Output when using FIRST and LAST - sas

Question

Say we have the SAS code:
data t1 (keep=KEY COUNT C_AMT2 C_AMT);
SET t1;
BY key;
RETAIN COUNT C_AMT;
IF FIRST.KEY THEN
DO;
COUNT=0;
C_AMT2=0;
END;
COUNT+1;
C_AMT=SUM(C_AMT2, C_AMT);
IF LAST.KEY THEN
OUTPUT;
RUN;
What would change here if I were to remove "IF LAST.KEY THEN OUTPUT;". The documentation says that output causes SAS to write to the datastep immediately, not at the end of the data step. Because here it is right before the end of the data step, would this mean removing it would cause no difference?

Answer 1

Removing it would cause a difference.
Then you would have a record for every value of key, assuming multiple values. Controlling the output means you'd have only the last record.
It looks like it's calculating a count and total so there are other ways to achieve this. I'm going to assume that there's some other code that you've suppressed.
The relevant section from the documentation that refers to this is in the link you have above
Implicit versus Explicit Output
By default, every DATA step contains an implicit OUTPUT statement at the end of each iteration that tells SAS to write observations to the data set or data sets that are being created. Placing an explicit OUTPUT statement in a DATA step overrides the automatic output, and SAS adds an observation to a data set only when an explicit OUTPUT statement is executed. Once you use an OUTPUT statement to write an observation to any one data set, however, there is no implicit OUTPUT statement at the end of the DATA step. In this situation, a DATA step writes an observation to a data set only when an explicit OUTPUT executes. You can use the OUTPUT statement alone or as part of an IF-THEN or SELECT statement or in DO-loop processing.
Here's some code that simulates your issue:
*Generate random data;
Data have;
do Key=1 to 2;
do i=1 to 3;
Amount=floor(rand('normal', 50, 5));
OUTPUT;
end;
end;
run;
data t1;
set have;
retain count C_Amt;
by Key;
if first.key then do;
count=0;
C_Amt=0;
end;
Count+1;
c_amt=sum(c_amt, amount);
if last.key then output;
run;
proc print data=t1;
run;
data t1;
set have;
retain count C_Amt;
by Key;
if first.key then do;
count=0;
C_Amt=0;
end;
Count+1;
c_amt=sum(c_amt, amount);
*if last.key then output;
run;
proc print data=t1;
run;
And the corresponding output:
With last.key then output
Obs Key i Amount count C_Amt
1 1 3 46 3 147
2 2 3 44 3 154
And with out last.key
Obs Key i Amount count C_Amt
1 1 1 47 1 47
2 1 2 54 2 101
3 1 3 46 3 147
4 2 1 61 1 61
5 2 2 49 2 110
6 2 3 44 3 154

Answer 2

Commas are an error here:
(keep=KEY, COUNT, C_AMT2, C_AMT)
Anyway:
RUN;
usually means:
output;
return;
But if SAS encounters an output statement in your code, the output at the end (enclosed in the run statement) will be ignored.
Hence, since your output statement is conditionally executed only IF LAST.KEY, in your dataset you will have only observations marked as last.key, because your RUN; will only mean return.
Something like:
data want; set have; output; run;
Is exactly the same to not explicit output:
data want; set have; output; run;
You can use output as you want:
data want01 want02;
set have;
if a then output want01;
if b then output want02;
run;
data want01;
var=var1;
output;
var=var2;
output;
run;