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 Path: eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!mx02.eternal-september.org!.POSTED!not-for-mail From: "G.B." Newsgroups: comp.lang.ada Subject: Re: Creating an empty file with Ada.Text_IO Date: Tue, 25 Aug 2015 18:45:08 +0200 Organization: A noiseless patient Spider Message-ID: References: <1rjn2z0rjbo2j$.1beneepusolhh.dlg@40tude.net> <1c1a3b38-4810-4002-b4f5-ec09b5f97041@googlegroups.com> Reply-To: nonlegitur@futureapps.de Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Tue, 25 Aug 2015 16:43:23 +0000 (UTC) Injection-Info: mx02.eternal-september.org; posting-host="b96887e80893c84a90c3007226ca0d1c"; logging-data="16510"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19rMABV9gPMCeBOPLUBM8o2A+dH+T2OlZY=" User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 In-Reply-To: <1c1a3b38-4810-4002-b4f5-ec09b5f97041@googlegroups.com> Cancel-Lock: sha1:De1S03KbpnC73RKro4Zc+G7cXVA= Xref: news.eternal-september.org comp.lang.ada:27610 Date: 2015-08-25T18:45:08+02:00 List-Id: On 25.08.15 17:26, Maciej Sobczak wrote: > >> procedure Write_String_To_File (File_Name : in String; S : in String) is >> A : Ada.Streams.Stream_Element_Array (1 .. S'Length); >> F : Ada.Streams.Stream_IO.File_Type; >> begin >> -- convert S to stream elements >> for I in S'Range loop >> A (Ada.Streams.Stream_Element_Offset (I)) := >> Ada.Streams.Stream_Element (Character'Pos (S (I))); >> end loop; > > Before anybody uses it in a rocket - there is no provision above that S'First will be 1 and the indices might not be compatible in the above assignment. > > Correct version is: > > for I in S'Range loop > A (Ada.Streams.Stream_Element_Offset (I - S'First + 1)) := > Ada.Streams.Stream_Element (Character'Pos (S (I))); > end loop; > > And there are actually several ways to write it. > Now this procedure can write String slices that do not begin at index 1. > > (Frankly, this part of Ada is inherently error-prone. I stumble on it repeatedly.) > subtypes seem helpful. If S can be made of a constraint subtype, too, then an unchecked conversion should work, too. (Does, using GNAT). with Ada.Characters.Latin_1; with Ada.Streams.Stream_IO; procedure Test is subtype S_E_Offset is Ada.Streams.Stream_Element_Offset; use type S_E_Offset; -- static assert: Is_Positive_in_S_E_Offset : constant array (Boolean) of Integer := ( S_E_Offset'First > S_E_Offset (Positive'First) => 123, S_E_Offset'Last >= S_E_Offset (Positive'Last) => 123 ); procedure Write_String_To_File (File_Name : in String; S : in String) is subtype Index is S_E_Offset range S_E_Offset (S'First) .. S_E_Offset (S'Last); A : Ada.Streams.Stream_Element_Array (Index); F : Ada.Streams.Stream_IO.File_Type; begin -- convert S to stream elements for I in S'Range loop A (S_E_Offset (I)) := Ada.Streams.Stream_Element (Character'Pos (S (I))); end loop; -- create a stream file and write the content (which might be empty) Ada.Streams.Stream_IO.Create (F, Ada.Streams.Stream_IO.Out_File, File_Name); Ada.Streams.Stream_IO.Write (F, A); Ada.Streams.Stream_IO.Close (F); end Write_String_To_File; begin -- empty file: Write_String_To_File ("file1.txt", ""); -- file with one, non-terminated line: Write_String_To_File ("file2.txt", String'(22 => 'a', 23 => 'b', 24 => 'c')); -- file with "one line and a half": Write_String_To_File ("file3.txt", "abc" & Ada.Characters.Latin_1.LF & "xyz"); end Test;