stata如何根据相同变量给重复出现的缺失值所在行赋值

* Example generated by -dataex-. To install: ssc install dataex
clear
input str10(hhid hhid_2015) float(year track) byte a4003 float Risk_attitude
"201100002" "2015000295" 2015 . 3 3
"201100002" "2015000295" 2017 1 . .
"201100006" "2015000299" 2015 . 3 3
"201100006" "2015000299" 2017 1 . .
"201100007" "2015000302" 2015 . 3 3
"201100007" "2015000302" 2017 1 . .
"201100008" "2015000304" 2017 1 . .
"201100008" "2015000304" 2015 . 5 5
"201100012" "2015000309" 2015 . 5 5
"201100013" "2015000315" 2017 1 . .
"201100013" "2015000315" 2015 . 4 4
"201100016" "2015000317" 2017 1 . .
"201100016" "2015000317" 2015 . 5 5
"201100017" "2015000318" 2015 . 4 4
"201100018" "2015000320" 2015 . 1 1
"201100018" "2015000320" 2017 1 . .
"201100022" "2015000322" 2015 . 4 4
"201100022" "2015000322" 2017 1 . .
"201100025" "2015000325" 2015 . 2 2
"201100030" "2015000327" 2015 . 5 5
"201100030" "2015000327" 2017 1 . .
"201100033" "2015000329" 2015 . 5 5
"201100033" "2015000329" 2017 1 . .
"201100034" "2015000331" 2015 . 6 6
"201100034" "2015000331" 2017 1 . .
"201100040" "2015000335" 2015 . 4 4
end
[/CODE]

请教各位！想要把hhid-_2015相同条件下，track为1时Risk的缺值，赋值为对应同一hhid_2015的a4003给Risk