Create a running sum in a UITableView

Question

I have a UITableView populated (previously saved via a button in navbar), with transactions. Each row has five UILabel: date, description, person, value (deposits and withdraws) balance. The table is sort by date. How can I obtain the daily balance? The balance is the sum (+ and -) from the first input up to the current row.

I have the following code (JournalDetail.m), but I only get the same amount as value, instead of a cumulative amount (balance)

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
static NSString *CellIdentifier = @"Cell";
GxPolizasL *cell = (GxPolizasL *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];

if (cell == nil) {
[[NSBundle mainBundle] loadNibNamed:@"GxPolizasL" owner:self options:nil];
cell = gxPolizasL;
self.gxPolizasL = nil;
GMDiario *gmDiarioy = (GMDiario *)[self.fetchedResultsController objectAtIndexPath:indexPath];
//Date value
NSDateFormatter* df = [[NSDateFormatter alloc] init];
[df setDateFormat:@"dd/MM/yy"];
cell.fechao.text = [df stringFromDate:gmDiarioy.pzFecha];
[df release];
//Description value
cell.referencia.text=gmDiarioy.pzAlias;
//Person value
cell.item.text = gmDiarioy.persona.prAlias;
//Transaction value
NSNumberFormatter* vf = [[NSNumberFormatter alloc] init];    
[vf setNumberStyle:NSNumberFormatterCurrencyStyle];
cell.valuem.text= [vf stringFromNumber: gmDiarioy.pzMont01];
[vf release];
//This is where the balance value is intended to be created
NSDecimalNumber *saldoAc = [NSDecimalNumber decimalNumberWithString:@"0.0"];
NSDecimalNumber *objectExpenseNumber = [gmDiarioy valueForKeyPath:@"pzMont01"];
saldoAc = [saldoAc decimalNumberByAdding:objectExpenseNumber];
NSLog(@"saldoAc: %@",saldoAc);
NSNumberFormatter* af = [[NSNumberFormatter alloc] init];
[af setNumberStyle:NSNumberFormatterCurrencyStyle];
cell.valuea.text= [af stringFromNumber: saldoAc];
[af release];
}
return cell;}

Could you help me to figure out which is the correct way to implement balance value? Thanks in advance. Your help is greatly appreciated.

This is the code for self.fetchedResultsController

- (NSFetchedResultsController *)fetchedResultsController
{
if (__fetchedResultsController != nil) {
return __fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[[NSFetchRequest alloc] init] autorelease];

//Diario is an entity that keeps all transactions
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Diario" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
[fetchRequest setFetchBatchSize:20];    

//This is to select only the journals for the current account (value passed at didSelectRowAtIndexPath from the previous UItableView
NSString *filtro=gmCuenta.cnAlias;  
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"cuenta.cnAlias like %@", filtro];
NSLog(@"NSPredicate1 %@", gmDiario.cuenta);
[fetchRequest setPredicate:predicate];
NSSortDescriptor *sortDescriptor = [[[NSSortDescriptor alloc] initWithKey:@"pzFecha" ascending:YES] autorelease];
NSArray *sortDescriptors = [NSArray arrayWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
NSFetchedResultsController *aFetchedResultsController = [[[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:nil] autorelease];
aFetchedResultsController.delegate = self;
self.fetchedResultsController = aFetchedResultsController;
NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error]) {
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();}
return __fetchedResultsController;
}

Answer 1

Basically what you want to do is this: Each time cellForRowAtIndexPath: gets called you want to loop through your data source array and sum all the values starting at the beginning of the array and ending with the array item equal to the row # (i.e. indexPath.row) of the cell in question. That will give you a running total (balance).

NSDecimalNumber *saldoAc = [NSDecimalNumber decimalNumberWithString:@"0.0"];
for (int i=0; i <= indexPath.row; i++) {
    NSIndexPath *indexPath = [NSIndexPath indexPathWithIndex:i];
    GMDiario *tempObj = (GMDiario *)[self.fetchedResultsController objectAtIndexPath:indexPath];
    NSDecimalNumber *objectExpenseNumber = [tempObj valueForKeyPath:@"pzMont01"];
    saldoAc = [saldoAc decimalNumberByAdding:objectExpenseNumber];
}
NSNumberFormatter* af = [[NSNumberFormatter alloc] init];
[af setNumberStyle:NSNumberFormatterCurrencyStyle];
cell.valuea.text= [af stringFromNumber: saldoAc];
[af release];

One other comment - you should be setting the values of your cell's subviews after you exit the if (cell == nil) block. Otherwise you will run into trouble when your tableView starts scrolling.