看看

You are confused by a macro variable scope.

A local macro variable lives within its scope. See comments below.

18   %let Mv=shoes;  ***Mv has a global scope;
19   %macro PRODUCT(Mv=bicycles);***Mv has a local scope;
20   %put Mv is &Mv;
21   %mend;
22
23   %PRODUCT(Mv=tents)
Mv is tents
24   %PRODUCT(Mv=clothes)
Mv is clothes
25   %PRODUCT(Mv=cars)
Mv is cars
26
27   %put Mv is &Mv;
Mv is shoes

很明显，你把局部宏变量宏程序中的宏变量进行了混淆，建议你将宏变理的运作原理再重新看一遍ADVANCE那本书中讲得很清楚

因为开头定义了全局的宏变量Mv, 因此在%product里再次定义mv时, 将全局mv的值修改为clothes, 打印出的结果为clothes. 在%product中加%local mv; 全局mv的值仍为shoes.
%let Mv=shoes;
%macro PRODUCT;
%local mv;
%let Mv=clothes;
%mend;
%PRODUCT;
%put Mv is &Mv;

请问ADWANCE 是哪一本，全名是什么？谢谢。