R：摘要特定分类值

Question

我正在尝试执行以下R功能：

summary(UNdata_2000_2014,UNdata_2000_2014$Year==2000)

我只想简要总结year==2000时的值。但我得到一个警告：

Warning messages:
1: In if (length(ll) > maxsum) { :
  the condition has length > 1 and only the first element will be used
2: In maxsum:length(ll) :
  numerical expression has 114 elements: only the first used
3: In if (length(ll) > maxsum) { :
  the condition has length > 1 and only the first element will be used
4: In maxsum:length(ll) :
  numerical expression has 114 elements: only the first used

任何帮助解释这个？