[R] Basic question on function "identical"

[(Ted Harding)]

On 15-Dec-03 Thomas Lumley wrote:
> 
> One reason that which.max() exists is that we cannot guarantee
> which(x==max(x)) to work. It is possible, though rather unlikely, for
> there to be no x such that x==max(x).  One reason is the unpredictable
> use of 10-byte wide floating point registers on Intel chips.

Hmmm ...

I'd be interested to learn more of what you mean. For instance,
in C-speak, to find the maximum of an array of double x[], of
length n, something like the following code could be written:

  xmax=x[1];
  for(i=1;i<n;i++) if(x[i+1]>x[i]) xmax=x[i+1];

Regardless of the accuracy of the comparison, each assignment
xmax = ... should make xmax an internally exact copy of the
thing on the righthand side. However, your reply suggests that this
may not happen, as a result of "unpredictable use of 10-byte wide
floating point registers on Intel chips".
Is this really the case? If so, how would a discrepancy arise?
(I know C programmers like to use "register variables" where
possible, for speed, but the copying should be faithful, surely).

On the other hand, R has to deal with things which may be a mixture
of types, so has to cope with the coercions implied by the hierarchy
of types; but for instance

> x<-c(-1.0, 0.5, TRUE, as.integer(0))
> which(x==max(x))
[1] 3
> 

still gives the answer one would expect.

But in any case, the usual sort of application is which(x==max(x))
where the values in x are doubles, so the first question remains.

Futhermore, "which(x==max(x))" has a useful feature which
"which.max(x))" does not have::

  > x<-c(0.1, 0.7, 5.1, 5.1, 1.5, 5.1, 3.4, 2.2, 4.3)
  > which(x==max(x))
  [1] 3 4 6
  > which.max(x)
  [1] 3

so I'd feel a bit deprived if the former were discouraged!

With thanks,
Ted.


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Date: 15-Dec-03                                       Time: 17:22:59
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