----- Original Message -----
From: "Stephen Liu" <sati...@yahoo.com>
To: <r-help@r-project.org>
Sent: Tuesday, August 17, 2010 8:25 AM
Subject: Re: [R] Missing values
----- Original Message ----
From: Michael Bedward <michael.bedw...@gmail.com>
To: Stephen Liu <sati...@yahoo.com>
Cc: r-help@r-project.org
Sent: Tue, August 17, 2010 3:57:05 PM
Subject: Re: [R] Missing values
NA is a value that you can use for "missing" or inapplicable. It is
also the value returned by R functions to indicate missing (e.g. if
you use the match function to search a vector for a particular value,
but the value isn't present, match will return NA).
H Michael,
I understand above explanation.
I don't understand follow;
The function is.na(x) gives a logical vector of the same size as x with
value
TRUE if and only if the corresponding element in x is NA.
z <- c(1:3,NA); ind <- is.na(z)
z <- c(1:3,NA); ind <- is.na(z)
# To see the logical vector the same size as the original print them out
# z has four elements, one of which is "not available" or "not applicable"
z
[1] 1 2 3 NA
# ind has four LOGICAL elements, the last of which is "not available"
# so the function is.na() returns TRUE
ind # logical elements
[1] FALSE FALSE FALSE TRUE
?is.na
....
Usage:
NA
is.na(x)
## S3 method for class 'data.frame':
is.na(x)
is.na(x) <- value
....
NaN means that the value wasn't missing but can't be expressed as a
number. 0/0 is NaN because the result is mathematically undefined
# Are you reading zero divided by zero in the line above?
(sometimes called 'indeterminate').
What is following expression;
Inf - Inf
# infinity and minus infinity
?
TIA
B.R.
Stephen L
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