From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-0.3 required=5.0 tests=BAYES_00, REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no version=3.4.4 X-Google-Thread: 103376,7b6305d0d57a9f34 X-Google-NewGroupId: yes X-Google-Attributes: gida07f3367d7,domainid0,public,usenet X-Google-Language: ENGLISH,ASCII-7-bit Received: by 10.68.73.233 with SMTP id o9mr1028487pbv.8.1320180468377; Tue, 01 Nov 2011 13:47:48 -0700 (PDT) Path: p6ni57776pbn.0!nntp.google.com!news2.google.com!news3.google.com!proxad.net!feeder1-2.proxad.net!newsfeed.straub-nv.de!news-1.dfn.de!news.dfn.de!news.uni-weimar.de!not-for-mail From: stefan-lucks@see-the.signature Newsgroups: comp.lang.ada Subject: Re: Normalizing array indices Date: Tue, 1 Nov 2011 21:43:27 +0100 Organization: Bauhaus-Universitaet Weimar Message-ID: References: Reply-To: stefan-lucks@see-the.signature NNTP-Posting-Host: medsec1.medien.uni-weimar.de Mime-Version: 1.0 X-Trace: tigger.scc.uni-weimar.de 1320180385 6002 141.54.178.228 (1 Nov 2011 20:46:25 GMT) X-Complaints-To: news@tigger.scc.uni-weimar.de NNTP-Posting-Date: Tue, 1 Nov 2011 20:46:25 +0000 (UTC) X-X-Sender: lucks@medsec1.medien.uni-weimar.de In-Reply-To: Xref: news2.google.com comp.lang.ada:14267 Content-Type: TEXT/PLAIN; charset=US-ASCII Date: 2011-11-01T21:43:27+01:00 List-Id: On Fri, 28 Oct 2011, Randy Brukardt wrote: > This is pretty typical. We've often talked about the need in Ada for > one-sided array subtypes, but we don't have any first class ones at this > point. > > I probably would constrain the parameter to have a lower bound of 1. Using > Ada 2012: > > generic > type Element_Type is private; > type Sort_Array_Type is array (Positive range <>) of Element_Type; > with function "<" (Left, Right: Element_Type) return Boolean is <>; > procedure Sort(A: in out Sort_Array_Type) > with Pre => A'First 1; Well, once I want to sort a slice X(Y .. Z) of X, this fails. The ease of slicing is one of the coolest features of Ada's arrays. In fact, the implementation of sort I mentioned would recursively perform Sort(A(A'First .. M)); and Sort(A(M+1 .. A'Last)); for M = (A'First+A'Last)/2 (with some exception treatment for A'First+A'Last>'Positive'Last and only if A'Length is above some threshold), followed by merging the two slices. So at some point of time, the slicing/renaming would become necessary -- either always at the beginning of Sort, or to re-slice A(M+1 .. A'Last). A better Ada-2012 precondition would actually be with Pre => A'Last < Positive'Last; > Then, if you had any calls that don't have the right bounds (which is likely > to be rare), I'd use a sliding trick similar to the one Adam showed. (But > I'd probably try hard to figure out how to avoid passing the slice in the > first place.) > > Of course, it's better to make the routine work for all possible bounds. And > if you do that, you certainly need to add that to the testing burden (it's > commonly forgotten and probably is one of the most common Ada bugs). I am relieved that I am not the only one who happened to suffer from such a problem. Still, I think a way to *specify* a formal subprogram parameter F such that F'First is a given constant, regardless of A'First if A is the actual parameter, would be ease testing and might be a good extension to Ada (2020?). So long Stefan -- ---- Stefan.Lucks (at) uni-weimar.de, University of Weimar, Germany ---- ------ I love the taste of Cryptanalysis in the morning! ------