Question

As a part of an assignment, I need to write two functions:

1. a function that sums up two natural numbers represented as a linked list
2. a functions that prints a number represented in the same way.

for some reason, both function work perfectly fine separately, but when I try to use the print function on the result of the sum function, it changes the value of the sum right in the beginning of the print function , and prints the wrong value. when I use printf to print the same value in the main, there is no problem. my code is detailed below. any ideas?

void main() 
{
  int a[1] = { 1 },
    b[1] = { 2 };
  int * *pa, **pb;
  List lst1, lst2;
  List sum;

  pa = (int * *) malloc(sizeof(int * )); * pa = &a[0];
  pb = (int * *) malloc(sizeof(int * )); * pb = &b[0];
  lst1 = arrToList(pa, 1);
  lst2 = arrToList(pb, 1);
  addNumbers(lst1, lst2, &sum);
  //printf("%d\n",*(sum.head->dataPtr));
  printNumber(sum);
}

//a function that recieves a number represented ad a list and prints it
void printNumber(List num) 
{
  ListNode * curr;
  int currData,
  i,
  number;

  if (isEmptyList(num) == TRUE) 
    printf("the input was an empty list, nothing to print");
  else 
  {
    i = 0;
    number = 0;
    curr = num.head;
    while (curr != NULL) 
    {
      currData = *(curr - >dataPtr);
      number = number + currData * ((int) pow(10, i));
      curr = curr - >next;
      i++;
    }
    printf("%d \n", number);
  }
}

// a function that sums in list 
// representation two numbers,
// each represented as a list 
void addNumbers(List n1, List n2, List * sum) 
{
  ListNode * currN1;
  ListNode * currN2;
  ListNode * currSum;
  int currN1N2Sum; //stores the sum of the current digits in n1 and n2 
  int carrier,
  prevCarrier; //current and previous  carriers that carries +1 to the 
  next digit of sum
  if the lst sum was bigger then 9

  if ((isEmptyList(n1) == TRUE) || (isEmptyList(n2) == TRUE)) 
    printf("bad input =(");
  else 
  {
    currN1 = n1.head;
    currN2 = n2.head; * sum = createEmptyList();
    carrier = 0;
    prevCarrier = 0;
    while ((currN1 != NULL) && (currN2 != NULL)) 
    {
      currN1N2Sum = *(currN1->dataPtr) + *(currN2->dataPtr) + prevCarrier;
      if (currN1N2Sum > 9) 
      {
        carrier = 1;
        currN1N2Sum = currN1N2Sum - 10;
      }
      currSum = creatNewListNode( & currN1N2Sum, NULL);
      insertNodeToEnd(sum, currSum);
      prevCarrier = carrier;
      carrier = 0;
      currN1 = currN1 - >next;
      currN2 = currN2 - >next;
    } //while ((currL1!=NULL)&&(currL2!=NULL))

    while (currN1 != NULL) 
    {
      currN1N2Sum = *(currN1 - >dataPtr) + prevCarrier;
      currN1 = currN1 - >next;
      if (prevCarrier != 0) prevCarrier = 0;
    }

    while (currN2 != NULL) 
    {
      currN1N2Sum = *(currN2 - >dataPtr) + prevCarrier;
      currN2 = currN2 - >next;
      if (prevCarrier != 0) prevCarrier = 0;
    }
  } // ! ((isEmptyList(n1)==TRUE)||(isEmptyList(n2)==TRUE))
}

here is the rest of the code:

typedef struct listNode{
int* dataPtr;
struct listNode* next;
} ListNode;

typedef struct list
{
ListNode* head;
ListNode* tail;
} List;

List createEmptyList()//creates and returns an empty linked list 
{
    List res;

    res.head = res.tail = NULL;

    return res;
}

Bool isEmptyList ( List lst )//checks if a given list is empty or not
{
    if (lst.head == NULL && lst.tail == NULL)
        return TRUE;
    else
        return FALSE;
}

void insertDataToEnd ( List * lst, int *dataPtr ) //inserts new data to the end of an existing linked list
{
    ListNode * newTail;
    newTail = creatNewListNode ( dataPtr, NULL );
    insertNodeToEnd(lst,newTail);
}

void insertNodeToEnd ( List * lst, ListNode * newTail )//insert an existing node to an existing linked list
{
    if (isEmptyList(*lst) == TRUE )
        insertNodeToStart ( lst,newTail );
    else
    {
        (*lst).tail -> next = newTail;
        newTail->next = NULL;
        (*lst).tail = newTail;
    }
}


ListNode * creatNewListNode ( int * dataPtr, ListNode * next )//inserts new node in an existing linked list
{
    ListNode * res;

    res = (ListNode *) malloc (sizeof(ListNode));

    res -> dataPtr  = dataPtr;
    res -> next     = next;

    return res;
}

void insertNodeToStart  ( List * lst, ListNode * newHead )//inserts node to the begining of a given linked list
{
    if ( isEmptyList( *lst ) == TRUE )
    {
        (*lst).head = newHead;
        (*lst).tail = newHead;
        newHead -> next = NULL;
    }
    else
    {
        newHead -> next = (*lst).head;
        (*lst).head = newHead; 
    }
}

Answer 1

The bug is in the function addNumbers.
When you add a node to store the sum you pass a pointer to the variable currN1N2Sum which is a local variable (stored on the stack). When the addNumbers function terminates, the storage of the local variable is set free. The value found at that location will remain unchanged and thus apparently valid as long as the storage is not reused.

This is why you had the impression the addNumbers function was correct. When calling the printNumber function the storage is overwritten and you find a different value in there.

This explain your bug.

There is another problem with addNumbers. When you will try to add two digit numbers, the content of the currN1N2Sum will be overwritten by a new value.

What you should do is allocate a buffer (malloc) and store the value contained into currN1N2Sum into it. Pass the pointer to the buffer into the new node.

BTW: you may change (*lst).head in lst->head. It will make your code more readable.