Must be non-negative and less than the size of the collection

Whats wrong with this code why this exception occur . 

System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index at System.ThrowHelper.ThrowArgumentOutOfRangeException() at System.Collections.Generic.List`1.get_Item(Int32 index) at CourseAcademics_ProgressReports.EvalResult(List`1[] a, List`1[] totalmarks, List`1 names, Int32 subjectcount, DataTable dt) in c:\Users\Administrator\Documents\Visual Studio 2010\Projects\RootsERPWeb\ERPWeb\CourseAcademics\ProgressReports.aspx.cs:line 315

public void EvalResult(List<double>[] a, List<double>[] totalmarks, List<string> names, int subjectcount, DataTable dt)
    {

        dt1 = new DataTable();
        try
        {
            for (int i = 0; i < a[0].Count; i++)
            {
                string[] xxx = new string[subjectcount + 4];
                xxx[0] = names[i];
                for (int j = 1; j <= subjectcount; j++)
                {
          double perc = Math.Round((a[j - 1][i] / totalmarks[j - 1][i]) * 100, 2);
                    xxx[j] = a[j - 1][i] + "/" + totalmarks[j - 1][i];
                }
                double obtainedmarkstotal = 0;
                double totalmark = 0;
                for (int k = 0; k < subjectcount; k++)
                {

                    obtainedmarkstotal += a[k][i];
                    totalmark += totalmarks[k][i];

                }
         double perctotal = Math.Round(obtainedmarkstotal / totalmark * 100, 2);
                xxx[subjectcount + 1] = obtainedmarkstotal + "/" + totalmark;
                xxx[subjectcount + 2] = perctotal.ToString() + "%";
                xxx[subjectcount + 3] = getgrade(perctotal);
                dt.Rows.Add(xxx);
            }
      
            gvforawards.Visible = true;
            Label1.Text = "";
            dt1 =(DataTable) dt;
            Session["result"]=dt1;
           gvforawards.DataSource = dt1;
           gvforawards.DataBind();
           int s = 0;
           int columns_count = gvforawards.Columns.Count;
           foreach (GridViewRow gvr in gvforawards.Rows)
           {
((HyperLink)gvr.Cells[0].Controls[0]).NavigateUrl="~/MarkSheet.aspx?;                             
               s++;
           }

        }
        catch(Exception ex)
        {
           
            gvforawards.Visible = false;
            Label1.ForeColor = Color.Red;
            Label1.Visible = true;
            Label1.Text = ex.ToString();
        }
    }