On Wed, 13 Aug 2008, Moshe Olshansky wrote:
Since 0 can be represented exactly as a floating point number, there is no
problem with something like x[x==0].
What you can not rely on is something like 0.1+0.2 == 0.3 to be TRUE.
As I tried to indicate in my previous email, it is more complicated than that.
Certainly 0 is exactly representable, the questions are whether x is exactly
zero and whether you want to test for exact zero
If x has been read in directly or computed by eg
addition/subtraction/multiplication of integers then x will be exactly zero
when it is approximately zero so there is no problem.
If you want to find the subset of x where 0/x will be NaN then you want to
test for exact zero and there is no problem even if x is not exactly zero
If x has been computed some other way and you want to find the values of x that
would be zero if the computations were done at infinite precision, then x==0
may not work.
-thomas
--- On Thu, 14/8/08, Roland Rau <[EMAIL PROTECTED]> wrote:
From: Roland Rau <[EMAIL PROTECTED]>
Subject: Re: [R] ignoring zeros or converting to NA
To: "rcoder" <[EMAIL PROTECTED]>
Cc: r-help@r-project.org
Received: Thursday, 14 August, 2008, 1:23 AM
Hi,
since many suggestions are following the form of
x[x==0] (or similar)
I would like to ask if this is really recommended?
What I have learned (the hard way) is that one should not
test for
equality of floating point numbers (which is the default
for R's numeric
values, right?) since the binary representation of these
(decimal)
floating point numbers is not necessarily exact (with the
classic
example of decimal 0.1).
Is it okay in this case for the value zero where all binary
elements are
zero? Or does R somehow recognize that it is an integer?
Just some questions out of curiosity.
Thank you,
Roland
rcoder wrote:
Hi everyone,
I have a matrix that has a combination of zeros and
NAs. When I perform
certain calculations on the matrix, the zeros generate
"Inf" values. Is
there a way to either convert the zeros in the matrix
to NAs, or only
perform the calculations if not zero (i.e. like using
something similar to
an !all(is.na() construct)?
Thanks,
rcoder
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Thomas Lumley Assoc. Professor, Biostatistics
[EMAIL PROTECTED] University of Washington, Seattle
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