为什么解析“％Y-％m”在R中使用strptime得到NA结果，但是“％Y-％m-％d”作品？ [英] Why is parsing "%Y-%m" using strptime in R giving an NA result, but "%Y-%m-%d" works?

问题描述

如果我使用 strptime 与一年和一年格式的％Y-％m（如2009-12），我得到NA结果。但是如果我添加一个2009-12-01一天，并相应地更改格式字符串，我会得到一个结果。示例：

 > strptime（2009-12，format =％Y-％m）
 [1] NA 
> strptime（2009-12-03，format =％Y-％m-％d）
 [1]2009-12-03
  pre> 
 
 
为什么这样？
 
 
 
 更新：我很好奇大概是为什么strptime不解析一年和一个月，因为它似乎不奇怪的原因是它不会这样做是因为它仅解析一年，或一年一次-day：
 > strptime（2009，format =％Y）＃年。作品。使用当前月份和日期作为默认值。 
 [1]2009-12-02
> strptime（2009-03，format =％Y-％d）＃年和日。作品。将当前月份用作默认值。 
 [1]2009-12-03
> strptime（2009-03，format =％Y-％m）＃年和月。不行？ 
 [1] NA 
  
 更新以解释为什么这不是重复的 
可能的重复在这个问题后几年被问到，它涉及到一个单独的API在 R ： asDate 函数。这个问题是关于 strptime 函数的一个怪癖，其中 R  3.1.3仍然适用。
解决方案
对我来说似乎是明智的行为。正如我所看到的，一个更好的问题将是为什么它允许你这样做： strptime（2009-03，format =％Y-％d）？
 
 
 
以下是解决我想要实现的解决方法（即，具有指定的月份和年份但今天的POSIXlt对象）：
  as.POSIXlt（paste（2009-12，days（Sys.Date（）），sep = - ） ）
  
 
I'm getting a result I don't understand in R.

If I use strptime with a year and day formatted %Y-%m (like "2009-12"), I get an NA result. But if I add a day, like "2009-12-01", and change the format string accordingly, I do get a result. Example:
> strptime("2009-12",format="%Y-%m")
[1] NA
> strptime("2009-12-03",format="%Y-%m-%d")
[1] "2009-12-03"
Why is that?

Update: The thing I'm curious about is why strptime doesn't parse a year and a month, and the reason it seems weird that it wouldn't do so is because it does parse a year only, or a year-and-a-day:
> strptime("2009",format="%Y") # year only. Works. Uses current month and day as defaults.
[1] "2009-12-02"
> strptime("2009-03",format="%Y-%d") # year and day. Works. Uses current month as default.
[1] "2009-12-03"
> strptime("2009-03",format="%Y-%m") # year and month. Doesn't work. ?
[1] NA
Update to explain why this is not a duplicate
The possible duplicate was asked a few years after this question and it is concerned with a separate API in R: the asDate function. This question is about a quirk of the strptime function that as of R 3.1.3 still applies.
解决方案
That seems like sensible behavior to me.  As I see it, a better question would be "why does it allow you to do this: strptime("2009-03",format="%Y-%d")?"

Here's a workaround to get what I think you're trying to achieve (i.e. a POSIXlt object with a specified month and year, but today's day):
as.POSIXlt(paste("2009-12", days(Sys.Date()), sep="-"))


                        
这篇关于为什么解析“％Y-％m”在R中使用strptime得到NA结果，但是“％Y-％m-％d”作品？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！