将字符串转换为R中的日期

答

已更新：通过 @进行了改进理查德·斯克里文（Richard Scriven）的 colClasses 和更简单的 as.Date（）建议

Updated: Improved with @Richard Scriven's colClasses and simpler as.Date() suggestions

这里有两种对我有用的方法，从包含 mmddyyyy 格式日期的csv到被R识别为日期对象的csv 。

Here are two similar methods that worked for me, going from a csv containing mmddyyyy format date, to getting it recognized by R as a date object.

首先从一个简单的文件tv.csv开始：

Starting first with a simple file tv.csv:

Series,FirstAir
Quantico,09272015
Muppets,09222015

方法1：全部作为字符串

在R内一次，

Method 1: All as string

Once within R,

> t = read.csv('tv.csv', colClasses = 'character')

• 导入 tv.csv 作为名为 t
的数据框

• colClasses ='character'）选项会使所有数据被视为 character 数据类型（而不是 Factor ， int 类型）


• imports tv.csv as a data frame named t
• colClasses = 'character') option causes all the data to be considered the character data type (instead of being Factor, int types)

检查其初始结构：

> str(t)
'data.frame':   2 obs. of  2 variables:
 $ Series  : chr  "Quantico" "Muppets"
 $ FirstAir: chr  "09272015" "09222015"

• R已将所有字符导入为字符串，在此表示为 chr


• R has imported all as strings of characters, indicated here as type chr

然后轻松地将 chr 或字符串转换为日期：

The chr or string of characters are then easily converted into a date:

> t$FirstAir = as.Date(t$FirstAir, "%m%d%Y")

• as.Date（）执行字符串到日期的转换


• ％m％ d％Y 指定如何解释 t $ FirstAir 中的输入。这些格式代码，至少在Linux上，可以通过运行 $ man date 找到，这会在 date 上提供手册。 >程序，其中包含格式代码列表。例如，它说％m月（01..12）


• as.Date() performs string to date conversion
• %m%d%Y specifies how to interpret the input in t$FirstAir. These format codes, at least on Linux, can be found with running $ man date which brings up the manual on the date program, where there is a list of formatting codes. For example it says %m month (01..12)

如果由于某种原因您不希望将所有字符全部转换，例如，一个包含多个变量的文件，并且希望保留R的自动类型识别功能，但仅修复一个日期变量，请遵循此方法。

If for some reason you don't want a blanket import conversion to all characters, for example a file with many variables and wish to leave R's auto type recognition in use but merely "fix" the one date variable, follow this method.

一旦在R内，

> t = read.csv('tv.csv')

• 导入 tv.csv 作为名为 t


• imports tv.csv as a data frame named t

检查其初始结构：

> str(t)
'data.frame':   2 obs. of  2 variables:
 $ Series  : Factor w/ 2 levels "Muppets","Quantico": 2 1
 $ FirstAir: int  9272015 9222015
>

• R会尽力猜测每个变量的变量类型


• 您可以立即看到一个问题，因为 FirstAir 变量R导入了 09272015 因为 int 表示整数，并且删除了前导零填充，所以09中的0对于稍后的日期转换很重要，而R却没有导入。因此，我们需要解决此问题。


• R tries its best to guess the variable type per variable
• As you can see an immediate problem is, for FirstAir variable R has imported 09272015 as int meaning integer, and dropped off the leading zero padding , the 0 in 09 is important later for date conversion yet R has imported it without. So we need to fix this.

这可以在一个命令中完成，但为清楚起见，我将其分为两个步骤。首先，

This can be done in a single command but for clarity I have broken this into two steps. First,

> t$FirstAir = sprintf("%08d", t$FirstAir)

• sprintf 是一种格式化函数


• 0 表示填充为零


• 8 表示确保8个字符，因为mmddyyyyy总共8个字符


• d 在输入为数字（当前为数字）时使用，请回想 str（）输出要求 t $ FirstAir 是 int 的意思是整数


• t $ FirstAir 是我们正在设置并用作输入的变量


• sprintf is a formatting function
• 0 means pad with zeroes
• 8 means ensure 8 characters, because mmddyyyy is total 8 characters
• d is used when the input is a number, which currently it is, recall str() output claimed the t$FirstAir is an int meaning integer
• t$FirstAir is the variable we are both setting and using as input

检查结果：

> str(t$FirstAir)
 chr [1:2] "09272015" "09222015"

• 已成功将其从 int 转换为 chr 类型，例如 9272015 变为 09272015


• it successfully converted from an int to a chr type, for example 9272015 became "09272015"

现在它是字符串或 chr 类型，然后我们可以进行转换，与方法1相同。

Now it is a string or chr type we can then convert, same as method 1.

> t$FirstAir = as.Date(strptime(t$FirstAir, "%m%d%Y"))

结果

我们做最后检查：

Result

We do a final check:

> str(t$FirstAir)
 Date[1:2], format: "2015-09-27" "2015-09-22"

在这两种情况下，文本文件中的原始值现在都已成功转换为R日期对象。

In both cases, what were original values in a text file are have now been successfully converted into R date objects.