Re: [R] diff, POSIXct, POSIXlt, POSIXt
Gabor == Gabor Grothendieck [EMAIL PROTECTED] on Sun, 23 Jul 2006 09:02:35 -0400 writes: Gabor Moving this to r-devel. [would have been a good idea ... but you didn't; I think it's too late now; rather keep the msg thread together] Gabor Looking at the diff.POSIXt Gabor code we see the problem is that it takes the length Gabor of the input using length which is wrong since in the Gabor case of POSIXlt the length is always 9 (or maybe Gabor length should be defined differently for POSIXlt?). Though I agree with Spencer that a user may expect length() to behave differently, but I don't think this would be a good idea. Yes, length() is generic, but its help() emphasizes that for lists, length() should be the number of list elements. Of course anyone one *can* define length() methods that behave differently for his/her classes, but then one would also want to make sure that e.g. x[length(x)] or 'x[length(x)] - value' works and -- in a case of simple S3 class built on a list, would work differently than if x was a the simple list. In my view, I would only consider redefing length() for non-basic S4 classes, i.e. those with slots, where no confusion is possible, since these objects are definitely not simple vectors nor lists (aka generic vectors). Gabor Try this which gives the same problem: Gabordts[-1] - dts[-length(dts)] Gabor We get a more sensible answer if length is calculated Gabor correctly: Gabor dts[-1] - dts[-length(dts[[1]])] Yes, thanks Gabor, and thanks to Patrick who is right that this is a bug and diff() should work for both kinds of POSIXt objects. I'll fix this for both R-patched and R-devel - but not via redefining length(POSIXlt). Martin Maechler, ETH Zurich Gabor On 7/23/06, Patrick Giraudoux Gabor [EMAIL PROTECTED] wrote: Try converting to POSIXct: That's what I did finally (see the previous e-mail). dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days What is the problem you are trying to solve? Actually, I don't understand why using diff() and POSIXct provides the expected result and not using POSIXlt. Both POSIXct and POSIXlt are of class POSIXt. The doc of diff() stresses that 'diff' is a generic function with a default method and ones for classes 'ts', 'POSIXt' and 'Date'. It does not mention differences between POSIXct and POSIXlt. Moreover, using diff() with POSIXlt has provided (wrong) numbers... and not an error. This may be difficult to detect sometimes along programme lines. Must one keep in mind that diff() is reliably applicable only on POSIXct? In this case, should not it bve mentionned in the documentation? All the best, Patrick jim holtman a écrit : Try converting to POSIXct: str(dts) 'POSIXlt', format: chr [1:10] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 ...dts [1] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 2005-07-15 [9] 2005-10-15 2006-04-15 dts - as.POSIXct(dts) dts [1] 2003-04-15 EDT 2003-07-15 EDT 2003-10-15 EDT 2004-04-15 EDT 2004-07-15 EDT 2004-10-15 EDT [7] 2005-04-15 EDT 2005-07-15 EDT 2005-10-15 EDT 2006-04-15 EDT diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days On 7/23/06, *Patrick Giraudoux* [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] wrote: Dear Listers, I have encountered a strange problem using diff() and POSIXt: dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - strptime(dts, %d/%m/%Y) class(dts) [1] POSIXt POSIXlt diff(dts) Time differences of 7862400, 7948800, 15811200, 7862400, 7948800, 15724800, 7862400, 7948800, 0 secs In this case the result is not the one expected: expressed in seconds and not in days, and the difference between the two last dates is not 0. Now, if one use a vector of 9 dates only (whatever the date removed), things come well: diff(dts[-1]) Time differences of 92, 183, 91, 92, 182, 91, 92, 182 days Also if one contrains dts to POSIXct dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days
Re: [R] diff, POSIXct, POSIXlt, POSIXt
Try converting to POSIXct: str(dts) 'POSIXlt', format: chr [1:10] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 ... dts [1] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 2005-07-15 [9] 2005-10-15 2006-04-15 dts - as.POSIXct(dts) dts [1] 2003-04-15 EDT 2003-07-15 EDT 2003-10-15 EDT 2004-04-15 EDT 2004-07-15 EDT 2004-10-15 EDT [7] 2005-04-15 EDT 2005-07-15 EDT 2005-10-15 EDT 2006-04-15 EDT diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days On 7/23/06, Patrick Giraudoux [EMAIL PROTECTED] wrote: Dear Listers, I have encountered a strange problem using diff() and POSIXt: dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - strptime(dts, %d/%m/%Y) class(dts) [1] POSIXt POSIXlt diff(dts) Time differences of 7862400, 7948800, 15811200, 7862400, 7948800, 15724800, 7862400, 7948800,0 secs In this case the result is not the one expected: expressed in seconds and not in days, and the difference between the two last dates is not 0. Now, if one use a vector of 9 dates only (whatever the date removed), things come well: diff(dts[-1]) Time differences of 92, 183, 91, 92, 182, 91, 92, 182 days Also if one contrains dts to POSIXct dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days Any rational in that? Patrick __ R-help@stat.math.ethz.ch 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. -- Jim Holtman Cincinnati, OH +1 513 646 9390 What is the problem you are trying to solve? [[alternative HTML version deleted]] __ R-help@stat.math.ethz.ch 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.
