From jkr@jkr.cc.rl.ac.uk Tue Mar 28 12:33:05 2000 Received: from nameserv.rl.ac.uk (nameserv.rl.ac.uk [130.246.135.129]) by dkuug.dk (8.9.2/8.9.2) with ESMTP id MAA15828 for ; Tue, 28 Mar 2000 12:33:04 +0200 (CEST) (envelope-from jkr@jkr.cc.rl.ac.uk) Received: from jkr.cc.rl.ac.uk (jkr.cc.rl.ac.uk [130.246.8.20]) by nameserv.rl.ac.uk (8.8.8/8.8.8) with ESMTP id LAA16983 for ; Tue, 28 Mar 2000 11:33:00 +0100 Received: (from jkr@localhost) by jkr.cc.rl.ac.uk (8.8.8+Sun/8.8.8) id LAA11190 for SC22WG5@dkuug.dk; Tue, 28 Mar 2000 11:34:06 +0100 (BST) Date: Tue, 28 Mar 2000 11:34:06 +0100 (BST) From: John Reid Message-Id: <200003281034.LAA11190@jkr.cc.rl.ac.uk> To: SC22WG5@dkuug.dk Subject: Interpretation 004 MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Date: 28th March 2000 To: J3 From: John Reid Subject: Interpretation 004 Here are drafts for the ANSWER and EDITS sections of 004. Also, I propose that the addendum be removed. For your convenience, the current 004 is attached. ANSWER: Processors may support values that are not present in the model of 13.7.1. IEEE -inf is an example of such a number and this should be returned on a machine that supports the IEEE standard. If the negative number of largest magnitude in the model had been intended, the model would have been mentioned as, for example, in the definition of HUGE (13.14.39). A simple example of its use is to test whether a set of numbers SET1 has a value greater than any value in the set SET2. Consider the expression MAXVAL(SET1)>MAXVAL(SET2). If SET1 is empty and SET2 is not, this value is (correctly) false even if all the values are outside the model with values less than -HUGE(SET1). It may be helpful to consider how MAXVAL might be coded for an array of rank one on an IEEE computer. The following code is suitable MAXVAL = IEEE_VALUE(1.0,IEEE_NEGATIVE_INF) DO I = 1, SIZE(ARRAY) MAXVAL = MAX(MAXVAL,ARRAY(I)) END DO EDITS: None. ------------------------------------------------------------------------------- NUMBER: 000004 TITLE: Value returned by MAXVAL/MINVAL KEYWORDS: MAXVAL, MINVAL DEFECT TYPE: Interpretation STATUS: X3J3 consideration in progress QUESTION: The Result Value section of the MAXVAL intrinsic function description uses the phrasing: or has the value of the negative number of the largest magnitude supported by the processor for numbers of the type and kind type parameter of ARRAY if ARRAY has size zero This phrasing has generated at least the two following views on the return value: * If the machine supports the IEEE standard then the implementation should return -inf. * For portability, the implementation should return -HUGE(ARRAY). These views lead to the following questions: 1. Is the intent of the standard to describe the result in terms of machine values rather than model values? 2. If the answer to 1 is "yes", how are programmers expected to use this intrinsic function portably? ANSWER: EDITS: SUBMITTED BY: Larry Rolison HISTORY: J3/97-240 m143 submitted - - - - - - - Date: Thu, 18 Sep 1997 13:38:50 -0500 [ From a coworker here at SGI/CRI ] The first question boils down to: on an IEEE-like hardware platform which has a bit pattern for -infinity, does this qualify as "a value ... supported by the processor"? The result of MAXVAL must be of the same type and kind as the argument. Since -infinity would only be possible if the arguments were of type real, one would have to conclude that -infinity was a valid real value. If that is true, then there would be at least one argument for which some intrinsics (EXPONENT, FRACTION) would fail. Based on this I would argue that returning -infinity should be invalid. I also think it is unwise, based on the discussion below. I think that the real problem is that there is no clean way to return a "failed" status from MAXVAL. Both the -huge() and the -infinity methods are defective attempts at solving that problem. It is not clear that the choice of -infinity is in any way superior to -HUGE(). If a machine allows -infinity, and all the tested elements in the array were -infinity, then you would expect that -infinity would be returned. But then is it the "answer" or is it an "error"? There is no way to tell. Returning -HUGE() suffers from exactly the same defect. I believe that there are a couple of drawbacks to the use of -infinity. 1) You might like to write code of the following form: answer = maxval(array, mask=larray) if (answer == xxxxx) then ! code for the case that all the elements in larray are .false. else ! code the the cae that answer is a meaningful result end if What do you use for xxxxx? In the current model, -HUGE(array) should be a workable and portable form. If the alternate -infinity version were used, then there would have to be a way in Fortran to represent -infinity. It's not clear that is always possible, and most likely not in a portable fashion. 2) The MAXVAL function allows either real or integer arguments. On an IEEE machine (for example) -infinity only has meaning for reals; the concept of -infinity makes no sense for integer arguments. It seems an unnecessary complication to have one scheme for reals and a different one for integers. -------------------------------------------------------------------------------