Calculate a difference from "baseline" defined by multi variables in a loop

[braverju]

Dear Community, 

I have the following dataset, that describes N persons who have up to 3 visits and took 3 measures (vars) at each visit. 

Person     Visit   Var1 Var2 Var3   

1               1      10    30     1    

1               2       5     20     3 

2               1       7     15     5     

2               2       5      13    5    

3               3       5      13     5       

3               1       6      12    4         

3               2       1      10    7     

3               3       2       8    6      

I would like to calculate a difference from the first visit for each Var. The resulting dataset should look like this: 

Person     Visit   Var1 Var2 Var3    Diff_1     Diff2    Diff3

1               1      10    30     1          .            .           .

1               2       5     20     3 ..        -5          -10       2

2               1       7     15     5          .           .           . 

2               2       5      13    5          -2         -17       0

3               3       5      13     5         .             .          .

3               1       6      12    4         1            -1         -1

3               2       1      10    7         -4          -3          2

3               3       2       8    6          -3          -5          1

I created the following code: 

-----------------------------------------------------------------

PROC SORT DATA = have;
      by Person VISIT;
RUN;

DATA want;
    SET have;
    by Person VISIT;
    RETAIN BL; //Baseline value = value for the first visit

ARRAY ITEM{3} VAR1 - VAR3;
ARRAY ITEM_sinceBL{3} Var1_sinceBL Var2_sinceBL Var3_sinceBL;

DO I=1 TO 3;
    if first.Person THEN BL = ITEM(I);
    ELSE ITEM_sinceBL(I) = ITEM(I) - BL;
END;

drop BL;
RUN;

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(I use look because in reality I have 18 variables.) But I don't get the right result. My BL seems to have the wrong value. 

I would appreciate your help very much

Julia

[gamotte]

Hello,

When you're on the baseline row, you set the BL variable three times so that, on the following rows you're only left with the last computed BL value, BL=ITEM(3).
You have to store baseline values in an array.

[RW9 · Accepted solution]

DONT CODE ALL IN UPPERCASE!

It hurts the eyes.

So what you want to do is:

get baseline record, merge this onto your data, then calculate.

data want;
  merge have 
        have (where=(visit=1) rename=var1-var_old1 var2=var_old2 var3=var_old3);
  by person;
  array var{3};
  array var_old{3};
  array diff_{3} 8.;
  do i=1 to 3;
    diff_{i}=var{i} - var_old{i};
  end;
run;
  

Not tested, post test data in the form of a datastep so we have something to work with.

[braverju]

Thank you very much! It worked for me.