Inserting a column into a data set

[jacksonan123]

If I have a data set A:

ID   Ratio

1      5

.       5

10     5

Data set B:

ID   Time  conc

1      0       0

.       1       0.5

24    24     0.01

The (.) means that there are several values for A that end at 10 while for B they end at 24.  The question is how can I introduce the constant ratio value 5 into data B from data A for all the rows in data set B?

[Tom]

I don't follow.  You want a constant?

data want;
  set B;
  ratio=5;
run;

You want the constant that is in dataset A?  Just take the first copy of it.

data want;
  set B;
  if _n_=1 then set A(keep=ratio);
run;

If you want something else you need to explain the rules in more detail.

[jacksonan123]

I tried your solution but it put in the column for ratio, but it was all
missing values. Let me try to explain again with my data set want.

Data a;

ID Ratio

1 2.5

. 2.5

10 2.5

Data B;

ID Time Conc

1 0 0

. 1 0.5

24 24 0.25

Data Want:

ID Time Conc Ratio

1 0 0 2.5

. 1 0.5 2.5

24 24 0.25 2.5

[Tom · Accepted solution]

Post actual data. Like this:

data a;
  do id=1 to 10;
    ratio=2.5;
    output;
  end;
run;

data b;
  id=0;
  do time=1 to 24 ;
    id=id+1;
    conc = round(ranuni(0),0.01);
    output;
  end;
run;

Now use that data:

data want;
  set b;
  if _n_=1 then set a(keep=ratio);
run;

Result:

Obs    id    time    conc    ratio

  1     1      1     0.31     2.5
  2     2      2     0.74     2.5
  3     3      3     0.52     2.5
  4     4      4     0.84     2.5
  5     5      5     0.09     2.5
  6     6      6     0.33     2.5
  7     7      7     0.11     2.5
  8     8      8     0.87     2.5
  9     9      9     0.61     2.5
 10    10     10     0.43     2.5
 11    11     11     0.92     2.5
 12    12     12     0.79     2.5
 13    13     13     0.91     2.5
 14    14     14     0.98     2.5
 15    15     15     0.41     2.5
 16    16     16     0.50     2.5
 17    17     17     0.21     2.5
 18    18     18     0.55     2.5
 19    19     19     0.31     2.5
 20    20     20     0.38     2.5
 21    21     21     0.43     2.5
 22    22     22     0.49     2.5
 23    23     23     0.71     2.5
 24    24     24     0.96     2.5

The only way that running that step would result in any missing values for RATIO would be if the variable already existed in dataset B.  In that case you could just drop it.

data want;
  set b(drop=ratio);
  if _n_=1 then set a(keep=ratio);
run;

It is still not clear why you made the A dataset have 10 observations instead of one.

[jacksonan123]

When data A was generated it had some other values that were needed other
than ratio.

Your answer was a solution and in the future when possible I will submit
data as you suggested.

I had always thought that when one set a data set meant that one data set
was placed on top of the other and merge was the way to combine but I stand
corrected by you code.

[Tom]

SET (and MERGE) are executable statements.  Each time it execute it reads another observation from the dataset(s).

That is why you can do code like this to process all of the records for a group in iteration of the data step.

do until(last.id);
  set have;
  by id;
 ....
end;

[jacksonan123]

Thanks for the explanation.