C LinkedList adding node to end

Question

I am trying to create a program which can add information to a linked list. It seems that once I add more than 2 nodes, previously added nodes get overwritten. Is there something I am missing in making this work?

I start with this function which either adds to the front of the list, or if there is already a node added to the front it calls a function addToBack

ListHeadPtr Front(ListHeadPtr theList, list * toBeAdded){

  printf("Received in addToFront\n");

  if(theList == NULL){
    theList = toBeAdded;
    return(theList);
  }

  else{
    theList = addToBack(theList,toBeAdded);
    /*toBeAdded->next = theList;
    theList = toBeAdded;*/
    return (theList);
  }

}

  ListHeadPtr back(ListHeadPtr theList, item * toBeAdded){

  if(theList == NULL){
    theList = addToFront(theList,toBeAdded);
    return(theList);
  }

  else{
    list *current;
    current = theList;
    while (current->next!=NULL){
      current = current->next;
    }
    current->next = toBeAdded;
    return(current);
  }

}

The item(toBeAdded is defined by this function

item *createItem(int time){

  item *itemPtr = (list *) malloc(sizeof(item));

  if(NULL == itemPtr)
    {
        printf("Unable to create item\n");
        return (NULL);
    }

  itemPtr->waitTime = time;
  itemPtr->next = NULL;

  return (itemPtr);
}

@MitchWheat Not sure how to do that. I'm still quite new to C. — user2225940
– user2225940, Commented Apr 1, 2013 at 1:44
Learning to use a debugger is very useful, but in many cases it's quicker to read the code and see if it's logically doing what you want. See my answer ... no debugger needed (yet). — Jim Balter
– Jim Balter, Commented Apr 1, 2013 at 1:48

Jim Balter · Accepted Answer · 2013-04-01 01:51:01Z

3

In your [addTo]Back function

return(current);

should be returning the head of the list instead. What you're doing is truncating the list to its last two elements.

One way to avoid such bugs is to be precise with your semantics. What is addToBack defined to return? The caller expects it to return the list, with the node added. There should be a documentation comment before the function, stating what it does and what it returns. That comment can guide you in writing the code ... and the code would be better, if there were just one statement at the end that returns the list, not several returns. Then this bug could not have happened.

edited Apr 1, 2013 at 1:51

answered Apr 1, 2013 at 1:45

Jim Balter

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7 Comments

user2225940 Over a year ago

Thanks so much! That has fixed it. I've run into another issue where for some reason, the while loop might get stuck and not be able to exit?

Jim Balter Over a year ago

The while loop will get stuck if you create a loop in your list. Having addToFront call addToBack and v.v. is suspicious ... neither function should call the other. addToBack should just return the node if the list is null or find the end of the list and append the node, and addToFront should just make the node's next ptr point to the list and then return the node.

user2225940 Over a year ago

Thanks for your input, I will inspect that.

user2225940 Over a year ago

to avoid the functions calling each other, I have implemented this check in the main: if(line==NULL) line2 = addToFront(line,toBeAdded); else line = addToBack(line,toBeAdded); however, now where it was once not infinitely looping, it is now.

Jim Balter Over a year ago

@user2225940 Well, I explained what addToFront and addToBack should do. Main should call one or the other depending on whether you want to treat the list like a stack (addToFront) or a queue (addToBack) ... it should not be conditional on whether the list is empty.

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