如何将数组转换为R中的data.table并返回？ [英] How to convert array to data.table in R and back?

问题描述

这是将数组转换为数据表的最直接方法吗？

require(data.table)
require(ggplot2)

# this returns a data.table with both array's dimensions and values
aaa <- array(rnorm(3*4*2), dim = c(3,4,2))
DT1 <- as.data.table(as.data.frame.table(aaa))

# the following does not work properly, because it only returns the array values
DT2 <- as.data.table(aaa)


# plot values aggregated by 3rd array dim
ggplot(DT1, aes(Var1, Freq, fill = Var3)) + geom_boxplot()
# sum values by 2nd array dim
DT1[ , sum(Freq), Var2]

EDIT1：
对不起，正确的意思是我得到的数据帧只有一列，因此我不知道值是从原始数组的哪个位置产生的。
的想法是将数组转换为平面表，这样更容易例如使用维度作为因子绘制变量，或按因子汇总值。

sorry, with "properly" I mean that I get a data frame with one column only, so that I don't know from which position in the original array a values has originated. The idea is to transform the array into a flat table, so that is easier to e.g. plot the variables using the dimensions as factors, or to aggregate values by factors. Would that be still possible with DT2?

EDIT2：
另一个有用的事情是将 data.table转换回原始数组。您是否知道通过定义用作维的列来将data.table强制转换为数组的函数？

one other useful thing would be to convert the data.table back into the original array. Do you know a function that coerces data.table to array, by defining which columns to use as dimensions?

aaa <- array(rnorm(3*4*2), dim = c(3,4,2), list(Var1 = LETTERS[1:3], Var2 = LETTERS[1:4], Var3 = LETTERS[1:2] ))

DT1 <- setDT(melt(aaa))

# convert DT1 back to aaa
array(data = DT1[ ,value],
      dim = c(length(unique(DT1[ ,Var1])),
              length(unique(DT1[ ,Var2])),
              length(unique(DT1[ ,Var3]))),
      dimnames = list(Var1 = unique(DT1[ ,Var1]),
                      Var2 = unique(DT1[ ,Var2]),
                      Var3 = unique(DT1[ ,Var3])))

谢谢！

推荐答案

仅适用于1.11.4和1.11.2版本，但不适用于某些较早的版本

返回相同的数据表，但 A = 1 ， B = 2 ， C = 3 以及行以不同的方式排序。因此，第二种方法是解决方法。

both approaches essentially return the same data.table but with A=1, B=2, C=3 in your second approach, and rows ordered in different ways. so the second approach is the way to go.

DT2 <- as.data.table(aaa)
head(DT2)
#   V1 V2 V3       value
#1:  1  1  1  0.32337516
#2:  1  1  2  1.59189589
#3:  1  2  1 -1.48751756
#4:  1  2  2 -0.86749305
#5:  1  3  1  0.01017255
#6:  1  3  2  2.66571093

#compare
DT[order(Freq), ]
#and 
DT2[order(value), ]

这篇关于如何将数组转换为R中的data.table并返回？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！