Assigning a value to a DataTable column

I want to sort a datatable by a combination of two columns, batch and seq. My strategy is
1. create a new column, track
2. concatenate each row's batch and seq value and assign to the new "track" column
3. sort by track
The problem is syntax error:
Cannot apply indexing with [] to an expression of type 'System.Data.DataTable'
I've tried a number of variations on that line.  Any ideas?  Thanks!

            private void InsertPMPT(DataTable dt)
            {
                    dt.Columns.Add( "track", typeof(string) );
                  //assigning batch+seq to all the rows' "track" field.
                  int icount = 0;
                  foreach(DataRow R in dt.Rows)
                  {
                                //PROBLEM HEREO!
                        dt[0].Rows[icount]["track"]=(String)R["batch"]+(String)R["seq"];
                        icount++;      
                  }
                  // Set PrimaryKey
                  //dt.Columns[ "track" ].Unique = true;
                  dt.PrimaryKey = new DataColumn[] { dt.Columns["track"] };
                  //now sort the dataset ascending by track column
                  string strExpr = "";
                  string strSort = "track";
                  DataRow[] foundRows;
                  // Use the Select method to find all rows matching the filter.
                  foundRows = dt.Select(strExpr,strSort);