-Usage
aggregate(x, by, FUN, ..., simplify = TRUE, drop = TRUE)
与tapply类似，但是x可以为data.frame或矩阵；
-Arguments
by
a list of grouping elements, each as long as the variables in the data frame x. The
elements are coerced to factors before use.
-Example
testDF
Vby1
by2
aggregate
-Usage
by(data, INDICES, FUN, ..., simplify = TRUE);
-Example
将warpbreaks的一二列，根据tension这一列的（L、M、H）运行summary
warpbreaks
by
aggregate与by的区别：aggregate是对x的每一列分别根据因子进行分组计算，而by是根据因子将data整个分成几个小的data.frame，然后进行运算；
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