Infopath repeating table - get field from current row

[ola]

I have a repeating table in Infopath. On a field change event, I need to execute a calculation to with data in the row in question. How do I go about this in the code-behind (C#) and Xpath?For example, I have a start date and end date. In the code-behind the do a calculation to calculate the number of hours in the span. This works for the first row, but not subsequent rows. I know the way I wrote it so far only works for the first row, but how do I make this work for all rows?