举个例子，现在有以下一些列数据（实际不止2行）
data a;
input data x $;
cards;
201401 A
201501 B
;
run;

data b;
input data y;
cards;
201501 3
;
run;

data c;
merge a b;
by data;
run;

proc transpose data=c out=d;
by data;
id x;
var y;
run;

想要把d数据组里的缺损值全部变成0。