Hi David,
I have probably 2 stupid questions regarding what you said but it might
be important to understand:
- why nrow() "would not make sens for a subsetted vector"?
On the help page of nrow(), it's written that we can apply it on a
vector, array or dataframe (basically everything...?). So what's the
difference between a "normal" vector (for which it would make sense and
work) and a subsetted vector?
- why assuming "that length() applied to dataframes would tell me how
many rows it had" would be a mistake? I mean in this case, length() is
calculated for each numerical variable (which are vectors, aren't they?).
I think these questions concern the way R handle the data and that's why
I think it might be important for me to understand these issues.
Thanks for your input.
Regards,
Ivan
Le 3/17/2010 16:39, David Winsemius a écrit :
On Mar 17, 2010, at 11:23 AM, Tony Laidig wrote:
Hello Everyone-
I'm calculating summary statistics on a dataset (~4000 records,
observations are not uniformly distributed) using summaryBy and trying
to add a column with the number of observations to the output as well.
What occurs to me is to use nrow(), but this doesn't appear to be
working
I'm able to replicate the same results with an example from the
summaryBy docs:
data(dietox)
dietox12<- subset(dietox,Time==12)
library(doBy)
#this one works
summaryBy(Weight+Feed~Evit+Cu,data=dietox12,FUN=c(mean,var,length))
#adding nrow doesn't give the number of rows
summaryBy(Weight+Feed~Evit+Cu,data=dietox12,FUN=c(mean,var,length,nrow))
I'm a bit puzzled. One of my many newbie mistakes was to assume that
length() applied to dataframes would tell me how many rows it had. It
appears that the authors of summaryBy have figured out how to get
length() to tell you the number of observations, presumably on a
subsetted vector where length would make sense. So ... it's not
clear why you also want nrow (which would not make sense for a
subsetted vector).
There must be a way to do this, but I can't figure it out. I suspect
there is another function that would be compatible with summaryBy.
Thanks in advance.
-Tony
David Winsemius, MD
West Hartford, CT
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