Same code giving different results in Online IDE and local IDE

Question

Below program is used to remove the duplicates from a sorted singly linked list. The code gives garbage values in online IDE. But when I comment the line .

delete curr;

The program works fine in online IDE itself. Here is the function I wrote. The other parts of the code are well-defined(not by me) by the online Judge.

And also the code without commenting the line delete curr; works fine in local IDE(codeblocks).

FULL PROGRAM:http://ideone.com/9bHab0

Why do I get the garbage values?

Node *removeDuplicates(Node *root)
{
 // your code goes here
    struct Node* curr = root,*prev = NULL;
    while(curr)
    {
        if(prev==NULL)
            prev = curr;
        else if(curr->data!=prev->data)
            prev = curr;
        else
        {
            prev->next = curr->next;
            delete curr;
            curr = prev->next;
        }
    }
    return root;
}

EDIT: One could see the pointer whose location is deleted, is reassigned immediately. Thus there couldn't be a dangling pointer here!

You *do* allocate your `Node` instances with `new`? *All* of them? What are their destructors doing? — Some programmer dude, Jun 30 '17 at 11:02
That's a happy mixture of C (`struct Node* curr`) and C++ (`delete curr`) code. Are you sure, that the caller is C++, and calls `operator new` to allocate instances? — IInspectable, Jun 30 '17 at 11:03
that will go into an infinite loop as soon as it has to go through the else if block - are you sure this is your actual code — Tom Tanner, Jun 30 '17 at 11:04
But it actually passes all test cases of online Judge, when I comment the particular line **delete curr**. — Pavithran Ravichandiran, Jun 30 '17 at 11:06
@IInspectable - the full code link shows the allocation is done via new. using `struct` is perfectly normal in C++ — Tom Tanner, Jun 30 '17 at 11:08
@JinxerAlbatross So? That doesn't mean it's good. It means you hid undefined behaviour by replacing it with a memory leak. — underscore_d, Jun 30 '17 at 11:08
@TomTanner: I don't look at off-site resources. Questions on SO are supposed to be self-contained. This is explained in the instructions on how to produce a [mcve]. — IInspectable, Jun 30 '17 at 11:09

Some programmer dude · Accepted Answer · 2017-06-30T11:13:33.383

Lets take a very simple example, with a two-node list, where you have e.g.

node1 -> node2

With the first iteration then prev is NULL so you do prev = curr. Now curr and prev points to the same node.

That means in the second iteration both if conditions are false (prev != NULL and curr->data == prev->data) you go into the else part, where you have

prev->next = curr->next;
delete curr;
curr = prev->next;

Here you delete curr but curr is pointing to the same memory as prev, leading to undefined behavior in the assignment curr = prev->next, as you now dereference the stray pointer prev.

To make matters worse, you then enter a third iteration where prev is still pointing to the deleted first node, and again dereference the invalid prev pointer (in your second if condition) and you once again end up in the else part where you continue the invalid dereferencing. And so on in infinity (or you get a crash).

And the solution is: Replace `prev = curr;` with `prev = curr; curr = curr->next;` in the first two conditional branches. — IInspectable, Jun 30 '17 at 11:12
Adding **curr = curr->next** to the **if part and else if part** solved the problem. Thanks for the solution. Should I change the header of the question to avoid unwanted attention towards this question? — Pavithran Ravichandiran, Jun 30 '17 at 11:24

Same code giving different results in Online IDE and local IDE

1 Answers1