static pointer in a recursive function?

This is a discussion on static pointer in a recursive function? within the C Programming forums, part of the General Programming Boards category; I am new to C and I'm trying to learn more about pointers. I have a working function that reverses ...

  1. #1
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    static pointer in a recursive function?

    I am new to C and I'm trying to learn more about pointers.

    I have a working function that reverses a linked list recursively.

    My working function looks like this:
    Code:
    /******************************************
     * Parameters: 
     * 	- Reference pointer to head of list
     * 	- Pointer to first node in list
     ******************************************/
    void recursive_reverse(struct node **head_ptr, struct node *first)
    {
    	struct node *dummy;
    	
    	if(first->next == NULL)
    		return;
    
    	dummy = first->next;
    	first->next = dummy->next;
    	dummy->next = *head_ptr;
    	*head_ptr = dummy;
    	recursive_reverse(head_ptr, first);
    }
    node is
    Code:
    struct node
    {
    	int data;
    	struct node *next;
    };
    My question is whether I can have a static pointer "first" (instead of the parameter "first") that gets initialised only once on the first itteration of the function and keeps track of where my original first node is?

    When I try:
    Code:
    void recursive_reverse2(struct node **head_ptr)
    {
    	static struct node *first = *head_ptr;
    	struct node *dummy;
    	
    	if(first->next == NULL)
    		return;
    
    	dummy = first->next;
    	first->next = dummy->next;
    	dummy->next = *head_ptr;
    	*head_ptr = dummy;
    	recursive_reverse2(head_ptr);
    }
    I get the compiler error:
    error: initializer element is not constant
    I don't know enough about the language to understand why I can't do this and would really appreciate any guidance.

    I'm sure there are other better ways to solve this specific problem but I would like to know if it's possible to adapt my solution with some kind of static pointer or if I'm wasting my time trying this route.

    many thanks

  2. #2
    cwr
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    You can't initialise a static declaration with a variable, the idea of static is that its initial value is known at compile time. A workaround to this would be to assign it to NULL, then check if it's NULL and assign it to *head_ptr:
    Code:
    static struct node *first = 0;
    if (!first)
        first = *head_ptr;
    Last edited by cwr; 09-19-2005 at 03:26 AM.

  3. #3
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    Thank you very much for your explanation and workaround.

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