与注释中一样，字符串 NA 不是 NA 值。因此 is.na（ NA）是 FALSE ，并且所有行均已选中。只需将您定义中的 NA 替换为 NA 。

As in the comments, the string "NA" is not an NA value. So is.na("NA") is FALSE and all rows are selected. Just replace "NA" in your definition with NA.

mydf <- expand.grid(var1 = c('type1', 'type2'), var2 = c(7, 6, NA), var3 = 9)
mydf$var3[!is.na(mydf$var2)] <- mydf$var2[!is.na(mydf$var2)]

请注意，您不能只用 mydf $ var2 替换左侧，因为它们现在的长度不相等-在您使用之前没有任何错误，因为什么都不是 NA 。

Note that you can't just replace the left hand side with just mydf$var2 because they now have unequal lengths - before you didn't have this error since nothing was NA.