Prevent infinite loop between objects referencing each other?

Question

Okay, so I wasn't completely sure what headline would fit my problem, but here goes the description:

I have objects than can reference other objects, to create dropdown lists where the content/values is dependant on what values is chosen in "parent" dropdowns.

My dropdown objects contain an id, and a parentId (and other stuff, not relevant here).

I want to prevent the users from making infinite loops, like this:

List 1 (Dependant on list 3)
List 2 (Dependant on list 1)
List 3 (Dependant on list 2)

I've tried writing a recursive method to prevent it, but I cannot figure out the logic.

Could anyone tell me how you would ensure that an object isn't referencing it self "down the line" ? Or provide an example perhaps.

Any help is much appreciated.

as per the conditions you mention wondering which would be the so called parent — V4Vendetta, Jul 15 '13 at 09:24

score 3 · Accepted Answer · answered Jul 15 '13 at 09:26

The simplest way I can think of is to create a flattened list. Recursively iterate the objects and store each reference in a list. As you find new objects check each one in the list.

You'll either encounter an object referencing itself or run out of objects to search.

This method being suitable will depend on your requirements, speed / memory/ number of items in the list.

Since all object contain an id the list could store/check that instead if you need to check value equality instead of reference equality

score 0 · Answer 2 · answered Jul 15 '13 at 09:28

If you have written a recursive function to manage those lists, one solution could be to create a list of elements and pass it as parameter into the recursive function and en each iteration add the current item to the list. To stop the recursive function, only check if the current item has been added previously to the list.

varocarbas · Answer 3 · 2013-07-15T10:03:50.737

If you iterate through the actual elements of each list by relying on specific counters for each list you shouldn't find any problem. The most likely way to provoke an infinite loop is changing the value of a counter from an external source. Example:

    for(int i = 0; i < max_i; i++)
    {
       if(val1[i] != null)
       {
         for(int j = 0; j < max_j; j++)
         {
            if(val2[j] != null)
            {
               //Delete or anything

               //YOU CANNOT AFFECT NEITHER i NOR j DIRECTLY.
            }
         }
      }

If you want to account for varying values of j in the internal part, you should rely on a different variable. Example:

if(val2[j] != null)
{             
    int j2 = j;

    //Do whatever with j2, never with j
}

By doing this (associating different counters to different loops), no endless loop will occur. An endless loop occurs when: i = 1, 2, 3, 4 and suddenly i is changed to 2 by an "external source"; thus solution: NEVER change i other than through the for loop.

score 0 · Answer 4 · answered Jul 15 '13 at 11:45

Thanks everyone for your input on this. I went with James suggestion using a list and ended up with the following code (which may or may not make sense for anyone else but me)

public static bool BadParent(int fieldId, int childId, List<int> list)
    {
        if (list == null)
            list = new List<int>();

        bool returnValue = true;

        var field = EkstraFelterBLL.getEkstraFeltUdfraEkstraFeltId(fieldId);

        if (field != null)
        {
            if (field.ParentEkstraFeltId == childId)
                returnValue = false; //loop reference, fail
            else if (list.Contains(field.EkstraFeltId))
                returnValue = false; //already been in the cycle, fail
            else
            {
                list.Add(field.EkstraFeltId);
                returnValue = BadParent(field.ParentEkstraFeltId, childId, list);
            }
        }

        return returnValue;
    }

Prevent infinite loop between objects referencing each other?

4 Answers4