想请教各位大神，如何用R的循环功能做下面的操作：
uk$date_1 <- as.Date(uk$sdate_1,"%d-%b-%y")
uk$date_2 <- as.Date(uk$sdate_2,"%d-%b-%y")
uk$date_3 <- as.Date(uk$sdate_3,"%d-%b-%y")
....
uk$date_212 <- as.Date(uk$sdate_212,"%d-%b-%y")
以及
uk$disease_date <- NA

uk$disease_date <- ifelse(uk$disease_1=="yes", uk$date_1, uk$disease_date)
uk$disease_date <- ifelse(uk$disease_2=="yes", uk$date_2, uk$disease_date)
uk$disease_date <- ifelse(uk$disease_3=="yes", uk$date_3, uk$disease_date)
uk$disease_date <- ifelse(uk$disease_4=="yes", uk$date_4, uk$disease_date)
...
uk$disease_date <- ifelse(uk$disease_212=="yes", uk$date_212, uk$disease_date)