Re: Cray style vectors

MitchAlsup1 <mitchalsup@aol.com> schrieb:

Thomas Koenig wrote:

However, what I did put in the paper (and what the subsequent
revision by a J3 subcommittee left in) is a prohibition against
using unsigneds in a DO loop.  The reason is semantics of
negative strides.

>

Currently, in Fortran, the number of iterations of the loop

>

    do i=m1,m2,m3
....
    end do

>

is (m2-m1+m3)/m3 unless that value is negative, in which case it
is zero (m3 defaults to 1 if it is not present).

>

So,

>

   do i=1,3,-1

>

will be executed zero times, as will

>

  do i=3,1

>

Translating that into arithmetic with unsigned integers makes
little sense, how many times should

>

    do i=1,3,4294967295

>

be executed?

>
3-1+4294967295 = 4294967297 // (m2-m1+m3)
>
4294967297 / 4294967295 = 1.0000000004656612874161594750863
>
So the loop should be executed one time. {{And yes I know 4294967295 ==
0x1,0000,0001}} What would you expect on a 36-bit machine (2s-complement)
where 4294967295 is representable naturally ??

Correct (of course).

The same result would be expected for

   do i=1u,3u,-1u

(asusming an u suffix for unsigned numbers).

The problem is that this violates a Fortran basic assumption since
FORTRAN 77, which is that DO loops can be zero-trip.

This is a can of worms that I would like to leave unopened.

Same goes for array slices.  Even assuming that no negative
indices are used, the slice a(1:3:-1) is zero-sized in Fortran,
as is a(3:1) .

For a(1u:3u:-1u) the same logic that you outlined above would apply,
making it a slice with one element.

Not going there :-)