r - 与 as.POSIXct 相比，为什么 lubridate 函数如此慢？

Question

正如标题所说。为什么 lubridate 函数这么慢？

library(lubridate)
library(microbenchmark)

Dates <- sample(c(dates = format(seq(ISOdate(2010,1,1), by='day', length=365), format='%d-%m-%Y')), 50000, replace = TRUE)

microbenchmark(as.POSIXct(Dates, format = "%d-%b-%Y %H:%M:%S", tz = "GMT"), times = 100)
microbenchmark(dmy(Dates, tz ="GMT"), times = 100)

Unit: milliseconds
expr                                                            min         lq          median      uq          max
1 as.POSIXct(Dates, format = "%d-%b-%Y %H:%M:%S", tz = "GMT")   103.1902    104.3247    108.675     109.2632    149.871
2 dmy(Dates, tz = "GMT")                                        184.4871    194.1504    197.8422    214.3771    268.4911

Answer 1

出于同样的原因，与 riding on top of rockets 相比，汽车速度较慢。增加的易用性和安全性使汽车比火箭慢得多，但被炸毁的可能性更小，而且汽车的启动、转向和制动也更容易。然而，在正确的情况下(例如，我需要登上月球)，火箭是完成这项工作的正确工具。现在，如果有人发明了一辆车顶绑着火箭的汽车，我们就会有所收获。

首先看看 dmy 正在做什么，您会看到速度的差异(顺便说一下，从您的基准来看，我不会说 lubridate 是慢得多，因为这些以毫秒为单位):

dmy #在命令行中键入此内容，您将得到:

>dmy
function (..., quiet = FALSE, tz = "UTC") 
{
    dates <- unlist(list(...))
    parse_date(num_to_date(dates), make_format("dmy"), quiet = quiet, 
        tz = tz)
}
<environment: namespace:lubridate>

我立即看到 parse_date 和 num_to_date 和 make_format。让人不禁好奇这些家伙到底是什么人。让我们看看:

parse_date

> parse_date
function (x, formats, quiet = FALSE, seps = find_separator(x), 
    tz = "UTC") 
{
    fmt <- guess_format(head(x, 100), formats, seps, quiet)
    parsed <- as.POSIXct(strptime(x, fmt, tz = tz))
    if (length(x) > 2 & !quiet) 
        message("Using date format ", fmt, ".")
    failed <- sum(is.na(parsed)) - sum(is.na(x))
    if (failed > 0) {
        message(failed, " failed to parse.")
    }
    parsed
}
<environment: namespace:lubridate>

num_to_date

> getAnywhere(num_to_date)
A single object matching ‘num_to_date’ was found
It was found in the following places
  namespace:lubridate
with value

function (x) 
{
    if (is.numeric(x)) {
        x <- as.character(x)
        x <- paste(ifelse(nchar(x)%%2 == 1, "0", ""), x, sep = "")
    }
    x
}
<environment: namespace:lubridate>

make_format

> getAnywhere(make_format)
A single object matching ‘make_format’ was found
It was found in the following places
  namespace:lubridate
with value

function (order) 
{
    order <- strsplit(order, "")[[1]]
    formats <- list(d = "%d", m = c("%m", "%b"), y = c("%y", 
        "%Y"))[order]
    grid <- expand.grid(formats, KEEP.OUT.ATTRS = FALSE, stringsAsFactors = FALSE)
    lapply(1:nrow(grid), function(i) unname(unlist(grid[i, ])))
}
<environment: namespace:lubridate>

哇，我们得到了strsplit-ting、expand-ing.grid-s、paste-ing、ifelse-ing、unname-ing 等，加上正在进行的全部错误检查(播放 Zep 歌曲)。所以我们这里有一些很好的语法糖。嗯，很好吃，但它是有代价的，速度。

将其与 as.POSIXct 进行比较:

getAnywhere(as.POSIXct)  #tells us to use methods to see the business
methods('as.POSIXct')    #tells us all the business
as.POSIXct.date          #what I believe your code is using (I don't use dates though)

as.POSIXct 进行了更多的内部编码和更少的错误检查，因此您必须问我想要轻松和安全还是速度和功能？取决于工作。