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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: <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
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
<[EMAIL PROTECTED]> wrote:
> On Tue, 5 Oct 2004 "[EMAIL PROTECTED]" (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.
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