小编use*_*392的帖子

假设我有一个具有3级A1,A2,A3和NA的因子A. 每个出现10例,因此共有40例.如果我做

subset1 <- df[df$A=="A1",]  
dim(subset1)  # 20, i.e., 10 for A1 and 10 for NA's
summary(subset1$A) # both A1 and NA have non-zero counts
subset2 <- df[df$A %in% c("A1"),] 
dim(subset2)  # 10, as expected
summary(subset2$A) # only A1 has non-zero count

用于子集化的变量类是因子还是整数是一样的.是否相等(和>,<)有效吗？那么我应该坚持%in%使用因素并始终包括!is.na在使用平等时？谢谢!