Creating groups of one variable using nonmissing values in a second variable

[Nicholas Stowell]

I have the following data:

* Example generated by -dataex-. To install: ssc install dataex
clear
input str1(var1 var2)
"a" "x"
"f" "" 
"e" "" 
"g" "" 
"d" "" 
"h" "" 
"b" "x"
"s" "" 
"e" "" 
"y" "" 
"f" "" 
"h" "x"
"h" "" 
"f" "" 
"d" "x"
"e" "" 
"r" "" 
"y" "" 
"e" "" 
"e" "" 
end

My goal is to create a third, numeric variable which groups var1 based on the position of "x" in var 2. My finished product should be:

* Example generated by -dataex-. To install: ssc install dataex
clear
input str1(var1 var2) float var3
"a" "x" 1
"f" ""  1
"e" ""  1
"g" ""  1
"d" ""  1
"h" ""  1
"b" "x" 2
"s" ""  2
"e" ""  2
"y" ""  2
"f" ""  2
"h" "x" 3
"h" ""  3
"f" ""  3
"d" "x" 4
"e" ""  4
"r" ""  4
"y" ""  4
"e" ""  4
"e" ""  4
end

Thanks!

[Nick Cox]

gen var3 = sum(var2 != "")