How to know which rows a currently displayed

My data is stored in a plain array. Is there a way to know the indices of the rows that are currently displayed. Also can I control the number of rows in the scrolling area. For example:
I have an array of 100 rows. I want to the grid to display 20 rows. I know this may be done by calculating the row height and grid height. I want to know if there is a better way.