如何获得xtabs来计算均值而不是R中的总和? [英] How can I get xtabs to calculate means instead of sums in R?

问题描述

我有一个数据框，其中每一行代表一个人.该数据框具有两个变量:年龄和年份.我想做一张每年的平均年龄表.我该怎么办?

I have a data frame where each line represents an individual. That data frame has two variables: age and year. I want to make a table of average ages per year. How can I do it?

我能想到的最好的方法是xtabs(age ~ year, dataframe)，但这给了我每年的年龄总和.

The best I could come up with was xtabs(age ~ year, dataframe), but this gives me the sum of ages per year.

推荐答案

使用aggregate:

xtabs(hp~cyl+gear,aggregate(hp~cyl+gear,mtcars,mean))
   gear
cyl        3        4        5
  4  97.0000  76.0000 102.0000
  6 107.5000 116.5000 175.0000
  8 194.1667   0.0000 299.5000

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