TL;DR:在数据表到字符列?
作为开场白,我怀疑这个问题可能会在其他地方得到答案,我正在努力寻找任何明确的答案,因此,如果有人能为我指出正确的方向,我将非常感激这一点,同时也希望得到详尽的答案。
我经常需要转换所有数据表因子列到字符列。看过Matt Dowle建议使用
set
为了这个任务
comment on this answer
然而,当我做一些测试时,我对结果感到惊讶。方法如下:
测试数据生成
library(data.table)
set.seed(1234)
## Create a vector of 1 million 4 character strings
## with 456,976 possible unique values
DiverseSize <- 1e6
Diverse <- paste0(sample(LETTERS,DiverseSize,replace = TRUE),
sample(letters,DiverseSize,replace = TRUE),
sample(letters,DiverseSize,replace = TRUE),
sample(letters,DiverseSize,replace = TRUE))
## Create a vector of 10 million single character strings
## with 26 possible unique values
CommonSize <- 1e7
Common <- sample(LETTERS,CommonSize,replace = TRUE)
## Mix them into a data.table columns, "x0" through "x9"
DT_Original<- data.table(x0 = as.factor(sample(c(Diverse,Common),size = CommonSize + DiverseSize, replace = FALSE)),
x1 = as.factor(sample(c(Diverse,Common),size = CommonSize + DiverseSize, replace = FALSE)),
x2 = as.factor(sample(c(Diverse,Common),size = CommonSize + DiverseSize, replace = FALSE)),
x3 = as.factor(sample(c(Diverse,Common),size = CommonSize + DiverseSize, replace = FALSE)),
x4 = as.factor(sample(c(Diverse,Common),size = CommonSize + DiverseSize, replace = FALSE)),
x5 = as.factor(sample(c(Diverse,Common),size = CommonSize + DiverseSize, replace = FALSE)),
x6 = as.factor(sample(c(Diverse,Common),size = CommonSize + DiverseSize, replace = FALSE)),
x7 = as.factor(sample(c(Diverse,Common),size = CommonSize + DiverseSize, replace = FALSE)),
x8 = as.factor(sample(c(Diverse,Common),size = CommonSize + DiverseSize, replace = FALSE)),
x9 = as.factor(sample(c(Diverse,Common),size = CommonSize + DiverseSize, replace = FALSE)))
DT1 <- copy(DT_Original)
DT2 <- copy(DT_Original)
DT3 <- copy(DT_Original)
功能
unfactorize <- function(df){
for(i in which(sapply(df, class) == "factor")) df[[i]] = as.character(df[[i]])
return(df)
}
set_unfactorize <- function(df){
for(col in names(df)[which(sapply(df, class) == "factor")]) set(df, j = col, value = as.character(df[[col]]))
}
执行
## Original
DT1 <- unfactorize(DT1)
## data.table::set version
set_unfactorize(DT2)
## Outside of function
for(col in names(DT3)[which(sapply(DT3, class) == "factor")]) set(DT3, j = col, value = as.character(DT3[[col]]))
分析结果
我对结果感到非常惊讶——BaseR版本似乎执行速度最快,使用的内存最少,尽管我希望它需要一个副本。是这样,还是我遗漏了什么?