On 2/12/2009 10:20 AM, Žroutík wrote:
Hi everybody,
given a fresh rgui.exe load on winxp OS, I enter (a minimal exaple)
n <- 12.357531
Then the following command:
n <- (n - floor(n))*10; n
gives the following outputs:
[1] 3.57531
[1] 5.7531
[1] 7.531
[1] 5.31
[1] 3.1
[1] 1 === still as expected
[1] 10 === not expected, count with me: 1 - floor(1) is zero, times
10 gives 0, not 10!!!!
You are assuming that when R prints "1", the value is exactly 1. But
try this:
> 0.999999999
[1] 1
> 0.999999999 == 1
[1] FALSE
R rounds values when it prints them, because people don't want to see
ugly output like this:
> options(digits=18):
> n <- 12.357531
> n <- (n - floor(n))*10; n
[1] 3.5753100
> n <- (n - floor(n))*10; n
[1] 5.75309999999998
> n <- (n - floor(n))*10; n
[1] 7.530999999999821
> n <- (n - floor(n))*10; n
[1] 5.30999999999821
> n <- (n - floor(n))*10; n
[1] 3.099999999982117
> n <- (n - floor(n))*10; n
[1] 0.999999999821171
And the reason you see such ugly output is because you are working with
a number that can't be represented exactly in binary floating point.
Duncan Murdoch
[1] 10 === should stay forever zero (0)
[1] 10
[1] 9.999998
[1] 9.999982
[1] 9.999821
[1] 9.998212
The sama happens with trunc().
e.g. (a minimal exaple)
n <- 0.245
n <- (n - trunc(n))*10; n
[1] 2.45
[1] 4.5
[1] 5
[1] 1.776357e-13 ===== zero expected!!!
[1] 1.776357e-12
And I'm asking "what the heck?!" and where is the bug in my examples? Any
suggestion well appreciated.
p.s. The expression with floor() and trunc() are to be implemented in a
function which gives a value equal precision order of the given number. e.g.
12.345 would have (-3), 12.1 would have (-1), 12 would have (0) and e.g.
12000 would have the order of the precision (4). Basically, it is the order
of the last given non-zero digit.
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and provide commented, minimal, self-contained, reproducible code.
