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

Wed Jun 21 14:51:16 EDT 2017

Sure. I agree with Q1 and Q2.

As for Q3,
1. is the statement standard conforming?
	y(:) = MATMUL( a(:,:), x(:) ) + 2*y(:)

2. if it is standard conforming, would it be equivalent to
	y(:) = 2*y(:)
   or it should be no code gen for the whole assignment?

Thanks,

Daniel

XL Fortran Development, Fortran Standard Representative
IBM Toronto Software Lab
Phone: 905-413-3056
Tie: 969-3056
Email: cdchen at ca.ibm.com
http://www.ibm.com/software/awdtools/fortran/xlfortran

From:	Walt Brainerd <walt.brainerd at gmail.com>
To:	fortran standards email list for J3 <j3 at mailman.j3-fortran.org>
Date:	06/21/2017 01:43 PM
Subject:	Re: (j3.2006) Question about zero-sized array.
Sent by:	j3-bounces at mailman.j3-fortran.org

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
  Tie: 969-3056
  Email: cdchen at ca.ibm.com
  http://www.ibm.com/software/awdtools/fortran/xlfortran

  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] 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
  > Tie: 969-3056
  > Email: cdchen at ca.ibm.com
  > http://www.ibm.com/software/awdtools/fortran/xlfortran
  >
  >
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