Thread: Help with variable scope in C !!

I wanted to find out the factors of a number. I decided later that i'd store all the factors of the number in a linked list so that i could get more familiar with them instead of using an array.
Here's the code

Code:

#include<stdlib.h>
#include<stdio.h>
int nextPrime(int num)
    {
        if(num==1 || num ==0)
            return 2;
        num++;
        int i;
        int flag;
        for(i=2;i<=num/2;++i)
        {
            if(num%i==0)
            {
                flag=0;
                break;
            }
        }
        if(flag==0)
            return nextPrime(num);
        else
            return num;
    }
    struct Node
    {
        int data;
        struct Node *link;
    };
    //typedef struct Node Node;
void append(struct Node *start,int value)
{
    while(start->link != NULL)
    {
        start=start->link;
    }
    start->link = (struct Node *)malloc(sizeof(struct Node));
    start=start->link;
    start->data=value;
    start->link=NULL;
}
void traverse(struct Node *start)
{
    while(start != NULL)
    {
        printf("%d\n",start->data);
        start=start->link;
    }
}
void calculateFactors(int number)
{
    int Den,reminder;
    Den = nextPrime(0);
    while(number > 1 )
    {
        reminder = number % Den;
        if(reminder != 0)
            Den=nextPrime(Den);
        else
           {
             number=number / Den;
             append(start,Den);   // Error here. Start undeclared first use in function.
           }
    }
}
void main()
{
    int number, i;
    struct Node *head, *start;
    head->link = NULL;
    start=head;
    scanf("%d",&number);
    calculateFactors(number);
    //append(start,10);
    //traverse(head);
}

I was wondering why it shows such an error as I have already initialized the node in the main program. Should it not be available in the functions as well? Can someone explain how to fix this and the scope of variable in this context

> struct Node *head, *start;
> head->link = NULL;
You need to initialise head before doing head->link.

Also main returns an int, not void.

By doing that, you provide head->link with an initial value. What is the value of head?

Originally Posted by megafiddle

At the point the errror occured, "start" has not yet been declared.

Good catch, I missed the comment.

Originally Posted by megafiddle

You could make it a global structure and declare it outside main, and before it is ever referenced.

NO! Do not do that. Rather, remove the declaration of start from the main function since it serves no purpose there: the head variable already does its job. Rather, change the declaration of the calculateFactors function to add another parameter named start, then change how you call that function in the main function.

Personally, I would pick either head or start as the naming convention then stick to it. You can have a local variable named head in the main function and also have a parameter named head in the calculateFactors function. Since the pointer is supposed to point to the head (or start) node either way, keeping the name consistent would be a good idea.

Originally Posted by megafiddle

Yes, but that structure is needed elsewhere in other functions.

That is why a pointer to the head is passed as an argument. This was done for the other functions that needed that pointer.

Originally Posted by megafiddle

The function "traverse()" is called from main (currently commented out). If it were actually called, how would main have access to the struct "start"? (or "head" if that replaced the "start")

Looking at the implementation of traverse and the clue given by its name, main already has access to the struct Node object, i.e., via the head pointer. It is traverse that needs access to that object by say, passing head as an argument when calling it from main (which is what was commented out).

Originally Posted by megafiddle

Ok, so the call within main(), to calculateFactors(), passes the address of "head" and then within calculateFactors(), "head" is passed on in the call to append()?

Yes. There's a problem in that when the linked list is empty, then the very first append should change the head node unless it is a dummy, but that's related to the problem that Salem noted in post #2.

Since you are appending to the Set, I suggest that you keep track of the tail of the linked list as it will be far more efficient that way.

Instead of set_create, I suggest a set_init function. The difference is that your set_create function calls malloc to create the set, whereas set_init will assume that the memory for the set has already been allocated, so it just initialises the set by setting the head (and tail) to a null pointer. This way, you can create a set local to the main function without using malloc, or use malloc in the main function if you wish. At the moment, your set_destroy function fails to call free for the set, but if you have set_init instead then it would be correct.

Originally Posted by thebenman

Turns out all the values are calculated fine but fails in the append statement in the calculateFactors function.

Where exactly does it fail in append, and what do you mean by fail?