2. Given the non-indexed SAS data set TEMP:
TEMP
X Y
P 52
P 45
A 13
A 56
R 34
R 12
R 78
The following SAS program is submitted:
proc print data = temp;
run;
Which BY statement completes the program, creates a listing report that is grouped by X, and completes without errors?
(A) by X;
(B) by X grouped;
(C) by X notsorted;
(D) by descending X;
这道题大家有什么想法，答案是c，能否解释一下！

昨天学习了kittyforever的关于macro的帖子，我现在这边也是班门弄斧的回答一下

%let rc = Begin;   （1）
%macro test;
data out;
set sashelp.prdsale nobs = totalobs;
if totalobs > 10 then do;
%let rc = high; （2a）
end;
else do;
%let rc = low; （2b）
end;
run;
%mend;
%let rc = Before Execution; （3）
%test;
%put &rc;


其中出现了3个给RC赋值的语句，其中1 和3 在外部，属于global赋值，2a和2b在marco内部，属于local赋值，因为macro中间有内部赋值，这样会覆盖掉1的外部赋值，此时执行到3之前，rc已经被赋值成low了，但是经过3之后，rc又被赋值成 Before Execution， 但是3之后又进行运算了macro，rc又被赋值回low，所以最后put rc就出来low了。



所以，也可以这样进行推演

%let rc = Begin;   （1）
%macro test;
data out;
set sashelp.prdsale nobs = totalobs;
if totalobs > 10 then do;
%let rc = high; （2a）
end;
else do;
%let rc = low; （2b）
end;
run;
%mend;
%let rc = Before Execution; （3）
%put &rc; (4)
%test;
%put &rc; (5)
我又新加了一步（4），这样（4）里面rc的值就为Before Execution， 而（5）的rc值还是被赋回low

再看
%let rc = Begin;   （1）
%macro test （rc=start) ******;
data out;
set sashelp.prdsale nobs = totalobs;
if totalobs > 10 then do;
%let rc = high; （2a）
end;
else do;
%let rc = low; （2b）
end;
run;
%mend;
%let rc = Before Execution; （3）
%put &rc；（4）
%test (rc=end)*****;
%put &rc; （5）
因为在marco中间对RC赋值，这样就表明这个rc是一个local赋值，不会影响到外部的rc的global赋值，而(3)的外部赋值将要覆盖掉（1）的外部赋值，所以最后 （4）或者（5）的赋值都将会是Before execution。希望能够帮助到你

%let rc = Begin;
%macro test;
data out;
   set sashelp.prdsale nobs = totalobs;
  if totalobs > 10 then  call symput('rc', 'high');
    else   call symput('rc', 'low');
run;
%mend test;
%let rc = Before Execution;
%test;
%put &rc;

output in the log
%put &rc;
high

or there is another case

%let rc = Begin;%macro test;
data out;
set sashelp.prdsale nobs = totalobs;
%if totalobs > 10 %then %do;
%let rc = high;
%end;
%else %do;
%let rc = low;
%end;
run;
%mend;
%let rc = Before Execution;
%test;
%put &rc;

Your macro is equivalent to,

%macro test;
%let rc = high;
%let rc = low;
data out;
set sashelp.prdsale nobs = totalobs;
if totalobs > 10 then do;
end;
else do;
end;
run;
%mend;

What you need to understand is the difference between SAS macro language and other SAS language. They are very different.