在 R 中，如何将数据帧的某些行与某些逻辑相加？

Question

嗨，有经验的 R 用户，

这是一件很简单的事情。我想和x通过Group.1根据一个可控变量。

x当我说这样的话时，我想通过对前两行进行分组来求和：number <- 2 如果我说3，它应该x对前三行求和Group.1

知道我如何解决这个问题吗？我应该写一个函数吗？提前谢谢大家。

  Group.1  Group.2      x
1       1     Eggs 230299
2       2     Eggs 263066
3       3     Eggs 266504
4       4     Eggs 177196

Answer 1

如果您想要的总和总是累积的，那么有一个函数，cumsum. 它是这样工作的。

> cumsum(c(1,2,3))
[1] 1 3 6

在这种情况下，您可能想要类似的东西

> mysum <- cumsum(yourdata$x)
> mysum[2] # the sum of the first two rows
> mysum[3] # the sum of the first three rows
> mysum[number] # the sum of the first "number" rows

Answer 2

你可以使用该by功能。

例如，给定以下 data.frame：

d <- data.frame(Group.1=c(1,1,2,1,3,3,1,3),Group.2=c('Eggs'),x=1:8)

> d
  Group.1 Group.2 x
1       1    Eggs 1
2       1    Eggs 2
3       2    Eggs 3
4       1    Eggs 4
5       3    Eggs 5
6       3    Eggs 6
7       1    Eggs 7
8       3    Eggs 8

你可以这样做：

num <- 3 # sum only the first 3 rows

# The aggregation function:
# it is called for each group receiving the 
# data.frame subset as input and returns the aggregated row
innerFunc <- function(subDf){
  # we create the aggregated row by taking the first row of the subset
  row <- head(subDf,1)
  # we set the x column in the result row to the sum of the first "num"
  # elements of the subset
  row$x <- sum(head(subDf$x,num))
  return(row)
}
# Here we call the "by" function:
# it returns an object of class "by" that is a list of the resulting
# aggregated rows; we want to convert it to a data.frame, so we call
# rbind repeatedly by using "do.call(rbind, ... )"
d2 <- do.call(rbind,by(data=d,INDICES=d$Group.1,FUN=innerFunc))

> d2
  Group.1 Group.2  x
1       1    Eggs  7
2       2    Eggs  3
3       3    Eggs 19