自己写的一段命令，keep if那句太长了看着就不爽，感觉可以改为循环语句，刚开始学stata比较菜不会，求大神赐教，万分感谢！
gen postcode1=real(postcode)
drop if postcode1==.
gen postcode2=postcode1/10000
gen postcode3=int(postcode2)
keep if postcode3==31 | postcode3==32 | postcode3==21 | postcode3==22 | postcode3==51 | postcode3==52 | postcode3==35 | postcode3==36 | postcode3==43 | postcode3==44 | postcode3==45 | postcode3==46 | postcode3==47