J3/15-142
To: J3
From: Malcolm Cohen
Subject: SELECT RANK
Date: 2015 February 23


1. Introduction

This paper contains formal requirements, specifications, and syntax for a
SELECT RANK construct as recently discussed on the J3 mailing list.


2. Formal requirements

That it be possible in Fortran, without using any C wrapper, to operate on
an assumed-rank array variable.  In particular, it shall be possible to
reference and define elements of the array.  Requiring additional "glue"
code would be acceptable when processing an assumed-rank array that is
eventually associated with an assumed-size array.


3. Specifications

There will be a construct to select a block based on the rank of an
assumed-rank array that is not associated with an assumed-size array:
- within each block there will be an associate-name of that rank associated
  with the selector;
- a single special block will catch all ranks of assumed-size, and in that
  block the associate-name will have shape (1:*); this can be reshaped
  using sequence association;
- a single special block will catch all non-matching ranks, and in that
  block the associate-name is assumed-rank.


4. Syntax

select-rank-construct ::= select-rank-stmt
                            [ select-rank-case ]
                          end-select-stmt

select-rank-stmt ::= SELECT RANK ( [ associate-name => ] selector )

Constraint: (select-rank-stmt) selector shall be the name of an
            assumed-rank array.

select-rank-case ::= rank-case-stmt
                     block

rank-case-stmt ::= RANK ( int-constant-expr )
               |   RANK ( * )
               |   RANK DEFAULT

ALTERNATIVE: Use "CASE whatever" instead of "RANK whatever".

Constraint: int-constant-expr shall be non-negative and less than or equal
            to the maximum rank supported by the processor.

5. Example

SUBROUTINE process(x)
  REAL x(..)
  !
  SELECT RANK(y=>x)
  RANK (0)
    y = 0
  RANK (1)
    y(::2) = 1
  RANK (2)
    y(:,2) = 2
  RANK (*)
    Do i=1,100
      If (y(i)==0) Exit
      y(i) = -y(i)
    End Do
  RANK DEFAULT
    Print *, 'I did not expect rank', RANK(y), 'shape', SHAPE(y)
  END SELECT


6. Assumed-size handling rationale and example

Because an assumed-size object starts out with sequence association, rank
is more of a movable feast with assumed-size than it is with other classes
of arrays, like assumed-shape, allocatables, and pointers.  For all other
array types, it is best to get the shape as if it were deferred shape, then
we get all the usual whole array operations.  If the assumed-size array is
wanted with any other rank or bounds than the default of (1:*), sequence
association can be used again...  for example

   RANK (*)
     IF (RANK(x)==2) THEN
       ! Special code for the rank two case.
       CALL seq_ass_2(y, LBOUND(x,1), UBOUND(x,1), LBOUND(x,2))
     ELSE
       ... same code as above for the non-rank-2 case.
     END IF
     ...
     SUBROUTINE seq_ass_2(a, lb1, ub1, lb2)
       REAL a(lb1:ub1,lb2:*)
       ... do whatever you like with assumed-size array A.


7. Edits

Edits will be a bit bigger than SELECT CASE (because of the name
association), and a bit smaller than SELECT TYPE (simpler selection
mechanism).  Deferred to the next revision of the paper.

===END===