as.POSIXct / as.POSIXlt和将字符向量转换为POSIXct / POSIXlt的strptime之间的区别 [英] Difference between as.POSIXct/as.POSIXlt and strptime for converting character vectors to POSIXct/POSIXlt
问题描述
我在这里提到了一些问题，询问如何将字符向量转换为datetime类。我经常看到2种方法，strptime和as.POSIXct / as.POSIXlt方法。我看了两个功能，但不清楚有什么区别。
I have followed a number of questions here that asks about how to convert character vectors to datetime classes. I often see 2 methods, the strptime and the as.POSIXct/as.POSIXlt methods. I looked at the 2 functions but am unclear what the difference is.
function (x, format, tz = "")
{
y <- .Internal(strptime(as.character(x), format, tz))
names(y$year) <- names(x)
y
}
<bytecode: 0x045fcea8>
<environment: namespace:base>
as.POSIXct
as.POSIXct
function (x, tz = "", ...)
UseMethod("as.POSIXct")
<bytecode: 0x069efeb8>
<environment: namespace:base>
as.POSIXlt
as.POSIXlt
function (x, tz = "", ...)
UseMethod("as.POSIXlt")
<bytecode: 0x03ac029c>
<environment: namespace:base>
进行微型基准测试以查看是否存在性能差异：
Doing a microbenchmark to see if there are performance differences:
library(microbenchmark)
Dates <- sample(c(dates = format(seq(ISOdate(2010,1,1), by='day', length=365), format='%d-%m-%Y')), 5000, replace = TRUE)
df <- microbenchmark(strptime(Dates, "%d-%m-%Y"), as.POSIXlt(Dates, format = "%d-%m-%Y"), times = 1000)
Unit: milliseconds
expr min lq median uq max
1 as.POSIXlt(Dates, format = "%d-%m-%Y") 32.38596 33.81324 34.78487 35.52183 61.80171
2 strptime(Dates, "%d-%m-%Y") 31.73224 33.22964 34.20407 34.88167 52.12422
strptime似乎稍快一些。那么给了什么？为什么会有两个类似的功能，或者我们错过了他们之间的差异？
strptime seems slightly faster. so what gives? why would there be 2 similar functions or are there differences between them that I missed?
推荐答案
那么这些功能做不同的事情。
Well, the functions do different things.
首先，有两个日期/时间的内部实现： POSIXct ，它存储自UNIX纪元（+一些其他数据）和 POSIXlt ，其存储日，月，年，小时，分钟，秒等的列表。
First, there are two internal implementations of date/time: POSIXct, which stores seconds since UNIX epoch (+some other data), and POSIXlt, which stores a list of day, month, year, hour, minute, second, etc.
strptime 是一种直接将各种格式的字符向量转换为 POSIXlt 格式的函数
strptime is a function to directly convert character vectors (of a variety of formats) to POSIXlt format.
as.POSIXlt 将各种数据类型转换为 POSIXlt 。它试图聪明，做出明智的事情 - 在人物的情况下，它作为一个包装器 strptime 。
as.POSIXlt converts a variety of data types to POSIXlt. It tries to be intelligent and do the sensible thing - in the case of character, it acts as a wrapper to strptime.
as.POSIXct 将各种数据类型转换为 POSIXct 。它也试图聪明，做一个明智的事情 - 在字符的情况下，它首先运行 strptime ，然后从 POSIXlt 到 POSIXct 。
as.POSIXct converts a variety of data types to POSIXct. It also tries to be intelligent and do the sensible thing - in the case of character, it runs strptime first, then does the conversion from POSIXlt to POSIXct.
有意义的是， strptime 更快，因为 strptime 仅处理字符输入，而其他人则尝试从输入类型中确定使用哪种方法。它也应该是一个更安全的，因为交付意外的数据只会给出一个错误，而不是试图做智能的事情，可能不是你想要的。
It makes sense that strptime is faster, because strptime only handles character input whilst the others try to determine which method to use from input type. It should also be a bit safer in that being handed unexpected data would just give an error, instead of trying to do the intelligent thing that might not be what you want.
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