Pointer Issues ( I think)

[kardon33]

Ok in short I have a linked. And my problem is that after the first node level the next pointer points to itself and loops forever.


But its more complicated, ill try to explain what im doing the best I can. I would post all the code but it would be way to many files.

I have multi dimensional array of objects declared like:

 Target targets[10][100]; 

The Target object has a bunch a data in but most importantly for my problem it has a pointer to another Target object. Which is the next variable in most linked lists. The pointer is declared like:

  Target* lastDetected; 

My code works like this.

Each column in the targets array (theres 10 of them) refers to frame or time t. So after each loop the columns are copied like so: 0->1->2...8->9. Each node or Target object is copied individually. Code that does that is:

    for (int i = 10- 1; i > 0; i--) {
        for (int j = 0; j < 100; j++) {  
          targets[i][j] = targets[i - 1][j];
        }
    }
 

Well else where in the code the next or lastDetected variable in each object in column 0 is set to another Target object in column 1 of the list of all targets.


However when I try to run in the Target object class. It only works correctly for one time then it goes into an infinite loop.

 
    Target *nTarget = this;
 
     while(nTarget->lastDetected != NULL)
 
         printf("->%d",nTarget->id);
         nTarget = nTarget->lastDetected;
 
    }
 

Any Thoughts?

[donbock]

Copying the target column breaks all of the embedded links -- they still point to the object in the old column.

Are you sure it is necessary to copy the columns? You could keep an int variable called thisFrame which holds the column index for the current frame. Each frame you increment thisColumn, being careful to wrap it around back to zero. To access older frames, call a priorFrame function that decrements its argument, being careful to wrap it around back to 9.

[kardon33]

"Copying the target column breaks all of the embedded links -- they still point to the object in the old column. "

Could you please explain why that is so. Is there some rule in C++ for why that cant happen? So I can avoid later mistakes.

So what you are saying is I should have a variable to that would start at zero and every frame it would increment one, but when I reach the end of the array (so 9) I bump it back down to zero and the previous frame would be 9.

Well thats at least what im going to try.


Thanks for your help.

[kardon33]

Just tested the concept, worked perfectly.

Thanks again.

[donbock]

@kardon33
There is no rule in C or C++ that will automatically protect you from having pointers that point someplace other than where you want them to point.

Suppose target[3][50] contains a pointer to target[3][65]. Then you copy the columns. The information that used to be in target[3][50] is now in target[4][50]; and the information that used to be in target[3][65] is now in target[4][65]. However, the pointer that was in target[3][50] (and now is in target[4][50]) still points to target[3][65], which now has completely different contents.

If your pointers never need to link to other columns, then you don't need an actual pointer. Instead, store the row index of the element you want to link to. Those will copy just fine from column to column.

[kardon33]

OK I see thanks, too bad I am always setting my pointers to the other columns creating a relation between each column.

[donbock]

@kardon33
OK. Then your link could be accomplished by two fields: an absolute row number (as described earlier) and a relative column number.

To follow the link:
add the relative column link value to the current column number (being careful to handle wrap-around properly);
and set the row to the absolute row link value.

This sort of linking would work well despite copying columns ... except for links that point to the column that just got shifted out of the array. There may be an easier way, but my first thought is that your column shifting activity has to also include a scan through all the array elements looking for links that point to the soon-to-be-disappeared column and set those links to a code that means no-link.

[weaknessforcats]

while(nTarget->lastDetected != NULL) 
 
         printf("->%d",nTarget->id); 
         nTarget = nTarget->lastDetected; 

In this code the loop ends with the printf. Looks very odd indeed. Maybe some missing braces?

[kardon33]

When I copied the source code the bracket was lost (im not sure how), its in my source code though. That would create a problem if it wasn't :).