Doubly linked list output validation

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I have coded the following method to add details to a linked list using pointers but I'm stuck at the final part of the method where I'm checking the condition of 'curr'.

if(curr-> newContact)
   {
       curr->next = newContact;
       newContact->prev = NULL;


   }
   else
   {
   curr->next = newContact;
   newContact->prev = curr;
   newContact->next = NULL;
   }

It doesn't add the contact.I'm not sure where I have gone wrong in the loop to check.This is the whole method.Anyone have any ideas?Thanks

C#

int addContact(struct contact *theList)
{
    struct contact *newContact, *curr;

    // create the new structure
    newContact = (struct contact *)malloc(sizeof(struct contact));
    if( newContact == NULL )
    {   // if true, then no memory left - oops
        return(0);
    }
    // find the end of the list
    curr = theList;
    // scroll through the list
    if(curr != NULL)
    {

        while( curr->next != NULL)
        {
            curr = curr->next;
        }

    }
    // now have the last contact
    // add the new one here.
    printf("\nEnter a surname: ");
    fflush(stdin);
    gets(newContact->sname);
    printf("\nEnter a first name: ");
    gets(newContact->fname);
    printf("\nEnter a phone: ");
    gets(newContact->phone);
    printf("\nEnter a company: ");
    gets(newContact->company);

    // need to hook the new contact to
    // end of list

    if(curr-> newContact)
    {
        curr->next = newContact;
        newContact->prev = NULL;


    }
    else
    {
    curr->next = newContact;
    newContact->prev = curr;
    newContact->next = NULL;
    }
    return(1);
}//add

This is the code where I call addContact:

C#

sts = addContact(head);
            sts = writeListToFile("test.csv",head);

            printf("\n\nUpdate Contact List");
            printf("\n%s,%s,%s,%s,%s",head->sname,head->fname,head->phone,head->company);
            head = head->next;

Posted 26-Apr-13 3:25am

johnmolo

Updated 26-Apr-13 6:13am

v3

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nv3 · Answer 1 · 2013-04-26T03:41:00

First of, the argument passing of theList is wrong. In case that the list is still empty, you were expecting to pass NULL for theList. But how do you then return the new element that is being allocated in that call?

The solution is to pass

C++

int addContact(struct contact **theList)
{
    ...
    curr = *theList;

Now to the end of your function. What you want to test is whether curr is NULL, hence the list is still empty. In that case you want to place the pointer to the first element into the list anchor:

C++

if (curr == 0)
{
    *theList = newContact;
    newContact->next = NULL;
    newContact->prev = NULL:
}
else
...

By the way, I doubt that your code will compile. Is newContact really a member of you contact structure?

[AMENDED]
And your calling code should look like this:

C++

struct contact* ptrHead = 0;
...
addContact (&ptrHead);
...
writeListToFile ("test.csv", ptrHead);

What I was wondering about is: Why are keeping a doubly linked list, but have only a single head pointer? The advantage of a doulbe linked is that you can traverse it in both directions. An additional pointer to the last element in the list would make that easy and also support your append operation; it would save you from looping over the entire list to find the last element. But first let's find the little bug that keeps it from working.