Re: [R] diff, POSIXct, POSIXlt, POSIXt
Try converting to POSIXct: That's what I did finally (see the previous e-mail). dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days What is the problem you are trying to solve? Actually, I don't understand why using diff() and POSIXct provides the expected result and not using POSIXlt. Both POSIXct and POSIXlt are of class POSIXt. The doc of diff() stresses that 'diff' is a generic function with a default method and ones for classes 'ts', 'POSIXt' and 'Date'. It does not mention differences between POSIXct and POSIXlt. Moreover, using diff() with POSIXlt has provided (wrong) numbers... and not an error. This may be difficult to detect sometimes along programme lines. Must one keep in mind that diff() is reliably applicable only on POSIXct? In this case, should not it bve mentionned in the documentation? All the best, Patrick jim holtman a écrit : Try converting to POSIXct: str(dts) 'POSIXlt', format: chr [1:10] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 ... dts [1] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 2005-07-15 [9] 2005-10-15 2006-04-15 dts - as.POSIXct(dts) dts [1] 2003-04-15 EDT 2003-07-15 EDT 2003-10-15 EDT 2004-04-15 EDT 2004-07-15 EDT 2004-10-15 EDT [7] 2005-04-15 EDT 2005-07-15 EDT 2005-10-15 EDT 2006-04-15 EDT diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days On 7/23/06, *Patrick Giraudoux* [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] wrote: Dear Listers, I have encountered a strange problem using diff() and POSIXt: dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - strptime(dts, %d/%m/%Y) class(dts) [1] POSIXt POSIXlt diff(dts) Time differences of 7862400, 7948800, 15811200, 7862400, 7948800, 15724800, 7862400, 7948800,0 secs In this case the result is not the one expected: expressed in seconds and not in days, and the difference between the two last dates is not 0. Now, if one use a vector of 9 dates only (whatever the date removed), things come well: diff(dts[-1]) Time differences of 92, 183, 91, 92, 182, 91, 92, 182 days Also if one contrains dts to POSIXct dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days Any rational in that? Patrick __ R-help@stat.math.ethz.ch mailto:R-help@stat.math.ethz.ch 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. -- Jim Holtman Cincinnati, OH +1 513 646 9390 What is the problem you are trying to solve? [[alternative HTML version deleted]] __ R-help@stat.math.ethz.ch 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.
