Question

I’ve frequently used a linear linked list construct in C

typedef struct _node {
    ...node guts...
    struct _node *next
} node;

And the enumeration idiom

for (node *each = headNode; each != NULL; each = each->next)

I’m in a situation right now, where a circular list is attractive to me (e.g. the next of the last node is set to the headNode). Naively, I thought I’d using something similar to the for expression there, and the more I’ve stared at it, I think I’ve convinced myself you can’t do something like that with a circular linked list.

It seems that no matter what kind of expression I come up with for the end conditional, I’ll have the basic problem that I want said conditional to evaluate true the first time and false the second time the same node is encountered. I could do something with a loop side effect:

for (BOOL traversed = FALSE, node *each = headNode;
    traversed && each != headNode;
    traversed = TRUE, each = each->next)

But that definitely loses the elegance/simplicity of the null terminated list approach. Is there some trick of logic that’s eluding me this late in the day?

Obviously I can use a while() construct, and maybe that’s the only way to do it.

Answer 1

Assuming the following:

• An empty list means NULL for the starting point.
• The last node will be the one who’s next pointer references your starting point, including a single-node list who’s next pointer self-references.
• NO next pointers are NULL.

Then the following will do what you want using a tertiary expression as the incremental step.

// node* start comes from "somewhere'
for (node *p=start; p; p = (p->next==start ? NULL : p->next))
{
    // do something with p
}

Note: p will be NULL when this exits, one way or another; start will remain unchanged, and a NULL start is acceptable, as is a single-self-referencing node.

That said, I’d do this with a while-loop, but since you specifically asked for a for-loop you gets that =P.