Checkmarks in a UITableView

Question

I create a tableview with 20 cells.
And when I select first row, it will show checkmark on it.

But when I scroll tableview, then it not only one checkmark on tableview cell.
It also show on another cell.

What's the problem with this?

self.dataAry = [NSArray arrayWithObjects:@"0",@"1",@"2",@"3",@"4",@"5",@"6",@"7",@"8",@"9",@"10",@"11",@"12",@"13",@"14",@"15",@"16",@"17",@"18",@"19",nil];
marks = [NSMutableArray new];
for (int i = 0 ; i < [self.dataAry count]; i++) {
    [marks addObject:@"NO"];
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {

static NSString *CellIdentifier = @"Cell";

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
}
cell.textLabel.text = [dataAry objectAtIndex:indexPath.row];
if ([[marks objectAtIndex:indexPath.row] isEqualToString:@"YES"]) {
    [cell setAccessoryType:UITableViewCellAccessoryCheckmark];
}else {
    [cell setAccessoryType:UITableViewCellAccessoryNone];
}
// Configure the cell.

return cell;}
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath{

UITableViewCell *thisCell = [tableView cellForRowAtIndexPath:indexPath];


if ([[tableView cellForRowAtIndexPath:indexPath] accessoryType] == UITableViewCellAccessoryCheckmark){
        [[tableView cellForRowAtIndexPath:indexPath] setAccessoryType:UITableViewCellAccessoryNone];
        [selectArray removeObject:[self.dataAry objectAtIndex:indexPath.row]];
        [marks replaceObjectAtIndex:indexPath.row withObject:@"NO"];
    }
    else {
        [[tableView cellForRowAtIndexPath:indexPath] setAccessoryType:UITableViewCellAccessoryCheckmark];
        [selectArray addObject:[self.dataAry objectAtIndex:indexPath.row]];
        [marks replaceObjectAtIndex:indexPath.row withObject:@"YES"];
    }
}

Answer 1

I think it is due to reusing the checkmarked cell. To correct that, write this:

*if ([[marks objectAtIndex:indexPath.row] isEqualToString:@"YES"]) {
[cell setAccessoryType:UITableViewCellAccessoryCheckmark];
}else {
[cell setAccessoryType:UITableViewCellAccessoryNone];*

Out of cell reusing space. That is After: ... } //Configure the cell.

just before

*return cell;*