Re: [R] diff, POSIXct, POSIXlt, POSIXt
Moving this to r-devel. Looking at the diff.POSIXt code we see the problem is that it takes the length of the input using length which is wrong since in the case of POSIXlt the length is always 9 (or maybe length should be defined differently for POSIXlt?). Try this which gives the same problem: dts[-1] - dts[-length(dts)] We get a more sensible answer if length is calculated correctly: dts[-1] - dts[-length(dts[[1]])] On 7/23/06, Patrick Giraudoux [EMAIL PROTECTED] wrote: Try converting to POSIXct: That's what I did finally (see the previous e-mail). dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days What is the problem you are trying to solve? Actually, I don't understand why using diff() and POSIXct provides the expected result and not using POSIXlt. Both POSIXct and POSIXlt are of class POSIXt. The doc of diff() stresses that 'diff' is a generic function with a default method and ones for classes 'ts', 'POSIXt' and 'Date'. It does not mention differences between POSIXct and POSIXlt. Moreover, using diff() with POSIXlt has provided (wrong) numbers... and not an error. This may be difficult to detect sometimes along programme lines. Must one keep in mind that diff() is reliably applicable only on POSIXct? In this case, should not it bve mentionned in the documentation? All the best, Patrick jim holtman a écrit : Try converting to POSIXct: str(dts) 'POSIXlt', format: chr [1:10] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 ... dts [1] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 2005-07-15 [9] 2005-10-15 2006-04-15 dts - as.POSIXct(dts) dts [1] 2003-04-15 EDT 2003-07-15 EDT 2003-10-15 EDT 2004-04-15 EDT 2004-07-15 EDT 2004-10-15 EDT [7] 2005-04-15 EDT 2005-07-15 EDT 2005-10-15 EDT 2006-04-15 EDT diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days On 7/23/06, *Patrick Giraudoux* [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] wrote: Dear Listers, I have encountered a strange problem using diff() and POSIXt: dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - strptime(dts, %d/%m/%Y) class(dts) [1] POSIXt POSIXlt diff(dts) Time differences of 7862400, 7948800, 15811200, 7862400, 7948800, 15724800, 7862400, 7948800,0 secs In this case the result is not the one expected: expressed in seconds and not in days, and the difference between the two last dates is not 0. Now, if one use a vector of 9 dates only (whatever the date removed), things come well: diff(dts[-1]) Time differences of 92, 183, 91, 92, 182, 91, 92, 182 days Also if one contrains dts to POSIXct dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days Any rational in that? Patrick __ R-help@stat.math.ethz.ch mailto:R-help@stat.math.ethz.ch 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. -- Jim Holtman Cincinnati, OH +1 513 646 9390 What is the problem you are trying to solve? [[alternative HTML version deleted]] __ R-help@stat.math.ethz.ch 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. __ R-help@stat.math.ethz.ch 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.
Re: [R] diff, POSIXct, POSIXlt, POSIXt
Hi, Gabor: For my 0.02 euros, I vote to make length(POSIXlt) = length of the series, NOT the length of the list = 9 always. I've stubbed my toe on that one many times. I always fix it by converting first to POSIXct. The key question is what would users naively expect to get from length(a_time_series)? I think most people not familiar with the POSIXlt format would expect the number of observations. After struggling for a while with code that did not perform as expected, I finally traced one such problem to the fact that length(a_time_series)= 9 if class(a_time_series) = POSIXlt, independent of the number of observations. How much code would break if this was changed? Each use of length(POSIXlt_object) would have to be replaced by something like length(as.list(POSIXlt_object)). However, since length(POSIXlt_object) is always 9, I doubt if length(POSIXlt_object) occurs very often. Currently to get the number of observations in a POSIXlt_object, you might find constructs like length(POSIXlt_object[[1]]). Or you will find people converting the POSIXlt to POSIXct and then computing the length. In either case, changing length(POSIXlt_object) to the number of observations would not break any of this code. Thanks for raising this question. Spencer Graves Gabor Grothendieck wrote: Moving this to r-devel. Looking at the diff.POSIXt code we see the problem is that it takes the length of the input using length which is wrong since in the case of POSIXlt the length is always 9 (or maybe length should be defined differently for POSIXlt?). Try this which gives the same problem: dts[-1] - dts[-length(dts)] We get a more sensible answer if length is calculated correctly: dts[-1] - dts[-length(dts[[1]])] On 7/23/06, Patrick Giraudoux [EMAIL PROTECTED] wrote: Try converting to POSIXct: That's what I did finally (see the previous e-mail). dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days What is the problem you are trying to solve? Actually, I don't understand why using diff() and POSIXct provides the expected result and not using POSIXlt. Both POSIXct and POSIXlt are of class POSIXt. The doc of diff() stresses