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.