[R] Strptime/ date time classes

[Mark Difford]

Hi Caroline,

Because POSIXlt is a complicated structure: you are dealing with a list, not
with what you think you are. Maybe this will help you to see more clearly.

strptime(19800604062759, format="%Y%m%d%H%M%S")
[1] "1980-06-04 06:27:59"

str(strptime(19800604062759, format="%Y%m%d%H%M%S"))
  POSIXlt[1:9], format: "1980-06-04 06:27:59"  ## length == 9

strptime(c(19800604062759,19800604062800), format="%Y%m%d%H%M%S")
[1] "1980-06-04 06:27:59" "1980-06-04 06:28:00"
str(strptime(c(19800604062759,19800604062800), format="%Y%m%d%H%M%S"))
  POSIXlt[1:9], format: "1980-06-04 06:27:59" "1980-06-04 06:28:00"  ##
length == 9

typeof(strptime(c(19800604062759,19800604062800), format="%Y%m%d%H%M%S"))
[1] "list"

length(unlist(strptime(c(19800604062759,19800604062800),
format="%Y%m%d%H%M%S")))
[1] 18  ## 9 * 2 == 18

HTH you on your way,
Mark.


Caroline Keef wrote:
> 
> Dear all,
> 
> I've come across a problem using strptime, can anyone explain what's
> going on?  I'm using version 2.7.0 on Windows XP.
> 
> Thank you
> 
> Caroline
> 
> First read in a data file using read.table
> 
> alldata = read.table(file, header=F, skip=4, colClasses =
> c("character","numeric"))
> 
> dim(alldata)
> [1] 223960      2
> 
> # inefficient, safe way of sorting out missing or dodgy data
> 
> alldata[,2][alldata[,2] < 0] = NA
> 
> # first ten lines of the data
> 
>  alldata[1:10,]
>                V1    V2
> 1  19800604062759    NA
> 2  19800604062800 0.271
> 3  19800604111900 0.286
> 4  19800604134300 0.362
> 5  19800604144400 0.465
> 6  19800604163300 0.510
> 7  19800604175400 0.518
> 8  19800604185100 0.526
> 9  19800611110900    NA
> 10 19800611110959    NA
> 
> #Then convert the first column using strptime
> 
> datetimes = strptime(alldata[,1],format="%Y%m%d%H%M%S")
>   
> #Then I want to get minimum and maximum, but some seem to be missing
> when they aren't.
> 
> length(as.POSIXlt(datetimes))  #also equal to length(datetimes)
> 
> [1] 9
>   
> # Why isn't this 223960?  Is it something to do with the class?
> 
> # This is the really puzzling bit (to me anyway)
> 
> a =(1:223960)[is.na(datetimes)]
> 
> # which gives 
> 1462  14295  18744  50499  50500  92472  92473  92474  92475  92476
> 137525 137526 137527 171066 171067 192353
> 
> # 16 values
> 
>  alldata[a,]
>                    V1    V2
> 1462   19810329012000 0.983
> 14295  19900325014300 0.219
> 18744  19920329014300 0.246
> 50499  19960331013000 0.564
> 50500  19960331015700 0.563
> 92472  19970330010200 0.173
> 92473  19970330011400 0.172
> 92474  19970330012700 0.172
> 92475  19970330014400 0.172
> 92476  19970330015500 0.172
> 137525 19980329011600 0.427
> 137526 19980329014100 0.427
> 137527 19980329015600 0.427
> 171066 19990328010300 0.223
> 171067 19990328011800 0.223
> 192353 20000326012800 0.189
> 
>  datetimes[a]
>  [1] "1981-03-29 01:20:00" "1990-03-25 01:43:00" "1992-03-29 01:43:00"
> "1996-03-31 01:30:00" "1996-03-31 01:57:00"  [6] "1997-03-30 01:02:00"
> "1997-03-30 01:14:00" "1997-03-30 01:27:00" "1997-03-30 01:44:00"
> "1997-03-30 01:55:00" [11] "1998-03-29 01:16:00" "1998-03-29 01:41:00"
> "1998-03-29 01:56:00" "1999-03-28 01:03:00" "1999-03-28 01:18:00" [16]
> "2000-03-26 01:28:00"
> 
> # They're all around the end of March!  I've looked at the data file and
> I can't see anything funny in it around these dates.
> 
> 
> 
> The first few lines of the data file look like
> 
> #TZUTC+0|*|SANR08002|*|SNAMENAUL|*|SWATERDELVIN|*|CNR98808|*|
> #CNAMEQ|*|CTYPEn-min-ip|*|CMW1440|*|RTIMELVLhigh-resolution|*|
> #CUNITm3/s|*|RINVAL-777|*|RNR-1|*|REXCHANGE98913|*|
> #RTYPEinstantaneous values|*|
> 19800604062759 -777.0  
> 19800604062800 0.271  
> 19800604111900 0.286  
> 19800604134300 0.362  
> 19800604144400 0.465  
> 19800604163300 0.510  
> 19800604175400 0.518  
> 19800604185100 0.526  
> 19800611110900 -777.0  
> 19800611110959 -777.0  
> 19800611111000 0.100  
> 19800611211400 0.096  
> 19800612000000 0.096  
> 19800612065000 0.098  
> 19800612133400 0.100  
> 
> 
> 
> 
> 
> Caroline KeefJBA Consulting
> South Barn, Broughton Hall, Skipton, North Yorkshire, BD23 3AE, UK
> t: +44 (0)1756 799919  f: +44 (0)1756 799449  
> 
> JBA Consulting now incorporates Maslen Environmental, the award winning
> environmental regeneration consultancy.
> http://www.maslen-environmental.com. 
> 
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> 
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