建立指标 [英] Create indicator

问题描述

我想为矩阵创建一个数字指示器，以便对于一个变量中的每个唯一元素，它根据另一个变量中的元素创建长度序列.例如:

I would like to create a numeric indicator for a matrix such that for each unique element in one variable, it creates a sequence of the length based on the element in another variable. For example:

frame<- data.frame(x = c("a", "a", "a", "b", "b"), y = c(3,3,3,2,2))
frame
  x y
1 a 3
2 a 3
3 a 3
4 b 2
5 b 2

指标z应该看起来像这样:

The indicator, z, should look like this:

  x y z
1 a 3 1
2 a 3 2
3 a 3 3
4 b 2 1
5 b 2 2

任何人和所有帮助都将不胜感激.谢谢.

Any and all help greatly appreciated. Thanks.

推荐答案

否ave?

frame$z <- with(frame, ave(y,x,FUN=seq_along) )
frame

#  x y z
#1 a 3 1
#2 a 3 2
#3 a 3 3
#4 b 2 1
#5 b 2 2

data.table版本可能类似于以下内容(感谢@mnel):

A data.table version could be something like below (thanks to @mnel):

#library(data.table)
#frame <- as.data.table(frame)
frame[,z := seq_len(.N), by=x]

我最初的想法是使用:

frame[,z := .SD[,.I], by=x]

其中，.SD是指data.table拆分by x的每个子集. .I返回整个data.table的行号.因此，.SD[,.I]返回每个组内的行号.尽管@mnel指出，与其他方法相比，这种方法效率低下，因为需要为每个组将整个.SD加载到内存中才能运行此计算.

where .SD refers to each subset of the data.table split by x. .I returns the row numbers for an entire data.table. So, .SD[,.I] returns the row numbers within each group. Although, as @mnel points out, this is inefficient compared to the other method as the entire .SD needs to be loaded into memory for each group to run this calculation.

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