[R] Rounding error in seq(...)

[Martin Batholdy]

hum,

can you explain that a little more detailed?
Perhaps I miss the background knowledge - but it seems just absurd to  
me.

0.1+0.1+0.1 is 0.3 - there is no rounding involved, is there?

why is
x <- 0.1 + 0.1 +0.1
not equal to
y <- 0.3

?



Am 30.09.2009 um 21:04 schrieb Duncan Murdoch:

> On 9/30/2009 2:50 PM, Michael Knudsen wrote:
>> On Wed, Sep 30, 2009 at 8:44 PM, Duncan Murdoch  
>> <murdoch at stats.uwo.ca> wrote:
>>> Why?  You asked for an increment of 1 in the second case (which is  
>>> exactly
>>> represented in R), then divided by 10, so you'll get the same as  
>>> 0.3 gives
>>> you.  In the seq() case you asked for an increment of a number  
>>> close to but
>>> not equal to 1/10 (because 1/10 is not exactly representable in  
>>> R), so you
>>> got something different.
>> Well, the problem is that I don't know how seq is implemented. I just
>> assumed that it wouldn't behave like this.
>
> It doesn't really matter how it is implemented:  if you ask to start  
> at 0.1 and increment by 0.1, you should expect the result to be the  
> same as
>
> 0.1
> 0.1+0.1
> 0.1+0.1+0.1
> etc.
>
> and I think it is.  The problem is that 0.1+0.1+0.1 is not the same  
> as 0.3, due to rounding.  Neither 0.1 nor 0.3 can be represented  
> exactly in the standard IEEE floating point formats that R uses.
>
> Duncan Murdoch
>
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