[R] length() misbehaving?

[Ott Toomet]

HI Dave,

 | From: "David Parkhurst" <parkhurs at indiana.edu>
 | Date: Fri, 14 Mar 2003 10:35:19 -0500
 | 
 | I'm having a weird problem with length(), in R1.6.1 under windows2000.  I have a
 | dataframe called byyr, with ten columns, the first of which is named cnd95.
 | summary(byyr) shows that byyr$cnd95 contains the factor level "tr" 66 times.  Also,
 | when I enter byyr$cnd95 at the command line, I can count 66 "tr" elements in the
 | resulting vector.  However, when I enter
 | 
 | n95trt <- length(byyr$cnd95[byyr$cnd95=="tr"])
 | n95trt
 | 
 | the result is 68!  Any ideas why this is happening, and how I can fix the miscount?
 | (That column also contains 69 entries of "c", and (relevantly?) two NA's.)

Yes, NA-s are relevant.  Try:

> a <- factor(c("a", "a", NA))
> a
[1] a    a    <NA>
Levels:  a 
> summary(a)
   a NA's 
   2    1 
> a=="a"
[1] TRUE TRUE   NA
# there are 3 elements in the vector, hence there is 3 in a[a=="a"] too.

> sum(a=="a", na.rm=T)
[1] 2

will give you the correct length.

perhaps it helps.

Ott

 | 
 | Thanks for any help.
 | 
 | Dave Parkhurst