Hello,
I am an advanced user of R. Recently I found out that apparently I do
not fully understand vectors and lists fully
Take this code snippet:
T = c("02.03.2008 12:23", "03.03.2008 05:54")
Times = strptime(T, "%d.%m.%Y %H:%M")
Times # OK
class(Times) # OK
is.list(Times) # sort of understand and not understand that
length(Times) # 9 ??? why is it length(Times[1]) ??
Times[1] # OK
Times[[1]] # Wrong
so Times is a vector-style thing of a non-atomic type.
What puzzles me is first that is.list returns true, and more importantly
that the length-query returns the length of the first object of that
apparent list and not how many elements are contained (i.e., I would
have expected 2 as result - and I do not know what syntax to use in
order to get the 2).
Moreover, if the whole thing is part of a data.frame:
DFTimes = as.data.frame(Times)
dim(DFTimes)
length(DFTimes$Times) # OK, 2
then everything is as expected.
Could anyone please clearify why is.list returns true and why length in
the first example returns 9 ? Is it that c() makes a list if the objects
to be concatenated are not representable directly by atomic types ?
thanks,
Thomas
Hello - a9804814 at unet.univie.ac.at wrote:> Hello, > > I am an advanced user of R. Recently I found out that apparently I do > not fully understand vectors and lists fully > Take this code snippet: > > > T = c("02.03.2008 12:23", "03.03.2008 05:54") > Times = strptime(T, "%d.%m.%Y %H:%M") > Times # OK > class(Times) # OK > is.list(Times) # sort of understand and not understand that?POSIXlt says Class '"POSIXlt"' is a named list of vectors representing 'sec' 0-61: seconds 'min' 0-59: minutes 'hour' 0-23: hours 'mday' 1-31: day of the month 'mon' 0-11: months after the first of the year. 'year' Years since 1900. 'wday' 0-6 day of the week, starting on Sunday. 'yday' 0-365: day of the year. 'isdst' Daylight savings time flag. Positive if in force, zero if not, negative if unknown.> length(Times) # 9 ??? why is it length(Times[1]) ??Because Times is a list with 9 elements, try typing names(Times) to see what those elements are called. That will help you understand.> > Times[1] # OK > Times[[1]] # WrongWhether or not this is 'wrong' has been I believe debated before on the list. See this message and the rest of the thread associated with it. http://tolstoy.newcastle.edu.au/R/help/06/07/31508.html - Erik Iverson
On Mar 5, 2008, at 2:56 PM, a9804814 at unet.univie.ac.at wrote:> Hello, > > I am an advanced user of R. Recently I found out that apparently I do > not fully understand vectors and lists fully > Take this code snippet: > > > T = c("02.03.2008 12:23", "03.03.2008 05:54") > Times = strptime(T, "%d.%m.%Y %H:%M") > Times # OKFar from OK, which is where your misunderstanding starts. Times is a POSIXlt object, and behaves as such, not as a vector and not as a list. Rather, a bit of both. The output you see is the result of print(Times), which calls print.POSIXlt(Times), which is: function (x, ...) { print(format(x, usetz = TRUE), ...) invisible(x) } <environment: namespace:base> This in turn calls format.POSIXlt(Times), which is pretty complicated and I'll omit here, but point is what you see when you type Times is not the object TImes itself, but only what the writer of print.POSIXlt wants you to see. Similar to the output of lm, which is much less than what lm really contains. Have a look at ?POSIXlt, which says: Class "POSIXlt" is a named list of vectors representing .... 9 vectors following. Try: names(Times) Times$sec Times$min .... Now perhaps this helps understand the answers?> class(Times) # OK > is.list(Times) # sort of understand and not understand that > length(Times) # 9 ??? why is it length(Times[1]) ?? > > Times[1] # OKNot OK at all. This calls [.POSIXlt, look at: `[.POSIXlt` In particular, try Times[2], Times[3] etc Times[[1]] is the same as Times$sec Times[[2]] is the same as Times$min and so on.> Times[[1]] # Wrong > > > so Times is a vector-style thing of a non-atomic type. > What puzzles me is first that is.list returns true, and more > importantly > that the length-query returns the length of the first object of that > apparent list and not how many elements are contained (i.e., I would > have expected 2 as result - and I do not know what syntax to use in > order to get the 2). > > Moreover, if the whole thing is part of a data.frame: > > DFTimes = as.data.frame(Times) > dim(DFTimes) > length(DFTimes$Times) # OK, 2 > > then everything is as expected. > > Could anyone please clearify why is.list returns true and why > length in > the first example returns 9 ? Is it that c() makes a list if the > objects > to be concatenated are not representable directly by atomic types ? > > thanks, > Thomas >Haris Skiadas Department of Mathematics and Computer Science Hanover College
Read R News 4/1 article on dates. On Wed, Mar 5, 2008 at 2:56 PM, <a9804814 at unet.univie.ac.at> wrote:> Hello, > > I am an advanced user of R. Recently I found out that apparently I do > not fully understand vectors and lists fully > Take this code snippet: > > > T = c("02.03.2008 12:23", "03.03.2008 05:54") > Times = strptime(T, "%d.%m.%Y %H:%M") > Times # OK > class(Times) # OK > is.list(Times) # sort of understand and not understand that > length(Times) # 9 ??? why is it length(Times[1]) ?? > > Times[1] # OK > Times[[1]] # Wrong > > > so Times is a vector-style thing of a non-atomic type. > What puzzles me is first that is.list returns true, and more importantly > that the length-query returns the length of the first object of that > apparent list and not how many elements are contained (i.e., I would > have expected 2 as result - and I do not know what syntax to use in > order to get the 2). > > Moreover, if the whole thing is part of a data.frame: > > DFTimes = as.data.frame(Times) > dim(DFTimes) > length(DFTimes$Times) # OK, 2 > > then everything is as expected. > > Could anyone please clearify why is.list returns true and why length in > the first example returns 9 ? Is it that c() makes a list if the objects > to be concatenated are not representable directly by atomic types ? > > thanks, > Thomas > > ______________________________________________ > R-help at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. >