Hi,  Is it possible to make a 'backward loop' and subtracting a value, ex. [i-4] in arrays and do loops?   In my data I have observations for a lot of different IDs and for each person I have a lot of different weeks and a lot of different events(also in weeks. If the person is at the event the dummy takes the value 1 and 0 for the rest of the event variables). All events occur in a given week in one year but I have observations for the years before and after the event for each person.   I want to make new variables where week0 is the week where the event occurs (ex event_201314=1) week1 the week after, week_2 is 2 wees before etc. I want to create both weeks before and after the event in my final data set. The new week variables should take the value of the original week_201301 but without referring to a specific date as the dates will be different for each person as the events occur over a year and each person only experience one event.   The data  looks something like this: ID week_201301 week_201302   week_201303     .....week_201452   event_201302  34             500                  200               200                       100                        1 23             0                      100               100                         0                          0 55             300                  100               100                         0                          0 ..   And I want it to look like this: ID week_201301 week_201302   week_201303     .....week_201452   event_201302    week_1    week0     week1 34             500             200                        200                       100                        1               500        200           200 23             0                 100                        100                       0                            0               ...            . ..              ... 55             300             100                        100                       0                            0               ...            ...               ...   ...   The week_ refers to weeks before the event and week to weeks after the event.   I have figured out how to make the weeks after the event, but not how to make the variables for weeks before the event. So essentially I have variables for a given week and I want to copy the value for the previous week into a new variable (depending on the first taking a value). My code looks like this:     data my_data; set my_data; array event(*) event_201301-event_201352; array work(*) week_201201-week_201252 week_201301-week_201352 week_201401-week_201452; array week(*) week_50-week_1 week0-week50; do j=1 to 52; if event[j]=1 then week_2=work[j-2]; if event[j]=1 then week_1=work[j-1]; .... if event[j]=1 then week0=work[j]; if event[j]=1 then week1=work[j+1]; .... if event[j]=1 then week25=work[j+25]; end; run;    The forward loop that uses j+1 to j+50 works fine, but is it possible to do something similar by subtracting a value? if event[j]=1 then week25=work[j+25]; When I run this part of the code I get an error: if event[j]=1 then week_2=work[j-2]; if event[j]=1 then week_1=work[j-1]; Is there some other way to do this than subtracting the values? Any help is much appreciated! Thanks in advance.   ... View more