dplyr中的summary函数by表示group by函数返回单个值，即使group by用一个因子填充。怎么了？
cols <- c("GGEES203","cl_kmeans")
y <- y[which(y$cl_kmeans>0),cols]
y$cl_kmeans <- as.factor(y$cl_kmeans)
str(y)
'data.frame': 3863 obs. of 2 variables:
$ GGEES203 : num 4 11 3 0 9 0 10 11 8 10 ...
$ cl_kmeans: Factor w/ 9 levels "1","2","3","4",..: 6 4 3 3 6 6 4 9 9 5 ...
gd <- y %>%
group_by(cl_kmeans) %>%
summarise(eenzaamheid_avg = mean(GGEES203, na.rm = TRUE))
eenzaamheid_avg
1 7.29
>