
printParents(w3)
}
func printParents(w *Widget) {
fmt.Printf("Parents of %d:", w.ID)
for _, w := range w.AllParents() {
fmt.Print(" ", w.ID)
}
fmt.Println()
}
Recursive solution
Of course it can be solved using recursion too:
func (w *Widget) AllParents() []*Widget {
if parent := w.Parent(); parent == nil {
return nil
} else {
return append(parent.AllParents(), parent)
}
}
This solution returns the "path" from the root leading to the direct parent.
Running the above test program with this implementation, output is:
Parents of 0:
Parents of 1: 0
Parents of 2: 0 1
Try this on the Go Playground.

Return value of a recursive function

In the following code, I'm trying to get a better hand at understanding how recursion actually work. I've always been a bit confused about it's actual working. I want to know what value does the inorder() function actually return in every step. From where does it get these values of 0,0,11,0,0,11,12,0,0,11 respectively. Could someone tell me the logic? It's a basic inorder tree traversal program.The reason why I'm trying to understand these outputs is because the same logic is somehow used to find the depth of the tree( I think) where with every recursion the value of depth increases without initialization.
#include<stdio.h>
#include<stdlib.h>
struct node
{
int data;
struct node *left;
struct node *right;
};
struct node* newNode(int data)
{
struct node* node=(struct node*)malloc(sizeof(struct node));
node->data=data;
node->left=NULL;
node->right=NULL;
return node;
}
int inorder(struct node *temp) {
if (temp != NULL) {
printf("\nleft %d\n",inorder(temp->left));
printf("\n%d\n", temp->data);
printf("\nright %d\n",inorder(temp->right));
}
}
int main()
{
struct node *root=newNode(1);
root->left=newNode(2);
root->right=newNode(3);
root->left->left=newNode(4);
root->left->right=newNode(5);
inorder(root);
getchar();
return 0;
}
This function should be changed to the following (the first and last print in the original code will only get you more confused!):
int inorder(struct node *temp) {
if (temp != NULL) {
inorder(temp->left);
printf("%d\n", temp->data);
inorder(temp->right);
}
}
The recursion starts with the left branch of a specific node (usually the "root") - printing recursively (in-order) all the nodes on that left-branch, then printing the current node, moving on to printing recursively (in-order) all the nodes in the right branch.
By the way, if you want to keep that tree "ordered" (meaning, all the nodes on the left branch are smaller than the node, and all the nodes on the right branch are bigger or equal to the node) you should change:
root->left->left=newNode(4);
root->left->right=newNode(5);
to:
root->right->right=newNode(4);
root->right->right->right=newNode(5);

Go linked list pointer assignment from non-pointer struct element

https://github.com/golang/go/blob/master/src/container/list/list.go#L49
I am having hard time why I am getting cannot assign to pointer error in Go.
Here's the code that works: http://play.golang.org/p/P9FjK8A-32 which is same as Go's original container/list code
type List struct {
root Element
len int
}
type Element struct {
next, prev *Element
list *List
Value interface{}
}
The original code has root as a value and reference it everytime it needs to be in pointer type but why not at first place define root as a pointer?
type List struct {
root *Element
len int
}
type Element struct {
next, prev *Element
list *List
Value interface{}
}
This give me an error: http://play.golang.org/p/1gCAR_rcx1 -> invalid memory address or nil pointer dereference
Why am I getting this error?
Why does Go define root as a non-pointer value when it defines next, and prev as pointers?
Thanks
A pointer is nil by default and needs to be initialized.
This:
// Init initializes or clears list l.
func (l *List) Init() *List {
l.root.next = l.root
l.root.prev = l.root
l.len = 0
return l
}
should become this:
// Init initializes or clears list l.
func (l *List) Init() *List {
l.root = new(Element) // necessary to avoid dereferencing a nil pointer
l.root.next = l.root
l.root.prev = l.root
l.len = 0
return l
}
Demo at http://play.golang.org/p/EYSscTMYnn
In the case of the standard library, it is not necessary to have root be a pointer, however, for prev and next it is necessary, otherwise the struct definition would be recursive, which is not allowed, because it would in theory cause a struct of infinite size...

