Mean of difftime vectors : "code infelicity" or intended behaviour ?

Dear list,

"+" (and "-") being defined for difftime class, I expected mean() to 
return something sensible. This is only half-true :
mean(c(1:5, 5:1),na.rm=TRUE)
[1] 3
mean(as.difftime(c(1:5, 5:1),unit="mins"),na.rm=TRUE)
Time difference of 3 mins

Fine so far. However :
mean(c(1:5, NA,5:1),na.rm=TRUE)
[1] 3
mean(as.difftime(c(1:5, NA,5:1),unit="mins"),na.rm=TRUE)
Time difference of NA mins

Ouch !

Curiously, var(), max() and min() behave as expected. What's so special 
with mean() ?
RSiteSearch("mean difftime")
[ ... doesn't return anything relevant ]

NB : this isn't done for the hell of it. I intended to replace some 
missing dates, with something computed from other dates and mean time 
intervals).

Any thoughs ?

					Emmanuel Charpentier
Oops. Clicked "Send" too fast (don't shoot, Brian !). I forgot :
sessionInfo()
R version 2.8.1 (2008-12-22) 
i486-pc-linux-gnu 

locale:
LC_CTYPE=fr_FR.UTF-8;LC_NUMERIC=C;LC_TIME=fr_FR.UTF-8;LC_COLLATE=fr_FR.UTF-8;
LC_MONETARY=C;LC_MESSAGES=fr_FR.UTF-8;LC_PAPER=fr_FR.UTF-8;LC_NAME=C;
LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=fr_FR.UTF-8;LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices datasets  utils     methods   base     

loaded via a namespace (and not attached):
[1] grid_2.8.1         lattice_0.17-20    lme4_0.999375-28   
Matrix_0.999375-21
[5] nlme_3.1-90

On Tue, 17 Mar 2009 06:53:20 +0000, Emmanuel Charpentier wrote?:
Dear list,

"+" (and "-") being defined for difftime class, I expected mean() to
return something sensible. This is only half-true :

mean(c(1:5, 5:1),na.rm=TRUE)
[1] 3
mean(as.difftime(c(1:5, 5:1),unit="mins"),na.rm=TRUE)
Time difference of 3 mins

Fine so far. However :

mean(c(1:5, NA,5:1),na.rm=TRUE)
[1] 3
mean(as.difftime(c(1:5, NA,5:1),unit="mins"),na.rm=TRUE)
Time difference of NA mins

Ouch !

Curiously, var(), max() and min() behave as expected. What's so special
with mean() ?

RSiteSearch("mean difftime")
[ ... doesn't return anything relevant ]

NB : this isn't done for the hell of it. I intended to replace some
missing dates, with something computed from other dates and mean time
intervals).

Any thoughs ?

					Emmanuel Charpentier

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