doubt in recursion

[tubby123]

I have a doubt in a question on "Merge 2 sorted linked lists" .
I tried really hard before posting, i tested on paper many times - it comes out correct, but there is something wrong with my code.

Basically, i am not getting the recursion right.
Also, is there some software/tool where i can see the interiors of the stack when recursion is going on?

My code is as follows

#include<stdio.h>
#include<conio.h>

typedef struct node
{
    int data;
    struct node* link;
}Node;
Node* start1;
Node* start2;
Node* start;
Node* node;
Node* result;

int main(void)
{
    printf("\nEnter the elements of the first ll , Enter -1 to stop entering data");
    start1=form();
    printf("\nEnter the elements of the second ll , Enter -1 to stop entering data");
    start2=form();

    printf("\nLinked list 1 contents are ");
    display(start1);
    printf("\nLinked list 2 contents are ");
    display(start2);

    start=NULL;
    merge(start1,start2);
    display(start);

    printf("\nThe value stored in result = %d",result->data);

    getch();
    return 0;
}

int merge(Node* node1, Node* node2)
{
  if(node1==NULL)
    return node2;
  if(node2==NULL)
    return node1;

  if(node1!=NULL &&  node2!=NULL)
  {
   if(node1->data <= node2->data)
   {
    if(start == NULL)
    start = node1;
    result=node1;
    result->link = merge(node1->link,node2);
   }


   else
   {
    if(start == NULL)
    start=node2;
    result=node2;
    result->link = merge(node1,node2->link);
   }

   return result;
  }
  //return result;
}


int display(Node* node)
{
    while(node)
    {
        printf("%d ",node->data);
        node = node->link;
    }
}

int form()
{
    int num=0;
    Node* ptr;
    Node* last;
    Node* start=NULL;

    start=NULL;

    while(1)
    {
        scanf("%d",&num);
        if(num == -1)
        {
            printf("Linked list formation over ");
            return start;
        }
        else
        {
            ptr=(Node*)malloc(sizeof(Node));
            ptr->data = num;
            ptr->link = NULL;

            if(start == NULL)
            {
                start=ptr;
                last=ptr;
            }
            else
            {
                    last->link = ptr;
                    last=ptr;
            }
        }
    }
}

Thanks
Aftab

[AndrewHunter]

1. Global variables are bad, get rid of them.
2. You have start declared locally and globally, that is bad. Additionally, you pass start1 and start2 to your function but you have them declared globally.
3. You have been told before, you do not need to cast the return of malloc.
4. You should always check the return of malloc to ensure it worked.
5. Forget about recursion for now, get the program working using a loop, then adjust to a recursive solution.

EDIT: Someone nicer than me will probably debug what you have, however this code is too broke for me to care about.

[tubby123]

Having a look at the merge function alone would be sufficient to point out my mistake.

I have written int merge(),
I have corrected that to Node* merge, still doesnt work

[tubby123]

@AndrewHunter, sir, i dint get that part about not having to cast the return type of malloc.
I need a pointer of type Node right, but malloc returns (void*).
Sorry sir, i am relatively new to programming.
Yes, i shall correct the global variables.

Many people have told me global variables are bad, but why is that ???

[AndrewHunter]

Read: Why Global Variables are Bad. Especially in your case since based on some of the items I posted in my original post.

EDIT: Also your form() function should have a return type of Node*

EDIT EDIT: Here this is what your form function should look like and how you should handle your list:

#include <stdio.h>
#include <stdlib.h>

struct Node{
	int data;
	struct Node *next;
};

void form(struct Node**);

int main(void){

	struct Node *List = NULL;

	form(&List);
	while(List){
		printf("%d ", List->data);
		List = List->next;
	}


	getchar();
	return (0);
}
void form(struct Node **newList){

	struct Node *head=NULL, *add=NULL;
	int num = 0;

	printf("Enter a value(-1 to quit): ");
	scanf("%d", &num);
	
	while(num != -1){
		add = malloc(sizeof(*add));
		if(!add){
			printf("Error with malloc");
			exit(1);
		}
		add->data = num;
		add->next = NULL;
		if(*newList == NULL){
			*newList = add;
			head = *newList;
		}
		else{
			(*newList)->next = add;
			*newList = (*newList)->next;
		}
		printf("Enter a value(-1 to quit): ");
		scanf("%d", &num);
	}
	*newList = head;
}