R data.table fread-读取列为日期 [英] R data.table fread - read column as Date

问题描述

我想从 data.table 中读取具有 fread 的文件，该文件的列为 YYYY- MM-DD格式的日期。默认情况下， fread 将该列读取为 chr 。但是，我希望将列设置为 Date ，与应用 as.Date 时获得的方式相同。

I would like to read a file with fread from data.table that has a column with "YYYY-MM-DD" format dates. By default, fread reads the column as chr. However, I would like to have the column as Date, the same way I would obtain when applying as.Date.

我尝试使用

dt[,starttime.date := as.Date(starttime.date)]

，但是运行时间很长（我有大约4300万行）。

but it takes very long to run (I have approx. 43 million rows).

推荐答案

使用 fasttime 包，根据 fread文档中的建议，比 as.Date 或 as.IDate 快100倍：

Using the fasttime package, as suggested in the fread documentation, is approximately 100x faster than as.Date or as.IDate:

library(data.table)
library(fasttime)

dt[,starttime.date := fastPOSIXct(starttime.date)]

基准测试结果：

library(microbenchmark)
library(fasttime)
DT <- data.table(start_date = paste(sample(1900:2018, 100000, replace = T), 
                                    sample(1:12, 100000, replace = T),
                                    sample(1:28, 100000, replace = T),
                                    sep = "-"))
microbenchmark(
  as.Date(DT$start_date),
  as.IDate(DT$start_date),
  fastPOSIXct(DT$start_date)
)

> Unit: milliseconds
>                        expr    mean 
>      as.Date(DT$start_date)  383.89
>     as.IDate(DT$start_date)  405.89
>  fastPOSIXct(DT$start_date)    4.59 

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