小编Cra*_*idt的帖子

请考虑以下代码.当您没有NA在您的条件中明确测试时,该代码将在以后失败,然后您的数据会发生变化.

>   # A toy example
>   a <- as.data.frame(cbind(col1=c(1,2,3,4),col2=c(2,NA,2,3),col3=c(1,2,3,4),col4=c(4,3,2,1)))
>   a
  col1 col2 col3 col4
1    1    2    1    4
2    2   NA    2    3
3    3    2    3    2
4    4    3    4    1
>   
>   # Bummer, there's an NA in my condition
>   a$col2==2
[1]  TRUE    NA  TRUE FALSE
> 
>   # Why is this a good thing to do?
>   # It NA'd the whole row, and kept it
>   a[a$col2==2,]
   col1 col2 …