Path: utzoo!utgpu!water!watmath!clyde!rutgers!cmcl2!beta!unm-la!unmvax!nmtsun!john
From: john@nmtsun.nmt.edu (John Shipman)
Newsgroups: comp.lang.fortran
Subject: Modifying iteration variables in DO loops
Keywords: X3 standard FORTRAN 77
Message-ID: <1309@nmtsun.nmt.edu>
Date: 1 Feb 88 19:07:01 GMT
Organization: Zoological Data Processing
Lines: 36

In section 11.10 of X3.9-1978, the FORTRAN 77 standard (pp. 11-5 to
11-9), there is no statement about prohibiting the modification of a
DO variable within a loop.  The standard defines how this works for
     "DO ... <iter> = <e1>, <e2> [, <e3> ]":

(1) INITIALIZATION.  Compute the values of expressions <e1>, <e2>,
    and <e3>, and call the results M1, M2, and M3; if <e3> is
    omitted, set M3 to 1.  Compute the number of loops:
        <count> = MAX ( INT ( ( M2 - M1 + M3 ) / M3 ), 0 )
    and store this value in a hidden temporary.  Set <iter> to M1.    

(2) TEST.  If <count> is zero, branch to (5).

(3) LOOP BODY.

(4) MODIFICATION.  Add M3 to <iter>.  Decrement <count>.  Go to (2).

(5) (The statement after the end of the loop)

For example, with some mythical standard-conforming compiler, this loop
would be executed twice, and the final value of COUNT would be 5:

           do 86 count = 1, 2
             count = count + 1
      86   continue

I bet NOBODY has a standard-conforming compiler.  From a compiler
writer's point of view, this is inefficient because you have to modify
both <iter> and <count> at the bottom of the loop; this tends to make
your benchmarks look bad.  I bet the politics behind this decision
were pretty convoluted---anybody know the history of this one?

-- 
John Shipman/Zoological Data Processing/Socorro, New Mexico
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