(j3.2006) Question about zero-sized array.

Wed Jun 21 13:43:35 EDT 2017

Let me try to help (maybe it won't) with a different point of view.
I always thought it was not the best to say that x is always defined,
but since the standard does . . .

For Q1, I would say the value of x is the empty array.

For Q2, I think you are asking the wrong question.
An *element* of the array can have the value 0.0 or 1.0, but
x has no elements, and the question is meaningless.
Also Q3.

Crystal clear?

On Wed, Jun 21, 2017 at 9:37 AM, Daniel C Chen <cdchen at ca.ibm.com> wrote:

> Thanks John and Malcolm for your response. I didn't make the questions
> clear enough so I will try again.
>
> As for Q1,
> non-allocatable case:
> real :: x(0) ! x is always defined by the standard. What is its value at
> this point?
> allocatable case:
> real, allocatable :: x(:)
> allocate(x0)) ! at this point, x is defined by allocation. Again, what is
> the value after allocation?
>
> As for Q2, I actualy meant to ask 'x' rather than 'a'.
> x = 0.0 ! Does x have value 0.0?
> x = 1.0 ! x Does x have value 1.0?
>
> As for Q3, if x in Q2 is defined with the value 0.0 or 1.0 and used in the
> following statement respectively,
> a(:,:) = 1.0
> y(:) = MATMUL( a(:,:), x(:) ) + 2*y(:) ! Does the value of x affect the
> result?
>
> Thanks,
>
> Daniel
>
> XL Fortran Development, Fortran Standard Representative
> IBM Toronto Software Lab
> Phone: 905-413-3056 <(905)%20413-3056>
> Tie: 969-3056
> Email: cdchen at ca.ibm.com
> http://www.ibm.com/software/awdtools/fortran/xlfortran
>
> [image: Inactive hide details for "Malcolm Cohen" ---06/20/2017 07:52:35
> PM---Daniel Chen asks: > The standard specifies, "Zero-sized a]"Malcolm
> Cohen" ---06/20/2017 07:52:35 PM---Daniel Chen asks: > The standard
> specifies, "Zero-sized arrays and zero-length strings are
>
> From: "Malcolm Cohen" <malcolm at nag-j.co.jp>
> To: "'fortran standards email list for J3'" <j3 at mailman.j3-fortran.org>
> Date: 06/20/2017 07:52 PM
> Subject: Re: (j3.2006) Question about zero-sized array.
> Sent by: j3-bounces at mailman.j3-fortran.org
> ------------------------------
>
>
>
> Daniel Chen asks:
> > The standard specifies, "Zero-sized arrays and zero-length strings are
> > always defined".
> > It also says "Allocation of a zero-sized array or zero-length character
> > variable causes the array or variable to become defined"
> >
> > Question1: Does it have initial value at all?
>
> John Reid opines:
> >Well, yes. It always has the same value, including initially.
>
> No.  Being allocatable or a pointer it is initially undefined.
> ?initially? means at the beginning of execution; at that point the array
> does not have any shape.
>
> Daniel continues::
> > allocate(a(7,0),x(0),y(7))
> > a(:,:) = 0.0 !Q2: Is this legal? What is the value of a?
>
> John replies:
> >Yes, you are broadcasting a scalar to every element of the array. a has
> >the value it always has when the size is zero.
>
> ?it always has? could be a little misleading here; being an array, the
> shape is part of the value, so the value is the 7x0 empty array and not the
> 0x23 empty array.  I?m not sure we have all this explicitly written down
> anywhere though, except for derived types, where we say the array bounds of
> a component are part of the value of the structure.  Outside of a structure
> it is the shape that?s important rather than the bounds.
>
> Cheers,
> --
> ..............Malcolm Cohen, NAG Oxford/Tokyo.
>
> *From:* j3-bounces at mailman.j3-fortran.org [mailto:j3-bounces at mailman.j3-
> fortran.org <j3-bounces at mailman.j3-fortran.org>] *On Behalf Of *John Reid
> *Sent:* Wednesday, June 21, 2017 12:53 AM
> *To:* fortran standards email list for J3 <j3 at mailman.j3-fortran.org>
> *Subject:* Re: (j3.2006) Question about zero-sized array.
>
>
>
> Daniel C Chen wrote:
> > Hello,
> >
> > The standard specifies, "Zero-sized arrays and zero-length strings are
> > always defined".
> > It also says "Allocation of a zero-sized array or zero-length character
> > variable causes the array or variable to become defined"
> >
> > Question1: Does it have initial value at all?
>
> Well, yes. It always has the same value, including initially.
>
> > Question2: Can it appear as the LHS of an assignment?
>
> Yes.
>
> > Question3: Can it be used in an expression and have a defined result?
>
> Yes.
> >
> > Please consider the following example:
> >
> > program test
> > real, dimension(:,:), allocatable:: a
> > real, dimension(:), allocatable :: x,y
> >
> > allocate(a(7,0),x(0),y(7))
> > a(:,:) = 0.0 !Q2: Is this legal? What is the value of a?
>
> Yes, you are broadcasting a scalar to every element of the array. a has
> the value it always has when the size is zero.
>
> > x(:) = 0.0
>
> Yes, you are again broadcasting a scalar to every element of the array.
>
> > y(:) = 1.0
> > y(:) = MATMUL( a(:,:), x(:) ) !Q3: Is this legal? what is the value of y?
>
> Yes. Every element of y has the value 0.0.
>
> > end program test
> >
> > Non-allocatable version:
>
> Nothing is reallocated in the allocatable version, so this is the same.
>
> Cheers,
>
> John.
>
> > program test2
> > real :: a(7,0)
> > real :: x(0),y(7)
> >
> > a = 0.0 !Q2: is this legal? What is the value of a?
> > x = 0.0
> > y = 1.0
> > y = MATMUL( a, x ) !Q3: is this legal? what is the value of y?
> > end program test2
> >
> > Thanks,
> >
> > Daniel
> >
> > XL Fortran Development, Fortran Standard Representative
> > IBM Toronto Software Lab
> > Phone: 905-413-3056 <(905)%20413-3056>
> > Tie: 969-3056
> > Email: *cdchen at ca.ibm.com* <cdchen at ca.ibm.com>
> > *http://www.ibm.com/software/awdtools/fortran/xlfortran*
> <http://www.ibm.com/software/awdtools/fortran/xlfortran>
> >
> >
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Walt Brainerd
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