数据为分类变量0，1
#先转换为factor

xz$Overweight<-factor(xz$Overweight)
xz$alc<-factor(xz$alc)
xz$Current_smoke<-factor(xz$Current_smoke)
#生成新变量
xz$dm[xz$Overweight==0 & xz$alc==0]<-0
xz$dm[xz$Overweight==1 & xz$alc==0]<-1
xz$dm[xz$Overweight==0 & xz$alc==1]<-2
xz$dm[xz$Overweight==1 & xz$alc==1]<-3
xz$dm<-factor(xz$dm)
结果报错
invalid factor level, NA generated
请问是什么原因啊