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SAS Code Fragments
How can I "fill down" a variable in SAS? 
OR
How can I do a one-record-to-many merge in SAS?

Sometimes you want to create a new variable that is a constant, but you do not know the exact numerical value of the constant.  For example, if you want to create a variable that is the mean of another variable, you could do a proc means and then a data step to create the new variable.  The code fragment below shows how to output the mean (or whatever statistic that you like) to a data set, and then merge that data set with your original data set.  This is not as straightforward as it might seem at first, because the two data sets do not have a common variable on which to do the merge.  Another way to think of this is as merging a data file with only one record to a data file with many records.

data test;
input studentid class score1 score2;
cards;
1 1 34 24
2 1 39 25
3 1 34 26
4 1 38 20
5 1 32 21
1 2 45 36
2 2 43 30
3 2 48 39
4 2 41 37
5 2 40 31
1 3 50 46
2 3 51 49
3 3 57 48
4 3 50 40
5 3 57 46
;
run;

proc means data = test mean;
var score1 score2;
output out = test1 mean=m1 m2;
run;

data test2;
merge test test1;
retain mean1 mean2;
if _n_ = 1 then do;
mean1 = m1; 
mean2 = m2; 
end;
drop _freq_ _type_ m1 m2;
run;

proc print data = test2;
run;

Obs    studentid    class    score1    score2     mean1      mean2

  1        1          1        34        24      43.9333    34.5333
  2        2          1        39        25      43.9333    34.5333
  3        3          1        34        26      43.9333    34.5333
  4        4          1        38        20      43.9333    34.5333
  5        5          1        32        21      43.9333    34.5333
  6        1          2        45        36      43.9333    34.5333
  7        2          2        43        30      43.9333    34.5333
  8        3          2        48        39      43.9333    34.5333
  9        4          2        41        37      43.9333    34.5333
 10        5          2        40        31      43.9333    34.5333
 11        1          3        50        46      43.9333    34.5333
 12        2          3        51        49      43.9333    34.5333
 13        3          3        57        48      43.9333    34.5333
 14        4          3        50        40      43.9333    34.5333
 15        5          3        57        46      43.9333    34.5333

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