将所有因子级别按名称作为新列从三列中返回data.table [R] [英] Return all factor levels by name as new columns from a three column data.table [R]

问题描述

以任何方式使用data.table或dplyr解决以下问题？

Any way to use data.table or dplyr to solve the below?

library(data.table)

(DT = data.table(a = LETTERS[c(1, 1:3, 8)], b = c(2, 4:7), 
                 c = as.factor(c("bob", "mary", "bob", "george", "alice")), key="a"))

返回：

#    a b      c
# 1: A 2    bob
# 2: A 4   mary
# 3: B 5    bob
# 4: C 6 george
# 5: H 7  alice

想得到这个：

#        alice bob george  mary 
# 1: A    NA   2    NA     NA
# 2: A    NA   NA   NA     4
# 3: B    NA   5    NA     NA
# 4: C    NA   NA   6      NA
# 5: H    7    NA   NA     NA

推荐答案

类似于创建虚拟变量。

uc <- sort(unique(as.character(DT$c)))
DT[,(uc):=lapply(uc,function(x)ifelse(c==x,b,NA))][,c('b','c'):=NULL]

我听说过 ifelse 的错误，所以更快的路线可能是

---

I've heard bad things about ifelse, so a speedier route may be

uc <- sort(unique(as.character(DT$c)))
is <- 1:nrow(DT)
js <- as.character(DT$c)
vs <- DT$b

DT[,(uc):=NA_integer_]
for (i in is) set(DT,i=is[i],j=js[i],value=vs[i])

DT[,c('b','c'):=NULL]

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