swirl学习之十四——Dates and Times

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1: R Programming
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 1: Basic Building Blocks      2: Workspace and Files     
 3: Sequences of Numbers       4: Vectors                 
 5: Missing Values             6: Subsetting Vectors      
 7: Matrices and Data Frames   8: Logic                   
 9: Functions                 10: lapply and sapply       
11: vapply and tapply         12: Looking at Data         
13: Simulation                14: Dates and Times         
15: Base Graphics             

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| R has a special way of representing dates and times, which
| can be helpful if you're working with data that show how
| something changes over time (i.e. time-series data) or if
| your data contain some other temporal information, like
| dates of birth.

...

  |=                                                   |   3%

| Dates are represented by the 'Date' class and times are
| represented by the 'POSIXct' and 'POSIXlt' classes.
| Internally, dates are stored as the number of days since
| 1970-01-01 and times are stored as either the number of
| seconds since 1970-01-01 (for 'POSIXct') or a list of
| seconds, minutes, hours, etc. (for 'POSIXlt').

...

  |===                                                 |   6%

| Let's start by using d1 <- Sys.Date() to get the current| date and store it in the variable d1. (That's the letter| 'd' and the number 1.)> d1<-Sys.Date()| That's a job well done!  |====                                                |   9%| Use the class() function to confirm d1 is a Date object.> class(d1)
| date and store it in the variable d1. (That's the letter
| 'd' and the number 1.)

>

| That's a job well done!

  |====                                                |   9%

| Use the class() function to confirm d1 is a Date object.

>
[1] "Date"

| You nailed it! Good job!

  |======                                              |  11%

| We can use the unclass() function to see what d1 looks like
| internally. Try it out.

> unclass(d1)
[1] 16571

| You got it!

  |=======                                             |  14%

| That's the exact number of days since 1970-01-01!

...

  |=========                                           |  17%

| However, if you print d1 to the console, you'll get today's
| date -- YEAR-MONTH-DAY. Give it a try.

> d1
[1] "2015-05-16"

| Keep working like that and you'll get there!

  |==========                                          |  20%

| What if we need to reference a date prior to 1970-01-01?
| Create a variable d2 containing as.Date("1969-01-01").

> d2<-as.Date("1969-01-01")

| You nailed it! Good job!

  |============                                        |  23%

| Now use unclass() again to see what d2 looks like
| internally.

> unclass(d2)
[1] -365

| You are amazing!

  |=============                                       |  26%

| As you may have anticipated, you get a negative number. In
| this case, it's -365, since 1969-01-01 is exactly one
| calendar year (i.e. 365 days) BEFORE 1970-01-01.

...

  |===============                                     |  29%

| Now, let's take a look at how R stores times. You can
| access the current date and time using the Sys.time()
| function with no arguments. Do this and store the result in
| a variable called t1.

> t1<-Sys.time()

| All that hard work is paying off!

  |================                                    |  31%

| View the contents of t1.

> t1
[1] "2015-05-16 11:08:48 CST"

| That's correct!

  |==================                                  |  34%

| And check the class() of t1.

> class(t1)
[1] "POSIXct" "POSIXt" 

| You're the best!

  |===================                                 |  37%

| As mentioned earlier, POSIXct is just one of two ways that
| R represents time information. (You can ignore the second
| value above, POSIXt, which just functions as a common
| language between POSIXct and POSIXlt.) Use unclass() to see
| what t1 looks like internally -- the (large) number of
| seconds since the beginning of 1970.

> unclass(t1)
[1] 1431745729

| Excellent work!

  |=====================                               |  40%

| By default, Sys.time() returns an object of class POSIXct,
| but we can coerce the result to POSIXlt with
| as.POSIXlt(Sys.time()). Give it a try and store the result
| in t2.

> t2<-as.POSIXlt(Sys.time())

| Excellent job!

  |======================                              |  43%

| Check the class of t2.

> class(t2)
[1] "POSIXlt" "POSIXt" 

| Keep working like that and you'll get there!

  |========================                            |  46%

| Now view its contents.

> t2
[1] "2015-05-16 11:12:57 CST"

| Nice work!

  |=========================                           |  49%

| The printed format of t2 is identical to that of t1. Now
| unclass() t2 to see how it is different internally.

> unclass(t2)
$sec
[1] 57.47574

$min
[1] 12

$hour
[1] 11

$mday
[1] 16

$mon
[1] 4

$year
[1] 115

$wday
[1] 6

$yday
[1] 135

$isdst
[1] 0

$zone
[1] "CST"

$gmtoff
[1] 28800

attr(,"tzone")
[1] ""    "CST" "CDT"

| Nice work!

  |===========================                         |  51%

| t2, like all POSIXlt objects, is just a list of values that
| make up the date and time. Use str(unclass(t2)) to have a
| more compact view.

