dplyr与plyr的冲突
在做分组计算时，正常情况下使用dplyr就可以，如下：
library(dplyr)
zou=data.frame(n=rep(c(1,2),10),x=1:20)
zou%>%group_by(n)%>%summarise(z=mean(x))
# # A tibble: 2 x 2
# n z
# <dbl> <dbl>
# 1 10
# 2 11
但是，如果在把plyr添加进去，就是这样的：
library(dplyr)
library(plyr)
zou=data.frame(n=rep(c(1,2),10),x=1:20)
zou%>%group_by(n)%>%summarise(z=mean(x))
# z
# 1 10.5
原因是什么呢？R是有提示的，需要先加载plyr再加载dyplyr就可以了：
> library(plyr)
----------------------------------------------------------------------------------------
You have loaded plyr after dplyr - this is likely to cause problems.
If you need functions from both plyr and dplyr, please load plyr first, then dplyr:
library(plyr); library(dplyr)
----------------------------------------------------------------------------------------
载入程辑包：‘plyr’
The following objects are masked from ‘package:dplyr’:
arrange, count, desc, failwith, id, mutate, rename, summarise, summarize
The following object is masked from ‘package:purrr’:
compact