table cell drawing on same position - iphone

Question

I have to populate numbers of row. when i uses
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
this line, after scrolling to some rows it redrawing rows on same position.see below screen
if i make cell nil i.e.
UITableViewCell *cell = nil;
then works fine,but it takes unexpecedly more memory. now my question is why its happening.
what is the relation of making cell nil to redrawing on same position?
This is my code:
- (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];
}
UIButton *questionButton=[UIButton buttonWithType:UIButtonTypeCustom];
questionButton.backgroundColor=[UIColor clearColor];
questionButton.tag=indexPath.row+1;
[questionButton setContentHorizontalAlignment:UIControlContentHorizontalAlignmentLeft];
UIImageView *statusImageView=[[UIImageView alloc]initWithFrame:CGRectMake(190,25,23,23)];
NSString *statusImageName=[[NSString alloc]init];
statusImageName=#"Right";
[statusImageView setImage:[UIImage imageNamed:statusImageName]];
[questionButton addSubview:statusImageView];
if(indexPath.row%2==0)
{
[questionButton setBackgroundImage:[UIImage imageNamed:#"TopicsbarLeft"] forState:UIControlStateNormal];
[questionButton setBackgroundImage:[UIImage imageNamed:#"TopicsbarLetSelected"] forState:UIControlStateHighlighted];
CGRect topicButtonFrame=CGRectMake(0, 10, 250, 70);
questionButton.frame=topicButtonFrame;
NSLog(#"Even Rows =%d",indexPath.row);
NSString *questionLabel=[NSString stringWithFormat:#" Question %d :",indexPath.row+1];
[questionButton setTitle:questionLabel forState:UIControlStateNormal];
//questionButton.titleLabel.textAlignment=NSTextAlignmentLeft;
[questionButton.titleLabel setTextAlignment:UITextAlignmentLeft];
}
else
{
[questionButton setBackgroundImage:[UIImage imageNamed:#"TopicsbarRight"] forState:UIControlStateNormal];
[questionButton setBackgroundImage:[UIImage imageNamed:#"TopicsbarRightSelected"] forState:UIControlStateHighlighted];
CGRect topicButtonFrame=CGRectMake(100, 10, 250, 70);
questionButton.frame=topicButtonFrame;
NSString *questionLabel=[NSString stringWithFormat:#" Question %d :",indexPath.row+1];
[questionButton setTitle:questionLabel forState:UIControlStateNormal];
questionButton.titleLabel.textAlignment=UITextAlignmentRight;
[cell addSubview:questionButton];
}
[cell addSubview:questionButton];
cell.selectionStyle=UITableViewCellSelectionStyleNone;
return cell;
}

Answer 1

The purpose of dequeueReusableCellWithIdentifier is to use less memory. If the screen can fit 4 or 5 table cells, then with reuse you only need to have 4 or 5 table cells allocated in memory even if the table has 1000 entries.
So when you do this UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier]; and it returns an old cell to reuse. It is not a new cell instance. It contains UI elements of the cell initialised to the old data. To display it again you need to reset all the UI elements and put in your new values.
The second way when you put UITableViewCell *cell = nil; there is no reuse. There is no advantage in the second way over just using an array of table cells. If your table has 1000 entries then you will have 1000 cells allocated in memory. If you are going to do that you would put them in an array and just index the array with the row number and return the cell. This should explain your spike in memory usage as it is not reusing any cell instance.
For small tables with fixed cells second solution may be an reasonable solution, for dynamic or large tables reusing is the best way to go...
UPDATE: since you have asked for some code. Also note that you are doing 2 types of rending depending on odd or even. So the cell that comes out of reuse might be odd when you want to show even numbered question. In that case you would need to reset the cell just after the cell is got (either from reuse or created new) and let your normal logic flow through...
[questionButton setBackgroundImage:[UIImage imageNamed:#"default.png"]];
[questionButton setTitle:#"" forState:UIControlStateNormal];
CGRect topicButtonFrame=CGRectMake(0, 0, 250, 70);
questionButton.frame=topicButtonFrame;

Answer 2

The problem is here :
[cell addSubview:questionButton];
}
[cell addSubview:questionButton];
You are adding the questionButton twice. Please remove one in else section and try.
Update :
Can you try this just for the confirmation what is the problem:
if(indexPath.row%2==0)
{
UIButton *questionButton=[UIButton buttonWithType:UIButtonTypeCustom];
questionButton.backgroundColor=[UIColor clearColor];
questionButton.tag=indexPath.row+1;
[questionButton setContentHorizontalAlignment:UIControlContentHorizontalAlignmentLeft];
UIImageView *statusImageView=[[UIImageView alloc]initWithFrame:CGRectMake(190,25,23,23)];
NSString *statusImageName=[[NSString alloc]init];
statusImageName=#"Right";
[statusImageView setImage:[UIImage imageNamed:statusImageName]];
[questionButton addSubview:statusImageView];
[questionButton setBackgroundImage:[UIImage imageNamed:#"TopicsbarLeft"] forState:UIControlStateNormal];
[questionButton setBackgroundImage:[UIImage imageNamed:#"TopicsbarLetSelected"] forState:UIControlStateHighlighted];
CGRect topicButtonFrame=CGRectMake(0, 10, 250, 70);
questionButton.frame=topicButtonFrame;
NSString *questionLabel=[NSString stringWithFormat:#" Question %d :",indexPath.row+1];
[questionButton setTitle:questionLabel forState:UIControlStateNormal];
//questionButton.titleLabel.textAlignment=NSTextAlignmentLeft;
[questionButton.titleLabel setTextAlignment:UITextAlignmentLeft];
[cell addSubview:questionButton];
}
else
{
UIButton *questionButton=[UIButton buttonWithType:UIButtonTypeCustom];
questionButton.backgroundColor=[UIColor clearColor];
questionButton.tag=indexPath.row+1;
[questionButton setContentHorizontalAlignment:UIControlContentHorizontalAlignmentLeft];
UIImageView *statusImageView=[[UIImageView alloc]initWithFrame:CGRectMake(190,25,23,23)];
NSString *statusImageName=[[NSString alloc]init];
statusImageName=#"Right";
[statusImageView setImage:[UIImage imageNamed:statusImageName]];
[questionButton addSubview:statusImageView];
[questionButton setBackgroundImage:[UIImage imageNamed:#"TopicsbarRight"] forState:UIControlStateNormal];
[questionButton setBackgroundImage:[UIImage imageNamed:#"TopicsbarRightSelected"] forState:UIControlStateHighlighted];
CGRect topicButtonFrame=CGRectMake(100, 10, 250, 70);
questionButton.frame=topicButtonFrame;
NSString *questionLabel=[NSString stringWithFormat:#" Question %d :",indexPath.row+1];
[questionButton setTitle:questionLabel forState:UIControlStateNormal];
questionButton.titleLabel.textAlignment=UITextAlignmentRight;
[cell addSubview:questionButton];
}