that 'diff' is a generic function with a default method and ones for classes 'ts', 'POSIXt' and 'Date'. It does not mention differences between POSIXct and POSIXlt. Moreover, using diff() with POSIXlt has provided (wrong) numbers... and not an error. This may be difficult to detect sometimes along programme lines. Must one keep in mind that diff() is reliably applicable only on POSIXct? In this case, should not it bve mentionned in the documentation? All the best, Patrick jim holtman a écrit : Try converting to POSIXct: str(dts) 'POSIXlt', format: chr [1:10] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 ... dts [1] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 2005-07-15 [9] 2005-10-15 2006-04-15 dts - as.POSIXct(dts) dts [1] 2003-04-15 EDT 2003-07-15 EDT 2003-10-15 EDT 2004-04-15 EDT 2004-07-15 EDT 2004-10-15 EDT [7] 2005-04-15 EDT 2005-07-15 EDT 2005-10-15 EDT 2006-04-15 EDT diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days On 7/23/06, *Patrick Giraudoux* [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] wrote: Dear Listers, I have encountered a strange problem using diff() and POSIXt: dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - strptime(dts, %d/%m/%Y) class(dts) [1] POSIXt POSIXlt diff(dts) Time differences of 7862400, 7948800, 15811200, 7862400, 7948800, 15724800, 7862400, 7948800,0 secs In this case the result is not the one expected: expressed in seconds and not in days, and the difference between the two last dates is not 0. Now, if one use a vector of 9 dates only (whatever the date removed), things come well: diff(dts[-1]) Time differences of 92, 183, 91, 92, 182, 91, 92, 182 days Also if one contrains dts to POSIXct dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days Any rational in that? Patrick __ R-help@stat.math.ethz.ch mailto:R-help@stat.math.ethz.ch 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
Re: [R] diff, POSIXct, POSIXlt, POSIXt
Just one more comment. It is possible to define length.POSIXlt yourself in which case diff works with POSIXlt objects. length.POSIXlt - function(x) length(x[[1]]) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days On 7/23/06, Gabor Grothendieck [EMAIL PROTECTED] wrote: Moving this to r-devel. Looking at the diff.POSIXt code we see the problem is that it takes the length of the input using length which is wrong since in the case of POSIXlt the length is always 9 (or maybe length should be defined differently for POSIXlt?). Try this which gives the same problem: dts[-1] - dts[-length(dts)] We get a more sensible answer if length is calculated correctly: dts[-1] - dts[-length(dts[[1]])] On 7/23/06, Patrick Giraudoux [EMAIL PROTECTED] wrote: Try converting to POSIXct: That's what I did finally (see the previous e-mail). dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days What is the problem you are trying to solve? Actually, I don't understand why using diff() and POSIXct provides the expected result and not using POSIXlt. Both POSIXct and POSIXlt are of class POSIXt. The doc of diff() stresses that 'diff' is a generic function with a default method and ones for classes 'ts', 'POSIXt' and 'Date'. It does not mention differences between POSIXct and POSIXlt. Moreover, using diff() with POSIXlt has provided (wrong) numbers... and not an error. This may be difficult to detect sometimes along programme lines. Must one keep in mind that diff() is reliably applicable only on POSIXct? In this case, should not it bve mentionned in the documentation? All the best, Patrick jim holtman a écrit : Try converting to POSIXct: str(dts) 'POSIXlt', format: chr [1:10] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 ... dts [1] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 2005-07-15 [9] 2005-10-15 2006-04-15 dts - as.POSIXct(dts) dts [1] 2003-04-15 EDT 2003-07-15 EDT 2003-10-15 EDT 2004-04-15 EDT 2004-07-15 EDT 2004-10-15 EDT [7] 2005-04-15 EDT 2005-07-15 EDT 2005-10-15 EDT 2006-04-15 EDT diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days On 7/23/06, *Patrick Giraudoux* [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] wrote: Dear Listers, I have encountered a strange problem using diff() and POSIXt: dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - strptime(dts, %d/%m/%Y) class(dts) [1] POSIXt POSIXlt diff(dts) Time differences of 7862400, 7948800, 15811200, 7862400, 7948800, 15724800, 7862400, 7948800,0 secs In this case the result is not the one expected: expressed in seconds and not in days, and the difference between the two last dates is not 0. Now, if one use a vector of 9 dates only (whatever the date removed), things come well: diff(dts[-1]) Time differences of 92, 183, 91, 92, 182, 91, 92, 182 days Also if one contrains dts to POSIXct dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days Any rational in that? Patrick __ R-help@stat.math.ethz.ch mailto:R-help@stat.math.ethz.ch 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. -- Jim Holtman Cincinnati, OH +1 513 646 9390 What is the problem you are trying to solve? [[alternative HTML version deleted]] __ R-help@stat.math.ethz.ch 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. __ R-help@stat.math.ethz.ch 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.
