计算每组的行数并将结果添加到原始数据帧 [英] Count number of rows per group and add result to original data frame

问题描述

说我有一个 data.frame 对象：

df <- data.frame(name=c('black','black','black','red','red'),
                 type=c('chair','chair','sofa','sofa','plate'),
                 num=c(4,5,12,4,3))

现在，我想计算名称和类型的每种组合的行数（观察数）。可以这样完成：

Now I want to count the number of rows (observations) of for each combination of name and type. This can be done like so:

table(df[ , c("name","type")])

，或者也可以与 plyr ，（尽管我是不确定如何）。

or possibly also with plyr, (though I am not sure how).

但是，如何将结果合并到原始数据框中？这样结果将如下所示：

However, how do I get the results incorporated into the original data frame? So that the results will look like this:

df
#    name  type num count
# 1 black chair   4     2
# 2 black chair   5     2
# 3 black  sofa  12     1
# 4   red  sofa   4     1
# 5   red plate   3     1

其中 count 现在存储聚合结果。

where count now stores the results from the aggregation.

使用 plyr 的解决方案也可能很有趣，尽管我想看看如何用基数R完成此操作。 / p>

A solution with plyr could be interesting to learn as well, though I would like to see how this is done with base R.

推荐答案

另一种概括更多的方法：

Another way that generalizes more:

df$count <- unsplit(lapply(split(df, df[c("name","type")]), nrow), df[c("name","type")])

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