objective c - function to check if there is any empty cell in a multicolumn nstableview not working -

hi have written function check if there empty cell in multicolumn nstableview below.however function not working , checks columns in first row only.. suggestions.and right way check

  -(bool)istablerowscontainingemptycells     {            for(int row=0;row< [[self.myarray arrangedobjects] count];row++)         {         nstablecolumn *column1 = [self.formfieldvalues                                   tablecolumnwithidentifier:@"a"];         nstablecolumn *column2 = [self.formfieldvalues                                   tablecolumnwithidentifier:@"b"];         nscell *cell1 = [column1 datacellforrow:row];         nscell *cell2 = [column2 datacellforrow:row];             nslog(@"cell1 %@ cell2 %@",cell1,cell2);         if([[cell1 stringvalue] isequaltostring:@""]||[cell1 stringvalue]==nil||[[cell2 stringvalue] isequaltostring:@""]||[cell2 stringvalue]==nil)         {             return yes;         }         }         return no;     } 

try

nscell *cll=[tableview preparedcellatcolumn:0 row:0];             nsstring *str=[cll stringvalue]; nsinteger len=[str length]; if (len ==0) {     nslog(@"no data") }