segfault after return 0;

I wrote a program to test my binary tree and when I run it, the program seems to crash (btree.exe has stopped working, Windows is checking for a solution ...).
When I ran it through my debugger and placed the breakpoint on the function I suspect is causing it, destroy_tree(), it seemed to run as expected and returned back to the main function. Main, in turn, returned from the program but then the cursor jumped back to destroy_tree() and looped recusively within itself.
The minimal code sample is below so it can be ran instantly. My compiler is MinGW and my debugger is gdb (I'm using Code::Blocks).
#include <iostream>
using namespace std;
struct node
{
int key_value;
node *left;
node *right;
};
class Btree
{
public:
Btree();
~Btree();
void insert(int key);
void destroy_tree();
private:
node *root;
void destroy_tree(node *leaf);
void insert(int key, node *leaf);
};
Btree::Btree()
{
root = NULL;
}
Btree::~Btree()
{
destroy_tree();
}
void Btree::destroy_tree()
{
destroy_tree(root);
cout<<"tree destroyed\n"<<endl;
}
void Btree::destroy_tree(node *leaf)
{
if(leaf!=NULL)
{
destroy_tree(leaf->left);
destroy_tree(leaf->right);
delete leaf;
}
}
void Btree::insert(int key, node *leaf)
{
if(key < leaf->key_value)
{
if(leaf->left!=NULL)
insert(key, leaf->left);
else
{
leaf->left = new node;
leaf->left->key_value = key;
leaf->left->left = NULL;
leaf->left->right = NULL;
}
}
else if (key >= leaf->key_value)
{
if(leaf->right!=NULL)
insert(key, leaf->right);
else
{
leaf->right = new node;
leaf->right->key_value = key;
leaf->right->left = NULL;
leaf->right->right = NULL;
}
}
}
void Btree::insert(int key)
{
if(root!=NULL)
{
insert(key, root);
}
else
{
root = new node;
root->key_value = key;
root->left = NULL;
root->right = NULL;
}
}
int main()
{
Btree tree;
int i;
tree.insert(1);
tree.destroy_tree();
return 0;
}
As an aside, I'm planning to switch from Code::Blocks built-in debugger to DDD for debugging these problems. I heard DDD can display visually pointers to objects instead of just displaying the pointer's address. Do you think making the switch will help with solving these types of problems (data structure and algorithm problems)?
Your destroy_tree() is called twice, you call it once and then it gets called after the execution leaves main() from the destructor.
You may think it should work anyway, because you check whether leaf!=NULL, but delete does not set the pointer to NULL. So your root is not NULL when destroy_tree() is called for the second time,
Not directly related (or maybe it is) to your problem, but it's good practice to give structs a constructor. For example:
struct node
{
int key_value;
node *left;
node *right;
node( int val ) : key_val( val ), left(NULL), right(NULL) {}
};
If you do this, your code becomes simpler, because you don't need worry about setting the pointers when you create a node, and it is not possible to forget to initialise them.
Regarding DDD, it;'s a fine debugger, but frankly the secret of debugging is to write correct code in the first place, so you don't have to do it. C++ gives you a lot of help in this direction (like the use of constructors), but you have to understand and use the facilities it provides.
Btree::destroy_tree doesn't set 'root' to 0 after successfully nuking the tree. As a result, the destructor class destroy_tree() again and you're trying to destroy already destroyed objects.
That'll be undefined behaviour then :).
Once you destroy the root.
Make sure it is NULL so it does not try to do it again (from the destructor)
void Btree::destroy_tree(node *leaf)
{
if(leaf!=NULL)
{
destroy_tree(leaf->left);
destroy_tree(leaf->right);
delete leaf;
leaf = NULL; // add this line
}
}

Resources