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The goal is to remove adjacent duplicates in a linked list. For example, the input aaabccf will be reduced to bccf (removing adjacent duplicate a's) ---> bf (removing adjacent duplicate c's) and should give output as bf. If input is aaa , output is NULL.

My C code works fine for most for the cases, except for cases like aaa , abb , aabcdd etc where the one of the adjacent duplicate node to be removed is at the end and points to NULL. Please tell me where I have made mistake.

What I have tried:

This is my code for removing adjacent duplicates-


C
void remove_adj_dup(struct node** head)
{
    struct node *temp=NULL, *current=NULL, *temp2=NULL;
    int flag=1;

    while(flag)
    {
        current = *head;
        flag=0;

        while(current->next!=NULL)
        {
            if(current->data==current->next->data)
            {
                temp=current;
                flag=1;
                while(current->data==current->next->data && current->next!=NULL)
                {
                    current=current->next;
                }
                
                if(temp== *head) *head=current->next;
                
                else
                {
                    temp2= *head;
                    while(temp2->next!=temp) temp2=temp2->next;
                    temp2->next=current->next;
                }
                //printf("# %c\n",current->next->data);
                //printList(*head); 
            }
            
            current=current->next;
        }
    }
}
Posted
Updated 5-Dec-22 9:35am

In the function remove_adj_dup() there are some places where null pointers are accessed. At the very beginning it would make sense to check the variable head.
C
if ((head == NULL) || (*head == NULL))
  return;

There are two more places where it goes wrong with null pointers.
C
//while (current->next != NULL)
while ((current != NULL ) && (current->next != NULL))

Before you can check current->next you should also be sure that current is not a null pointer. One more place I leave for practice with the debugger.
The flag is not needed and can be deleted completely, as well as the corresponding while loop.
C
current = *head;
// while (flag)
// {
//   flag = 0;
 
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Comments
CPallini 5-Dec-22 16:59pm    
5.
merano99 5-Dec-22 17:55pm    
Thx
Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:
Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16
Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:
C
int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!
 
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Comments
OriginalGriff 5-Dec-22 12:23pm    
That['s what the debugger is *for* - it lets you look at your code while it is running, view (and change) variable contents, execute your code line by line to follow exactly what it is doing, and why.

So break out the debugger, and start looking at what exactly your code is doing, and what the data it is working with looks like.

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