10. The SAS data set Sashelp.Prdsale contains the variables Region and Salary with 4 observations per Region. Sashelp.Prdsale is sorted primarily by Region and within Region by Salary in descending order. The following program is submitted:
data one;
set sashelp.prdsale;
retain temp;
by region descending salary;
if first.region then
do;
temp=salary;
output;
end;
if last.region then
do;
range=salary-temp;
output;
end;
run;
For each region, what is the number of observation(s) written to the output data set?
答案是2个！为什么呢?retain在这里的作用是什么呢？temp和range是2个不同的变量，怎么就变成了2个观测呢？