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From: Ted Dennison <dennison@telepath.com>
Subject: Re: raised CONSTRAINT_ERROR
Date: 2000/07/24
Message-ID: <8lhldq$ock$1@nnrp1.deja.com>#1/1
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References: <86lmytp5i0.fsf@book.mteege.de>
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Date: 2000-07-24T00:00:00+00:00
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In article <86lmytp5i0.fsf@book.mteege.de>,
  Matthias Teege <matthias@mteege.de> wrote:
> now I have a problem that I didnt understand. The followig
> code gives me a raised CONSTRAINT_ERROR (in Line:
> StrArg := Ada.Command_Line.Argument(I)) and I didnt know
> why.
...
> StrArg : String (1 .. 10);
...
>       StrArg := Ada.Command_Line.Argument(I);

That's why. Unless your command line argument happens to be *exactly* 10
characters long, you will get a constraint error here.

Check out the following entry in our FAQ:
http://www.adapower.com/lab/adafaq/24.html


Ada's string handling is very powerful. But its also very different from
most other languages. The important thing to remember is that its length
is defined by the variable's *declaration*, not by its contents.
Generally you either want to set the string's size with an
initialization:

   loop
      declare
         StrArg : constant String := Ada.Command_Line.Argument(I);
      begin
         ...
      end;
   end loop;

...or you want to declare one string big enough to hold the entire
argument, no matter how big it could reasonably get, then assign into a
correctly-sized slice of it.

      Str_Arg_Length := Ada.Command_Line.Argument(I)'length;
      StrArg (1..Str_Arg_Length) := Ada.Command_Line.Argument(I);

      Put_Line (StrArg(1..Str_Arg_Length));

Since Ada strings don't rely on a terminator to define their length, the
'length operation does *not* iterate through the string. It might not
even generate any code at all if you have optimizations on.

Another alternative is to use a dynamic string type, like
Ada.Strings.Unbounded.Unbounded_String. Or you can use the source string
in-situ (like you did in your second example)

--
T.E.D.

http://www.telepath.com/~dennison/Ted/TED.html


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