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=-1.9 required=5.0 tests=BAYES_00 autolearn=ham autolearn_force=no version=3.4.4 X-Google-Thread: 103376,7728b533f7ab5fb6,start X-Google-Attributes: gid103376,domainid0,public,usenet X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news1.google.com!news4.google.com!proxad.net!feeder1-2.proxad.net!feeder.erje.net!feeder2.news.saunalahti.fi!feeder1.news.saunalahti.fi!nntp.inet.fi!central1.inet.fi!inet.fi!read4.inet.fi.POSTED!53ab2750!not-for-mail Sender: AWI003@FIW9752 Newsgroups: comp.lang.ada Subject: Avoiding side effects From: Anders Wirzenius Message-ID: User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.3 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Date: Tue, 30 Sep 2008 05:50:18 GMT NNTP-Posting-Host: 194.251.142.2 X-Complaints-To: abuse@inet.fi X-Trace: read4.inet.fi 1222753818 194.251.142.2 (Tue, 30 Sep 2008 08:50:18 EEST) NNTP-Posting-Date: Tue, 30 Sep 2008 08:50:18 EEST Organization: Sonera Customer Xref: g2news1.google.com comp.lang.ada:2150 Date: 2008-09-30T05:50:18+00:00 List-Id: Another thread about functions and side effects triggered me to ask advice how to solve a specific problem without using side effects. Problem: Two files A and B containing one code per line (actually a material number). I want to have the difference between those files (A-B), i.e. remove those lines in A that are mentioned in B. B may contain codes that are not present in A. A and B contain up to 1.5 million lines each. My solution: I sort the files alphabetically and remove possible duplicates (Unix commands sort and uniq). I create two functions, Strip and Advance. function Strip (Remove_This : in String) return String -- Displays rows until row > Remove_This. function Advance (Candidate : in String) return String -- Advance in file until row > Candidate. The main program is then merely a loop where I alternate between advancing lines in the two files until the row in the file is greater than the value to compare against: ... loop Trans_String := To_Unbounded_String (Strip (To_String (Trans_String))); Trans_String := To_Unbounded_String (Advance (To_String (Trans_String))); end loop; The problem is how to deal with the situation when I reach the end of the file. I have done it using a side effect: EOF_Reached : Boolean := False; loop Trans_String... exit when EOF_Reached; Trans_String... exit when EOF_Reached; end loop; -- Display possible tail of file A. EOF_Reached is set to True in the functions Strip and Advance as a side effect. Is there a better way without using the side effect EOF_Reached? -- Anders