如题，peter2是一个空数据集，如何将其type值设为god？
> peter = data.frame(type = c('male','male','female','female'),code = c('m01','m02','f01','f02'))
>
> str(peter)
'data.frame': 4 obs. of 2 variables:
$ type: Factor w/ 2 levels "female","male": 2 2 1 1
$ code: Factor w/ 4 levels "f01","f02","m01",..: 3 4 1 2
>
> peter
type code
1 male m01
2 male m02
3 female f01
4 female f02
>
> peter2 = peter[peter$type == 'other',]
>
> peter2
[1] type code
<0 rows> (or 0-length row.names)
>
> peter2$type = 'god'
Error in <code>$<-.data.frame</code>(<code>*tmp*</code>, "type", value = "god") :
replacement has 1 row, data has 0
>
