R help - Oct 2004 - correct my method of estimating mean of two POSIXlt data frames

Hello,  I searched the archives but could not come to a solution.  I
have to two columns of information

t_start_cdt looks like:
> t_start_cdt[1:4]
[1] "2003-07-09 11:02:25" "2003-07-09 11:10:25"
"2003-07-09 11:30:25"
[4] "2003-07-09 12:00:25"
> class(t_start_cdt)
[1] "POSIXt"  "POSIXlt"

t_end_cdt looks like:
> t_end_cdt[1:4]
[1] "2003-07-09 11:02:35" "2003-07-09 11:10:35"
"2003-07-09 11:30:35"
[4] "2003-07-09 12:00:35"
> class(t_end_cdt)
[1] "POSIXt"  "POSIXlt"


I'd like to estimate the mean of each "pair".  For example, the
mean
of (t_start_cdt[1] and t_end_cdt[1]).  The only way I could do this
is:
hi <- cbind(as.matrix(as.POSIXct(t_start_cdt)),
as.matrix(as.POSIXct(t_end_cdt)))
hi <- apply(hi, MARGIN=1, FUN=mean)
class(hi) <- c("POSIXt", "POSIXct")
t_mean_cdt <- as.POSIXlt(hi)
rm(hi)

What am I missing conceptually about POSIXlt, and what is the better method?
thanks,
Mike

<gotrout <at> gmail.com> writes:

: 
: Hello,  I searched the archives but could not come to a solution.  I
: have to two columns of information
: 
: t_start_cdt looks like:
: > t_start_cdt[1:4]
: [1] "2003-07-09 11:02:25" "2003-07-09 11:10:25"
"2003-07-09 11:30:25"
: [4] "2003-07-09 12:00:25"
: > class(t_start_cdt)
: [1] "POSIXt"  "POSIXlt"
: 
: t_end_cdt looks like:
: > t_end_cdt[1:4]
: [1] "2003-07-09 11:02:35" "2003-07-09 11:10:35"
"2003-07-09 11:30:35"
: [4] "2003-07-09 12:00:35"
: > class(t_end_cdt)
: [1] "POSIXt"  "POSIXlt"
: 
: I'd like to estimate the mean of each "pair".  For example, the
mean
: of (t_start_cdt[1] and t_end_cdt[1]).  The only way I could do this
: is:
: hi <- cbind(as.matrix(as.POSIXct(t_start_cdt)),
: as.matrix(as.POSIXct(t_end_cdt)))
: hi <- apply(hi, MARGIN=1, FUN=mean)
: class(hi) <- c("POSIXt", "POSIXct")
: t_mean_cdt <- as.POSIXlt(hi)
: rm(hi)
: 
: What am I missing conceptually about POSIXlt, and what is the better method?
: thanks,


If a.lt and b.lt are the two vectors of POSIXlt dates then try 
converting each to POSIXct and unclassing to make each numeric.
Take the mean of the two numeric vectors and convert them back to
POSIXct and finally to POSIXlt:

a <- unclass(as.POSIXct(a.lt))
b <- unclass(as.POSIXct(b.lt))
as.POSIXlt(structure((a+b)/2, class = c("POSIXt",
"POSIXct")))

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If instead of a data frame of POSIXlt objects you use a matrix
of POSIXct objects then that should work.   That is, replace

   tmp <- data.frame(t.start.cdt, t.end.cdt)

with

   tmp <- cbind(t.start.cdt+0, t.end.cdt+0)

in your code.

Date:   Tue, 5 Oct 2004 14:56:41 -0700 
From:   <gotrout at gmail.com>
To:   <r-help at stat.math.ethz.ch> 
Subject:   Re: [R] correct my method of estimating mean of two POSIXlt data
frames

 
My apology for not properly quoting someone.

Let me amend my original email. In R version 1.8.1, if I have two
objects that look like:
> t.start.cdt[1:4]
[1] "2003-07-09 11:02:25" "2003-07-09 11:10:25"
"2003-07-09 11:30:25"
[4] "2003-07-09 12:00:25"
> class(t.start.cdt)
[1] "POSIXt"  "POSIXlt"
> t.end.cdt[1:4]
[1] "2003-07-09 11:02:35" "2003-07-09 11:10:35"
"2003-07-09 11:30:35"
[4] "2003-07-09 12:00:35"
> class(t.end.cdt)
[1] "POSIXt"  "POSIXlt"

I could estimate the mean of "pairs" of times (such as t.start.cdt[1]
and t.end.cdt[1]) using these commands:
tmp <- data.frame(t.start.cdt, t.end.cdt)
tmp <- apply(tmp, MARGIN=1, FUN=mean)
class(tmp) <- c("POSIXt", "POSIXct")
t.mean.cdt <- as.POSIXlt(tmp)

In version 1.9.1, I get these warnings
Warning messages:
1: longer object length
is not a multiple of shorter object length in: cl == c("Date",
"POSIXct", "POSIXlt")
2: longer object length
is not a multiple of shorter object length in: cl == c("Date",
"POSIXct", "POSIXlt")
3: argument is not numeric or logical: returning NA in:
mean.default(newX[, i], ...)
4: argument is not numeric or logical: returning NA in:
mean.default(newX[, i], ...)

I think I wrote a rather convoluted way around this problem, which I
stated in my original email, but I don't "understand" why the
method
stated here doesn't work. Can you explain what I'm missing.

Also, please note that in my original message, I used underscore
instead of period in the variable names.

Mike
Berkeley, California, USA



On Tue, 5 Oct 2004 21:13:05 +0100 (BST), Prof Brian Ripley
<ripley at stats.ox.ac.uk> wrote:
> On Tue, 5 Oct 2004 "gotrout at gmail.com" (but with no name nor
signature)
> wrote:
> 
> Quoting someone without credit (and therefore in breach of their
> copyright)
> 
> 
> 
> > > If a.lt and b.lt are the two vectors of POSIXlt dates then try
> > > converting each to POSIXct and unclassing to make each numeric.
> > > Take the mean of the two numeric vectors and convert them back to
> >
> > I see. I'm a little confused with the use of class/unclass versus
> > as.XX. For example, instead of using unclass, why wouldn't I use
> > as.numeric? Could someone explain the difference?
> 
> That advice is wrong: you should not be unclassing before forming the
> mean as mean() has a method for POSIXct.