About the value of each week

I have an rpt in crosstab that view the total sales of each salesman in every week. This is the rpt looks like:

Week 35 Week 36 Week 37 Week 38
Eric 100 200.25 300 500
Vlad 320 400 0 400

My problem is, if a salesman sales in a week is equal to zero, the sales that must be viewed will be the sale last week.

Here's the rpt must look like this..

Week 35 Week 36 Week 37 Week 38
Eric 100 200.25 300 500
Vlad 320 400 340 400use a formula for displaying value

something like

if sales>0 then
sales
else
previous(sales)|||Thanx! i will try