r语言 - plyr：:d dply 错误消息：类(输出[[var]])中的错误 <- 类(值)：尝试在 NULL 上设置属性

Question

不到一个月前这还行得通。有什么变化吗?(添加简单样本):

adk47sample

PeakName Elevation
Marcy      5344
Marcy      5344
Marcy      5344
Marcy      5344
Marcy      5344
Marcy      5344
Marcy      5344
Algonquin      5114
Algonquin      5114
Algonquin      5114
> require(plyr)
Loading required package: plyr
> adk.need.fr.round.6 <- ddply(adk47sample, .(PeakName, Elevation),
+   summarize, needThese=if(length(PeakName) < 6) "needThis")
Error in class(output[[var]]) <- class(value) : 
  attempt to set an attribute on NULL

对于这个简单的例子，我期望:

> adk.need.fr.round.6
      PeakName     Elevation needThese
1     Algonquin      5114     needThis

Answer 1

它适用于我的plyr版本1.8，如果我给character(0)的else结果@joran建议(与else NULL它给出相同的错误):

adk.need.fr.round.6 <- ddply(adk47sample, 
.(PeakName, Elevation),summarize, 
  needThese=if(length(PeakName) < 6) "needThis" else character(0))