R help - Aug 2002 - is.numeric()

Dear R-list

I am having troubles understanding how the function is.numeric() works.
Any help appreciated.

Some commands are given here (the output and my comments are given
below)


vec <- c(1.4, NA, NA, NA)
sapply(vec,FUN=is.numeric)
is.numeric(vec[2])
is.na(vec[2])
is.numeric(NA)

vec <- c(TRUE, FALSE, NA, NA)
sapply(vec,FUN=is.numeric)
is.numeric(vec[2])
is.numeric(vec[3])
is.numeric(FALSE)

vec <- c(1.3, FALSE, NA, NA)
sapply(vec,FUN=is.numeric)
is.numeric(vec[2])
is.numeric(vec[3])


###
output + comments :
###

> vec <- c(1.4, NA, NA, NA)
> sapply(vec,FUN=is.numeric)
[1] TRUE TRUE TRUE TRUE
> is.numeric(vec[2])
[1] TRUE
> is.na(vec[2])
[1] TRUE
> is.numeric(NA)
[1] FALSE


I would have expected that is.numeric() on vec[2], vec[3] and vec[4]
would give three times FALSE.

My attempt to try to vectorize is.numeric() using sapply() might be
nonsense. Is it possible to vectorize is.numeric() ?

> vec <- c(TRUE, FALSE, NA, NA)
> sapply(vec,FUN=is.numeric)
[1] TRUE TRUE TRUE TRUE
> is.numeric(vec[2])
[1] FALSE
> is.numeric(vec[3])
[1] FALSE
> is.numeric(FALSE)
[1] FALSE


Here I am surprised that is.numeric(vec[2]) doesn't return the same
result as in the first example. The element is NA in both cases.

Apparently my attempt to vectorize is.numeric doesn't give what I
expected. I would have expected that 
sapply(vec,FUN=is.numeric) would equal
c(is.numeric(vec[1],is.numeric(vec[2],is.numeric(vec[3],is.numeric(vec[4])



Cheers Ole

> version
         _                
platform i686-pc-linux-gnu
arch     i686             
os       linux-gnu        
system   i686, linux-gnu  
status                    
major    1                
minor    5.1              
year     2002             
month    06               
day      17               
language R  



-- 
Ole F. Christensen
Department of Mathematics and Statistics
Fylde College, Lancaster University 
Lancaster, LA1 4YF, England
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On Wed, 21 Aug 2002, Ole Christensen wrote:
> ###
> output + comments :
> ###
>
>
> > vec <- c(1.4, NA, NA, NA)
> > sapply(vec,FUN=is.numeric)
> [1] TRUE TRUE TRUE TRUE
> > is.numeric(vec[2])
> [1] TRUE
> > is.na(vec[2])
> [1] TRUE
> > is.numeric(NA)
> [1] FALSE
>
>
> I would have expected that is.numeric() on vec[2], vec[3] and vec[4]
> would give three times FALSE.
>
>
> My attempt to try to vectorize is.numeric() using sapply() might be
> nonsense. Is it possible to vectorize is.numeric() ?
You can vectorize anything.  You appear to be getting confused by the fact
that there is more than one sort of NA.

In the first case vec is a numeric vector, so the NAs in it are numeric
NAs. That is, they represent some number but you don't know which one
because you didn't measure it. This is numeric.

In the second case vec is a logical vector, so the NAs are logical NAs:
the value is TRUE or FALSE but you don't know which one.  This is not
numeric

You can also have character NAs, representing a missing character string.
This isn't numeric either.

The `default' NA produced when you type `NA' is a logical NA and so
isn't
numeric.

There's also NaN, which represents the value of an undefined mathematical
operation, eg 0/0.

We need different types of NA because all the elements in a vector must be
of the same type.


	-thomas

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