Newb question: Calculating values across a row in a NSTableView

I have an NSTableView with four columns in a document-based Cocoa app. This tableview uses an NSMutableArray to hold its values. The four columns are:

1: User's name
2: Starting Balance
3: Deductions
4: Current Balance

What I want to happen is every time columns 2 or 3 are changed, I want the program to recalculate the value displayed in column 4.

I'm not really sure how this is done since I'm still a big newb with Objective C. Could someone provide some information or some sample code? Thanks!


* Update: Okay, I figured out how to make that last column editable so I removed that section of the question. I didn't realize it was as easy as selecting the column and unchecking the "Editable" box.

 

Sorry it's been a while. ok so I'm making some progress here but I hit an issue I'm not quite sure I understand.

Code:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
	if ([keyPath isEqual:@"finalBalance"])
	{
		NSLog(@"No need to update for finalBalance.");
		return;
	}
	
	NSUndoManager *undo = [self undoManager];
	id oldValue = [change objectForKey:NSKeyValueChangeOldKey];
	
	if (oldValue == [NSNull null])
	{
		oldValue = nil;
	}
	
	if ([keyPath isEqual:@"usersName"])
	{
		NSLog(@"Don't update for usersName change");
	}
	else
	{
		// Update finalBalance
		int iStartValue = [object startingBalance];
		NSLog(@"The value of iStartValue is %d", iStartValue);
		
		int iAdjustment = [object deductionsFromBal];
		NSLog(@"The value of iAdjustment is %d", iAdjustment);
		
		int iNewValue = iStartValue - iAdjustment;
		NSLog(@"The value of iNewValue is %d", iNewValue);
		
		[object setValue:iNewValue forKeyPath:@"finalBalance"];
		NSLog(@"The finalBalance for %@ has been adjusted to %d", object, iNewValue);
	}
	
	[[undo prepareWithInvocationTarget:self] changeKeyPath:keyPath 
												  ofObject:object 
												   toValue:oldValue];
	
	[undo setActionName:@"Edit"];
}

At the line where I have [object setValue:iNewValue forKeyPath"finalBalance"];, I get this in the code:

warning: passing argument 1 of 'setValue:forKeyPath' makes pointer from integer without a cast

When I edit one of these columns in the program, it crashes to gdb. What am I doing wrong here?

 

Ugh, I'm an idiot. lol. I forgot all about that, eddietr. That works exactly as I intended it to. Thank you!