在R data.table中创建虚拟变量 [英] Creating dummy variables in R data.table

问题描述

我在R中使用了一个非常大的数据集，并且已经使用数据框进行操作，并决定切换到data.tables以帮助加快操作速度。我无法理解J操作，特别是我试图生成哑变量，但我不知道如何编写data.tables []中的条件操作。

I am working with an extremely large dataset in R and have been operating with data frames and have decided to switch to data.tables to help speed up with operations. I am having trouble understanding the J operations, in particular I'm trying to generate dummy variables but I can't figure out how to code conditional operations within data.tables[].

MWE：

test <- data.table("index"=rep(letters[1:10],100),"var1"=rnorm(1000,0,1))

do是通过添加列到 j 作为虚拟变量，使得列 a index ==a和时，c $ c>将具有值 1 0 。在data.frame环境中它看起来像：

What I would like to do is to add columns a through j as dummy variables such that column a would have a value 1 when the index == "a" and 0 otherwise. In the data.frame environment it would look something like:

test$a <- 0

test$a[test$index=='a'] <- 1

推荐答案

这似乎正在寻找：

inds <- unique(test$index)
test[, (inds) := lapply(inds, function(x) index == x)]

它提供

      index        var1     a     b     c     d     e     f     g     h     i     j
   1:     a  0.25331851  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
   2:     b -0.02854676 FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
   3:     c -0.04287046 FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
   4:     d  1.36860228 FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE
   5:     e -0.22577099 FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE
  ---                                                                              
 996:     f -1.02040059 FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE
 997:     g -1.31345092 FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE
 998:     h -0.49448088 FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE
 999:     i  1.75175715 FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE
1000:     j  0.05576477 FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE

这是另一种方式：

dcast(test, index + var1 ~ index, fun = length)
# or, if you want to preserve row order
dcast(test[, r := .I], r + index + var1 ~ index, fun = length)[, r := NULL]

另一个：

rs = split(seq(nrow(test)), test$index)
test[, names(rs) := FALSE ]
for (n in names(rs)) set(test, i = rs[[n]], j = n, v = TRUE )

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