[R] seq

[Barry Rowlingson]

Henric Nilsson wrote:

> I am, however, surprised of the fact that seq() prints it's internal 
> representation of 0 as 0.0 when `from = 0.5', but as -1.110223e-16 when 
> `from = 0.7'.

  These second one isnt a zero!

  > seq(from=0.5,by=-.1,len=6)==0
  [1] FALSE FALSE FALSE FALSE FALSE  TRUE

  - in this case the last element is exactly zero. Internally it will be 
a string of 0 bits or some other representation that means precisely 
zero. Zero. Zilch, nada, zip.

  > seq(from=0.7,by=-.1,len=8)==0
  [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE

  - in this case the last element _isnt_ exactly zero. At some point in 
the calculation floating point's lack of precision kicks in, and the 
last element in that seq() will be represented internally by a pattern 
of ones and zeroes that isn't the zero-pattern, but is the pattern for 
-1.110223e-16 instead. Quite where that happens is controlled by the 
mechanism that seq() uses to generate its numbers.

  I find it surprising that people still think that other people get 
surprised by this. Now that people start their programming with 
high-level languages like R, or Perl, or Visual Wotsit, they don't get 
exposed to internals, and think that numbers are just numbers. The idea 
that 0.1 != 1/10 would be surprising.

  Assembly language for everyone, I say....

BSR