How to check the missing element in the sequence

I am doing a project related to payroll where i will have some payperiodnumbers for each and every payroll that has been runned. I will show all the payrolls in a grid view with the corresponding pay period numbers.

Assume i get the following results when i binded to grid

Now from the grid if i select 1 and click on delete i would like to show an error message stating you have to delete max pay period first.

Like that if i had my max pay period number as 7 and if user selects 1,2,3,4,5,6 and try to delete i would like to display the same error. I am saving the selected ID's in a a开发者_C百科rraylist so can any one help me how can i check for my condition as specified. I can get the maximum payperiodid using the query but the remaining code i would like to do.

I am using 2.0 so no point of using LINQ here. Can any one help me

As Azodious pointed i am showing some condition that should work and some not

If max number is 7 and if i select 1,5,7 i would like to display an error message.

If i select 5,6,7 then it should delete that.

Something like this:

selectedNumbers.Sort();
selectedNumbers.Reverse();

int maxPeriodNumber = 5; // This you know
int lastValue = (int)selectedNumbers[0];

if (lastValue < maxPeriodNumber)
{
    // Highest selected number is smaller than required, warn user or throw exception
    return;
}

foreach (int val in selectedNumbers)
{
    if (val < (lastValue - 1))
    {
        // There is a gap in the numbering, warn user or throw exception
        return;
    }

    lastValue = val;
}

// When you end up here, everything is ok and you can delete the items whose numbers are in the list

A simple logic to display missing numbers

ArrayList a = new ArrayList();
List<int> lst = new List<int>();
lst.Add(1);
lst.Add(3);
lst.Add(5);
int fst = (int)lst[0];
int last = 0;
for (int i = 0; i < lst.Count; i++)
{
   last = (int)lst[i];
 }
for (int k = fst; k <= last; k++)
{
 if (k == fst | k == last)
   {
   }
 else
 {
    a.Add(k);
    a.Add(" ");
 }
}
   Label1.Text = "Missing Numbers are" + " " + System.String.Concat(a.ToArray());