使用 apply 和 data.table 在 R 中自动进行 num-to-char 转换

Question

我想计算我的 data.frame 两列的平均差，按三分之一分组。

apply 甚至不让我计算任何算术运算而不显式转换 已经是数字的列 .
data.table 进行操作和分组但 returns 一个字符向量 .
dplyr 语法 returns 数值正确。

为什么 apply() 将数值向量转换为字符？为什么 data.table 将结果转换为 char？

library(dplyr); library(data.table)
a <- letters[c(1,1:9)]
b <- (1:10)/10
c <- sin(1:10)
dat <- data.frame(a,b,c)
table(dat$a)
typeof(dat$b) #double
dat$bb <- apply(dat, 1,function(x) x["b"])
typeof(dat$bb) #character
dat$bb <- apply(dat, 1,function(x) x["b"]-x["c"])
# Error in x["b"] - x["c"] : non-numeric argument to binary operator
tidydat <- dat %>% group_by(a) %>% summarise(diffr = mean(b-c))
typeof(tidydat$diffr) #double
dt <- data.table(dat)
dt[,bb:=mean(b-c), by=a]
typeof(dt$bb) #character

> dt$bb
 [1] "-0.725384205816789" "-0.725384205816789" "0.158879991940133"  "1.15680249530793"   "1.45892427466314"  
 [6] "0.879415498198926"  "0.0430134012812109" "-0.189358246623382" "0.487881514758243"  "1.54402111088937"  
> tidydat$diffr
[1] -0.7253842  0.1588800  1.1568025  1.4589243  0.8794155  0.0430134 -0.1893582  0.4878815  1.5440211

EDIT 这个 data.table 部分是不真实的，我只是通过引用修改一个已经存在的 char 列，来自@Akrun

Answer 1

使用apply，将数据集从data.frame转换为matrix

> is.matrix(apply(dat, 1, I))
[1] TRUE

矩阵只能有一个class，即如果有一个字符元素，它会将整个数据转换为字符。而是使用 lapply（如果它是按列排列的）或者也可以在执行 apply

之前对 numeric 列进行子集化

out <- apply(dat[-1], 1,function(x) x["b"]-x["c"])

-输出

> out
 [1] -0.7414710 -0.7092974  0.1588800  1.1568025  1.4589243  0.8794155  0.0430134 -0.1893582  0.4878815  1.5440211
> str(out)
 num [1:10] -0.741 -0.709 0.159 1.157 1.459 ...

行为变化的原因是 vector 元素只有一个 class 并且在 data.frame/data.table/tibble 等中，列是 list 元素而不是行，即 class 特定于列而不是行

关于 data.table 案例

> library(data.table)
> dt <- as.data.table(dat)
> dt$bb <- NULL # in case if the character column was already created
> dt[,bb:=mean(b-c), by=a]
> str(dt)
Classes ‘data.table’ and 'data.frame':  10 obs. of  4 variables:
 $ a : chr  "A" "A" "B" "C" ...
 $ b : num  0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
 $ c : num  0.841 0.909 0.141 -0.757 -0.959 ...
 $ bb: num  -0.725 -0.725 0.159 1.157 0.704 ...

Answer 2

我认为提供了足够的信息来理解背后的原因。其实你可以试试下面的代码，看看当你使用 apply by rows

时发生了什么

> apply(dat, 1, str)
 Named chr [1:3] "a" "0.1" " 0.8414710"
 - attr(*, "names")= chr [1:3] "a" "b" "c"
 Named chr [1:3] "a" "0.2" " 0.9092974"
 - attr(*, "names")= chr [1:3] "a" "b" "c"
 Named chr [1:3] "b" "0.3" " 0.1411200"
 - attr(*, "names")= chr [1:3] "a" "b" "c"
 Named chr [1:3] "c" "0.4" "-0.7568025"
 - attr(*, "names")= chr [1:3] "a" "b" "c"
 Named chr [1:3] "d" "0.5" "-0.9589243"
 - attr(*, "names")= chr [1:3] "a" "b" "c"
 Named chr [1:3] "e" "0.6" "-0.2794155"
 - attr(*, "names")= chr [1:3] "a" "b" "c"
 Named chr [1:3] "f" "0.7" " 0.6569866"
 - attr(*, "names")= chr [1:3] "a" "b" "c"
 Named chr [1:3] "g" "0.8" " 0.9893582"
 - attr(*, "names")= chr [1:3] "a" "b" "c"
 Named chr [1:3] "h" "0.9" " 0.4121185"
 - attr(*, "names")= chr [1:3] "a" "b" "c"
 Named chr [1:3] "i" "1.0" "-0.5440211"
 - attr(*, "names")= chr [1:3] "a" "b" "c"
NULL

如您所见，当您运行 apply(dat,1,FUN = ...) 时，传递给 FUN 的数据被合并为一个字符向量，而不是 data.frame。

使用 apply 和 data.table 在 R 中自动进行 num-to-char 转换

Automatic num-to-char conversion in R using apply and data.table

r

apply

dplyr

data.table