# Re: [Rd] Surprising length() of POSIXlt vector (PR#14073)

From: <maechler_at_stat.math.ethz.ch>
Date: Fri, 20 Nov 2009 19:05:30 +0100 (CET)

PD> mark_at_celos.net wrote:
>> Arrays of POSIXlt dates always return a length of 9. This
>> is correct (they're really lists of vectors of seconds,
>> hours, and so forth), but other methods disguise them as
>> flat vectors, giving superficially surprising behaviour:
>>
>> strings <- paste('2009-1-', 1:31, sep='')
>> dates <- strptime(strings, format="%Y-%m-%d")
>>
>> print(dates)
>> # [1] "2009-01-01" "2009-01-02" "2009-01-03" "2009-01-04" "2009-01-05"
>> # [6] "2009-01-06" "2009-01-07" "2009-01-08" "2009-01-09" "2009-01-10"
>> # [11] "2009-01-11" "2009-01-12" "2009-01-13" "2009-01-14" "2009-01-15"
>> # [16] "2009-01-16" "2009-01-17" "2009-01-18" "2009-01-19" "2009-01-20"
>> # [21] "2009-01-21" "2009-01-22" "2009-01-23" "2009-01-24" "2009-01-25"
>> # [26] "2009-01-26" "2009-01-27" "2009-01-28" "2009-01-29" "2009-01-30"
>> # [31] "2009-01-31"
>>
>> print(length(dates))
>> # [1] 9
>>
>> str(dates)
>> # POSIXlt[1:9], format: "2009-01-01" "2009-01-02" "2009-01-03" "2009-01-04" ...
>>
>> print(dates[20])
>> # [1] "2009-01-20"
>>
>> print(length(dates[20]))
>> # [1] 9
>>
>> I've since realised that POSIXct makes date vectors easier,
>> but could we also have something like:
>>
>> length.POSIXlt <- function(x) { length(x\$sec) }
>>
>> in datetime.R, to avoid breaking functions (like the
>> str.POSIXt method) which use length() in this way?

PD> [You need "wishlist" in the title for this sort of stuff.]

```    PD> I'd be wary of this. Just the other day we found that identical() broke
PD> on some objects because a package had length() redefined as a class
PD> method. I.e. the danger is that something wants to use length() with its
PD> original low-level interpretation.

```

Yes, of course.
and Romain mentioned str(). Note that we have needed to define a "POSIXt" method for str(), partly just *because* of the current anomaly:
As Tony Plate, e.g., has argued, entirely correctly in my view, the anomaly is that length() and "[" are not compatible; and while I think no R language definition says that they should be, I still believe that you need very good reasons for them to be incompatible, as they are for POSIXlt.

In the current case, for me the only good reason is backwards compatibility.
My personal taste would be to change it and see what happens. I would be willing to clean up after that change within R 'base' and all packages I am coauthoring (quite a few), but of course there are still a thousand more R packages.. My strong bet would be that less than 1% would be affected, and my point guess for the percentage affected would be rather in the order of 1/1000.

The question is if we (you too!), the R community, are willing to bear the load of cleanup, after such a change which would really *improve* consistency of that small corner of R. For me, as I indicated above, I am willing to bear my share (and actually have got it ready for R-devel)

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https://stat.ethz.ch/mailman/listinfo/r-devel Received on Fri 20 Nov 2009 - 18:09:21 GMT

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