Question

The requirement is an input function that returns a Node* to the root of a linked list that input creates from the user’s input.
Like:

Node *input()
{
     Node *root; //not yet allocated
     while(1)
     {    
           //take in numbers from the user and correspondingly add them 
           //to the linked list and quit when anything else is entered

           if(input character is a number)
           {
                //allocate memory to a fresh pointer and push the number to it
                //and add this new Node to the LL
           }
     }
     return root;
}

-> There is the solution of pre-allocating memory to root and pushing the 1st number to it before the while loop body. But here, if the user won’t enter anything, that must deleted right away.

-> Also, there is a possible way of checking if root is NULL in the while loop, and if it is, allocating memory , so that it happens only once.

But I would like to know if there is a solution that will eliminate that odd situation arising from the root of the LL.

-> Maybe I could keep a dummy node at the 1st that doesn’t hold any values.

But apart from that?
If there is a better method of doing the whole thing, please suggest it.

Answer 1

You can do something like:

struct Node* root = 0; // don't forget to initialize

while (...) {
  if (...) {
    struct Node * item = ... ;  // allocate
    item->data = ...;
    item->next = root;
    root = item;
  }
}

return root;

And you’re done, no special case required for the first element. The end of list is indicated by a null next pointer.

If you want the list to be in insertion order, without traversing the whole list each time, you could:

struct Node* root = 0; // don't forget to initialize
struct Node* tail = 0;

while (...) {
  if (...) {
    struct Node * item = ... ;  // allocate
    item->data = ...;
    item->next = 0;

    if (tail)
      tail->next = item;
    tail = item;

    if (!root)
      root = item;
  }
}

return root;

No special-case allocation, but special-case assignments.