按名称返回所有因子级别作为来自三列数据的新列。表 [R]

Question

有什么方法可以使用 data.table 或 dplyr 来解决以下问题？

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

Answer 1

这类似于 creating dummy variables .

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

我听说过关于 ifelse 的坏消息，所以更快的路线可能是

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]