> str(unclass(t2))
List of 11
 $ sec   : num 57.5
 $ min   : int 12
 $ hour  : int 11
 $ mday  : int 16
 $ mon   : int 4
 $ year  : int 115
 $ wday  : int 6
 $ yday  : int 135
 $ isdst : int 0
 $ zone  : chr "CST"
 $ gmtoff: int 28800
 - attr(*, "tzone")= chr [1:3] "" "CST" "CDT"

| Great job!

  |============================                        |  54%

| If, for example, we want just the minutes from the time
| stored in t2, we can access them with t2$min. Give it a
| try.

> t2$min
[1] 12

| Keep up the great work!

  |==============================                      |  57%

| Now that we have explored all three types of date and time
| objects, let's look at a few functions that extract useful
| information from any of these objects -- weekdays(),
| months(), and quarters().

...

  |===============================                     |  60%

| The weekdays() function will return the day of week from
| any date or time object. Try it out on d1, which is the
| Date object that contains today's date.

> weekdays(d1)
[1] "Saturday"

| You got it right!

  |=================================                   |  63%

| The months() function also works on any date or time
| object. Try it on t1, which is the POSIXct object that
| contains the current time (well, it was the current time
| when you created it).

> months(t1)
[1] "May"

| Excellent work!

  |==================================                  |  66%

| The quarters() function returns the quarter of the year
| (Q1-Q4) from any date or time object. Try it on t2, which
| is the POSIXlt object that contains the time at which you
| created it.

> quarters(t2)
[1] "Q2"

| Keep up the great work!

  |====================================                |  69%

| Often, the dates and times in a dataset will be in a format
| that R does not recognize. The strptime() function can be
| helpful in this situation.

...

  |=====================================               |  71%

| strptime() converts character vectors to POSIXlt. In that
| sense, it is similar to as.POSIXlt(), except that the input
| doesn't have to be in a particular format (YYYY-MM-DD).

...

  |=======================================             |  74%

| To see how it works, store the following character string
| in a variable called t3: "October 17, 1986 08:24" (with the
| quotes).

> t3<-"October 17, 1986 08:24"

| You nailed it! Good job!

  |========================================            |  77%

| Now, use strptime(t3, "%B %d, %Y %H:%M") to help R convert
| our date/time object to a format that it understands.
| Assign the result to a new variable called t4. (You should
| pull up the documentation for strptime() if you'd like to
| know more about how it works.)

> ?strptime
> t4<-strptime(t3,"%B %d, %Y %H:%M")

| You are quite good my friend!

  |==========================================          |  80%

| Print the contents of t4.

> t4
[1] "1986-10-17 08:24:00 CST"

| Perseverance, that's the answer.

  |===========================================         |  83%

| That's the format we've come to expect. Now, let's check
| its class().

> class(t4)
[1] "POSIXlt" "POSIXt" 

| You're the best!

  |=============================================       |  86%

| Finally, there are a number of operations that you can
| perform on dates and times, including arithmetic operations
| (+ and -) and comparisons (<, ==, etc.)

...

  |==============================================      |  89%

| The variable t1 contains the time at which you created it
| (recall you used Sys.time()). Confirm that some time has
| passed since you created t1 by using the 'greater than'
| operator to compare it to the current time: Sys.time() > t1

> Sys.time() > t1
[1] TRUE

| That's a job well done!

  |================================================    |  91%

| So we know that some time has passed, but how much? Try
| subtracting t1 from the current time using Sys.time() - t1.
| Don't forget the parentheses at the end of Sys.time(),
| since it is a function.

> Sys.time() - t1
Time difference of 28.06015 mins

| You are amazing!

  |=================================================   |  94%

| The same line of thinking applies to addition and the other
| comparison operators. If you want more control over the
| units when finding the above difference in times, you can
| use difftime(), which allows you to specify a 'units'
| parameter.

...

  |=================================================== |  97%

| Use difftime(Sys.time(), t1, units = 'days') to find the
| amount of time in DAYS that has passed since you created
| t1.

> difftime(Sys.time(),t1,units='days')
Time difference of 0.02175513 days

| You're the best!

  |====================================================| 100%

| In this lesson, you learned how to work with dates and
| times in R. While it is important to understand the basics,
| if you find yourself working with dates and times often,
| you may want to check out the lubridate package by Hadley
| Wickham.

...

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