Dear all，

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input float(var1 var2 var3 var4)
1 . . .
. 1 . .
. . 1 .
. . . 1
1 1 . .
. 1 1 .
1 . 1 .
end

If I want to select the first observation from the example above , I have to use a statement like " if var==1&var2==.&var3==.&var4==." And if the condition become complex , the statement I have to write is going to be longer. I tried to find a way to simplify the if-statement but failed. Is there a book or something to introduce a solution to the problem like this one ？