r - data.table 相当于 tidyr 的完整/填充

Question

我有以下数据

library(tidyr)
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
library(data.table)
#> 
#> Attaching package: 'data.table'
#> The following objects are masked from 'package:dplyr':
#> 
#>     between, first, last

df <- structure(list(filename = c("PS92_019-6_rovT_irrad.tab", "PS92_019-6_rovT_irrad.tab", 
  "PS92_019-6_rovT_irrad.tab", "PS92_019-6_rovT_irrad.tab"), depth = c(5, 
  10, 20, 75), ps = c(3.26223404971255, 3.38947945477306, 3.97380593851983, 
  0.428074807655144)), row.names = c(NA, -4L), class = c("tbl_df", "tbl", 
  "data.frame"), .Names = c("filename", "depth", "ps"))

df
#> # A tibble: 4 x 3
#>                    filename depth        ps
#>                       <chr> <dbl>     <dbl>
#> 1 PS92_019-6_rovT_irrad.tab     5 3.2622340
#> 2 PS92_019-6_rovT_irrad.tab    10 3.3894795
#> 3 PS92_019-6_rovT_irrad.tab    20 3.9738059
#> 4 PS92_019-6_rovT_irrad.tab    75 0.4280748

在这个数据中，在深度 = 0 处有一个缺失的观察。使用 tidyr，
我可以完成它:

df %>% tidyr::complete(depth = c(0, unique(depth))) %>% fill(everything(), .direction = "up")  ## use the last observations to fill the new line
#> # A tibble: 5 x 3
#>   depth                  filename        ps
#>   <dbl>                     <chr>     <dbl>
#> 1     0 PS92_019-6_rovT_irrad.tab 3.2622340
#> 2     5 PS92_019-6_rovT_irrad.tab 3.2622340
#> 3    10 PS92_019-6_rovT_irrad.tab 3.3894795
#> 4    20 PS92_019-6_rovT_irrad.tab 3.9738059
#> 5    75 PS92_019-6_rovT_irrad.tab 0.4280748

问题是我必须在大型数据集上运行它，我发现
完成/填充功能有点慢。因此，我想给
使用 data.table 看看它是否可以加快速度。但是，我
无法理解它。任何帮助表示赞赏。

Answer 1

它没有特定的功能，但您可以通过以下方式实现相同的功能:

# load package
library(data.table)

# convert to a 'data.table'
setDT(df)

# expand and fill the dataset with a rolling join
df[.(c(0, depth)), on = .(depth), roll = -Inf]

这使:

                    filename depth        ps
1: PS92_019-6_rovT_irrad.tab     0 3.2622340
2: PS92_019-6_rovT_irrad.tab     5 3.2622340
3: PS92_019-6_rovT_irrad.tab    10 3.3894795
4: PS92_019-6_rovT_irrad.tab    20 3.9738059
5: PS92_019-6_rovT_irrad.tab    75 0.4280748

向@Frank 寻求改进建议。

旧解决方案:

df[CJ(depth = c(0,unique(depth))), on = 'depth'
   ][, c(1,3) := lapply(.SD, zoo::na.locf, fromLast = TRUE), .SDcols = c(1,3)][]