按组添加总和的新列 [英] Add a new column of the sum by group

问题描述

假设我有一个这样的数据框:

Suppose I have a data frame like this:

set.seed(123)
df <- as.data.frame(cbind(y<-sample(c("A","B","C"),10,T), X<-sample(c(1,2,3),10,T)))
df <- df[order(df$V1),]

是否有一个简单的函数可以将 V2 与 V1 相加(或任何 FUN)并将其作为新列添加到 df 中，例如:

Is there a simply function to sum (or any FUN) V2 by V1 and add to df as a new column, such that:

df$sum <- c(6,6,8,8,8,8,6,6,6,6)
df

我可能会为此编写一个函数，但我必须经常这样做，并且最好知道实现这一点的最简单方法.

I may write a function for that, but I have to do that frequently and be better to know the simplest way to realize that.

推荐答案

我同意 @mnel 至少在他的第一点.我在他引用的答案中没有看到 ave 的演示，我认为这是最简单"的 base-R 方法.使用该 data.frame(cbind( ...)) 构造应该被取缔，并且证明它的教师应该被剥夺他们的证书.

I agree with @mnel at least on his first point. I didn't see ave demonstrated in the answers he cited and I think it's the "simplest" base-R method. Using that data.frame(cbind( ...)) construction should be outlawed and teachers who demonstrate it should be stripped of their credentials.

set.seed(123)
 df<-data.frame(y=sample( c("A","B","C"), 10, T), 
                X=sample(c (1,2,3), 10, T))
  df<-df[order(df$y),]  # that step is not necessary for success.
df

 df$sum <- ave(df$X, df$y, FUN=sum)
 df
   y X sum
1  A 3   6
6  A 3   6
3  B 3   8
7  B 1   8
9  B 1   8
10 B 3   8
2  C 2   6
4  C 2   6
5  C 1   6
8  C 1   6

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