In a matrix, how I can replace values in empty cells with values of previous cells ?

Hi everyone!! I need help about this...

Measure of sales per year and in case there is no information for that year, show the sales of the previous year.

That is, given a series of sales per year for a series of customers, it would be necessary if, for a given year, that customer does not have a value, to fill the cell with the value of the last year that had value.

Example

Customer A (SALES 2017 = 1000, SALES 2016 = 1200, SALES 2015 = 900)

Client B (SALES 2016 = 500, SALES 2015 = 700)

In a matrix of sales of customer x year the crossing of Client B and 2017 would be empty and what I want is for it to take the value of the previous year that contains data, specifically 2016), that is to say that the table will remain as follows:

                 2015 2016 2017

Customer A 900 1200 1000

Client B 700 500 500

That value of 500 in 2017 I would have to put my measurement.

Thank you very much!!!!