Re: [R] diff, POSIXct, POSIXlt, POSIXt
OK. Got it. Thanks a lot everybody. I however feel that although the problem can be technically handled by any user aware of it, it should be fixed in R in a more general way, either by modifying the diff() code so that it really handles any kind of POSIXt (POSIXlt and POSIXct) with the same final result (as claimed in the documentation), or mentioning explicitely in de documentation that diff(), as it is written currently, can handle correctly only POSIXct (and not any POSIXt or POSIXlt). There is a kind of danger of wrong output for users (even those reading the documentation) if things are left as they are, and I have detected this problem just by chance. All the best, Patrick Gabor Grothendieck a écrit : Just one more comment. It is possible to define length.POSIXlt yourself in which case diff works with POSIXlt objects. length.POSIXlt - function(x) length(x[[1]]) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days On 7/23/06, Gabor Grothendieck [EMAIL PROTECTED] wrote: Moving this to r-devel. Looking at the diff.POSIXt code we see the problem is that it takes the length of the input using length which is wrong since in the case of POSIXlt the length is always 9 (or maybe length should be defined differently for POSIXlt?). Try this which gives the same problem: dts[-1] - dts[-length(dts)] We get a more sensible answer if length is calculated correctly: dts[-1] - dts[-length(dts[[1]])] On 7/23/06, Patrick Giraudoux [EMAIL PROTECTED] wrote: Try converting to POSIXct: That's what I did finally (see the previous e-mail). dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days What is the problem you are trying to solve? Actually, I don't understand why using diff() and POSIXct provides the expected result and not using POSIXlt. Both POSIXct and POSIXlt are of class POSIXt. The doc of diff() stresses that 'diff' is a generic function with a default method and ones for classes 'ts', 'POSIXt' and 'Date'. It does not mention differences between POSIXct and POSIXlt. Moreover, using diff() with POSIXlt has provided (wrong) numbers... and not an error. This may be difficult to detect sometimes along programme lines. Must one keep in mind that diff() is reliably applicable only on POSIXct? In this case, should not it bve mentionned in the documentation? All the best, Patrick jim holtman a écrit : Try converting to POSIXct: str(dts) 'POSIXlt', format: chr [1:10] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 ... dts [1] 2003-04-15 2003-07-15 2003-10-15 2004-04-15 2004-07-15 2004-10-15 2005-04-15 2005-07-15 [9] 2005-10-15 2006-04-15 dts - as.POSIXct(dts) dts [1] 2003-04-15 EDT 2003-07-15 EDT 2003-10-15 EDT 2004-04-15 EDT 2004-07-15 EDT 2004-10-15 EDT [7] 2005-04-15 EDT 2005-07-15 EDT 2005-10-15 EDT 2006-04-15 EDT diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days On 7/23/06, *Patrick Giraudoux* [EMAIL PROTECTED] mailto:[EMAIL PROTECTED] wrote: Dear Listers, I have encountered a strange problem using diff() and POSIXt: dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - strptime(dts, %d/%m/%Y) class(dts) [1] POSIXt POSIXlt diff(dts) Time differences of 7862400, 7948800, 15811200, 7862400, 7948800, 15724800, 7862400, 7948800,0 secs In this case the result is not the one expected: expressed in seconds and not in days, and the difference between the two last dates is not 0. Now, if one use a vector of 9 dates only (whatever the date removed), things come well: diff(dts[-1]) Time differences of 92, 183, 91, 92, 182, 91, 92, 182 days Also if one contrains dts to POSIXct dts-c(15/4/2003,15/7/2003,15/10/2003,15/04/2004,15/07/2004,15/10/2004,15/4/2005,15/07/2005,15/10/2005,15/4/2006) dts - as.POSIXct(strptime(dts, %d/%m/%Y)) diff(dts) Time differences of 91, 92, 183, 91, 92, 182, 91, 92, 182 days Any rational in that? Patrick __ R-help@stat.math.ethz.ch mailto:R-help@stat.math.ethz.ch 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. -- Jim Holtman Cincinnati, OH +1 513
