Question

I am trying to solve a problem. This program contains all the edge in a graph. The shortest path from source to destination is to find out. I have function named dotest as below.

 public void dotest()
    {
        List<edge> tlist;
        Int32  x;
        setall();
        Int32 ind;
        foreach (edge e1 in alltest)
        {
            tlist = new List<edge>(alledge);

            ind = 0;
            foreach (edge e2 in tlist)
            {
                if (e2.s == e1.s && e2.d == e1.d)
                {
                    break;
                }
                ind++;


            }
            tlist.RemoveAt(ind);



            x=shortpath(tlist, start, destination);
            if (x != -1)
                Console.WriteLine("{0}", x);
            else
                Console.WriteLine("Infinity");

        }


    }

Describing the above code. The code already contains list of alledge(all the edge or path). I have got series of input that contains list of edge to cut off and I have to find the shortest path of the new updated edge list. I compiled my test case and some of test case worked. But for some test case it have error message as.

Unhandled Exception: System.ArgumentOutOfRangeException: Argument is out of range.
Parameter name: index
at System.Collections.Generic.List1[ch_2_3_27.Solution+edge].RemoveAt (Int32 index) [0x00000] in :0
at ch_2_3_27.Solution.dotest () [0x00000] in :0
at ch_2_3_27.Solution.Main (System.String[] args) [0x00000] in :0
[ERROR] FATAL UNHANDLED EXCEPTION: System.ArgumentOutOfRangeException: Argument is out of range.
Parameter name: index
at System.Collections.Generic.List1[ch_2_3_27.Solution+edge].RemoveAt (Int32 index) [0x00000] in :0
at ch_2_3_27.Solution.dotest () [0x00000] in :0
at ch_2_3_27.Solution.Main (System.String[] args) [0x00000] in :0

I really cannot locate out error and I think all other parts works fine. Anybody can help??

And Edge(edge) above is a struct with members s,d,w(source, destination, weight all Int 32)

Answer 1

The error is pretty clear actually. You are trying to remove an item from tlist at a certain index. However, that index does not have a value.

If I were to guess, I would say that this only happens whenever nothing in your tlist matches if (e2.s == e1.s && e2.d == e1.d), so you end up with a +1 over the actual index of the tlist array.

To elaborate further, let’s assume for simplicity that tlist has 1 item, then the index to use that item will be 0. If your if does not work, then you will set ind++, thus setting ind to 1. When you try to remove from the index at 1, then you get your error because there is only an object in the 0 index, and nothing in the 1 index

I would change the code to something more like this

        ind = -1;
        foreach (edge e2 in tlist)
        {
            ind++;
            if (e2.s == e1.s && e2.d == e1.d)
            {
                break;
            }
        }
        if(ind != -1)
            tlist.RemoveAt(ind);

I would say to just do the RemoveAt inside of the if, however, that will result in a modified collection exception, so I believe this is the best solution.