Talk:Convex hull

I found some errors in the C code when I compare with c++ code : code C++

   // lower hull
   for (const auto& pt : p) {
       while (h.size() >= 2 && !ccw(h.at(h.size() - 2), h.at(h.size() - 1), pt)) {
           h.pop_back();
       }
       h.push_back(pt);
   }

code C :

  /* lower hull */
   for (i = 0; i < len; ++i) {
       while (hLen >= 2) {
           hptr = h;
           while (hptr->next->next != NULL) {
               hptr = hptr->next;
           }
           if (ccw(&hptr->data, &hptr->next->data, &p[i])) { <== mistake
               break;
           }

It should be

  /* lower hull */
   for (i = 0; i < len; ++i) {
       while (hLen >= 2) {
           hptr = h;
           while (hptr->next->next != NULL) {
               hptr = hptr->next;
           }
           if (ccw(&hptr->next->data, &hptr->data, &p[i])) {   <==== good code
               break;
           }

The same error is done in upper hull

Another error

C++ code

   // upper hull
   auto t = h.size() + 1;
   for (auto it = p.crbegin(); it != p.crend(); it = std::next(it)) {
       auto pt = *it;
       while (h.size() >= t && !ccw(h.at(h.size() - 2), h.at(h.size() - 1), pt)) {
           h.pop_back();
       }
       h.push_back(pt);
   }

C code

   /* upper hull */
                            <== t is fogotten
   for (i = len - 1; i >= 0; i--) {
       while (hLen >= 2) {
           hptr = h;
           while (hptr->next->next != NULL) {
               hptr = hptr->next;
           }
           if (ccw(&hptr->data, &hptr->next->data, &p[i])) {
               break;
           }

It should be

   int t = hLen + 1;
   for (i = len - 1; i >= 0; i--)	{
   while (hLen >= t) 
   	hptr = h;
       while (hptr->next->next != NULL){
           hptr = hptr->next;
       }
       if (ccw(&hptr->next->data, &hptr->data, &p[i]))	{
           break;
       }

Last error

   int comp(const void *lhs, const void *rhs) {
       Point lp = *((Point *)lhs);
       Point rp = *((Point *)rhs);
       if (lp.x < rp.x) return -1;
       if (rp.x < lp.x) return 1;
       return 0;
   }

It should be

   int comp(const void* lhs, const void* rhs) {
       Point lp = *((Point*)lhs);
       Point rp = *((Point*)rhs);
       if (lp.x < rp.x) return -1;
       if (rp.x < lp.x) return 1;
       return rp.y - lp.y ;
   }

You should test the code, and think about the results here. For example, "ccw" is a routine which determines whether the winding is clockwise or counterclockwise. But from which side? Anyways... since that's not a documented issue... ccw should work regardless of the winding direction -- as long as the winding direction is consistent. So... you have encountered a real issue here. But it's probably not a code correctness issue -- it's probably a lack of adequate documentation issue (which is a frequent problem for coders).

Similarly, on the second issue you brought up, just throwing code out -- without any documentation and without any test results -- does not adequately illustrate the issue.

That said, taking a close look at the implementation, like you have done here, is great. We all-too-often have had errors in code here on this site -- often for very understandable reasons. So double checking results is frequently a good thing. Thanks!

P.S. please sign your comments on the talk pages, using --~~~~. This helps readers comprehend talk pages. --Rdm (talk) 21:07, 22 December 2020 (UTC)

I'm sure of these errors because I tried this code for an exercice on Codingame (Encounter Surface) and I didn't get the good results.

I compare with C++ and Java codes, found the mistakes, correct then as I explain and now it works !

May be the code is correct for the data given but it's wrong !

Trap D (talk)

Hmm... color me surprised. Or maybe there was a mistake in how you applied the code (since you didn't manage to get your signature right, here, that sort of emphasizes that possibility). I might try it out myself one of these days... --Rdm (talk) 01:38, 23 December 2020 (UTC)

Ok just try with these points

   Point points[] = {
       {0,0}, {5,5}, {5,0}, {0,5}
   };

Trap D (talk)

That gives me the result

Convex Hull: [(0, 0), (5, 0), (5, 5), (0, 5)]

which is correct -- a square is its own convex hull. So I guess I do not understand what issue you are concerned about.

(Why do you not use --~~~~ to sign your comments? Having the date in your signature helps guide the eye and will still be useful, years later.) --Rdm (talk) 20:41, 23 December 2020 (UTC)