在data.table中索引唯一值 [英] Index unique values in data.table

问题描述

不知道如何用字来形成问题，但是如何为不同值出现时每个组增量创建一个data.table的索引列？

Not sure how to formulate the question in words, but how can I create an index-column for a data.table that per group increments when a different value appear?

这里是MWE

library(data.table)
in.data <- data.table(fruits=c(rep("banana", 4), rep("pear", 5)),vendor=c("a", "b", "b", "c", "d", "d", "e", "f", "f"))

代码应该生成

in.data[, wanted.column:=c(1,2,2,3,1,1,2,3,3)]

#    fruits vendor wanted.column
# 1: banana      a             1
# 2: banana      b             2
# 3: banana      b             2
# 4: banana      c             3
# 5:   pear      d             1
# 6:   pear      d             1
# 7:   pear      e             2
# 8:   pear      f             3
# 9:   pear      f             3

3，...每个水果。可能有一个非常简单的解决方案，但我被困住了。

So it labels each vendor 1, 2, 3, ... within each fruit. There is probably a very simple solution, but I'm stuck.

推荐答案

您可以使用嵌套组计数器：

I have a few ideas. You can use a nested group counter:

in.data[, w := setDT(list(v = vendor))[, g := .GRP, by=v]$g, by=fruits]

一个运行ID，取决于排序的数据（感谢@eddi），似乎浪费：

Alternately, make a run ID, which depends on sorted data (thanks @eddi) and seems wasteful:

in.data[, w := rleid(vendor), by=fruits]

base-R方法可能是：

The base-R approach would probably be:

in.data[, w := match(vendor, unique(vendor)), by=fruits]

# or in base R ...

in.data$w = with(in.data, ave(vendor, fruits, FUN = function(x) match(x, unique(x))))

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