Difference between two columns and blank data type question

[afleiderman1]

I got a quick question I got the following data

My rows are customer name

my column are weeks

my measure is sum of sales

and another measure from previous week sales 

                            Week 1                                                    Week 2                                                                          Week 3       

                       Previous    Week  diff           Previous        week                      diff                   PreviousWeek      Week          diff

James              blank           100          100                100            50                -50                       50                    60                  10

Fernando

Dresta

diff=week-previous

Since you see on the first week I have blank for the previous because I do not know what that value is because I have not brough it into the data set. Is there anyway to make my diff columns also blank but only for the first week.

[lbendlin · Accepted solution]

diff=if(isblank(previous),blank(),week-previous)