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From: "Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org>
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Subject: Re: basic basic ada question
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markww wrote:
> Ok, am I understanding this correctly - if I just '#include" the
> command line package:

"with" is significantly different from simple text inclusion. Most of 
what you know from C/++ is wrong for Ada. The sooner you can stop 
thinking in C/++ and start thinking in Ada, the easier you will find it.

>     with Ada.Command_Line; use Ada.Command_Line;

As a beginner, I would strongly recommend that you avoid "use", at least 
until you understand Ada's visibility rules better.

> then the package defines the functions:
> 
>     function Argument_Count return Natural;
>     function Argument (Number : in Positive) return String;
> 
> for me which I can use from then on?
> 
> So in my little app I can just use them like:
> 
>     Number_Of_Elements := Argument(0);
> 
> (assuming the 0th argument is the # of elements to allocate...)

As you quoted, Argument's parameter Number has subtype Positive. Zero is 
not a value of Positive. So such a call will raise Constraint_Error. In 
Ada, as in life, we usually count or number things starting from 1. 
Getting away from the low-level, offset-oriented C mindset is part of 
learning to think in Ada.

ARM A.15 says, "If the external execution environment supports passing 
arguments to a program, then Argument returns an implementation-defined 
value corresponding to the argument at relative position Number. If 
Number is outside the range 1..Argument_Count, then Constraint_Error is 
propagated."

So, if you have a single argument, it will be number 1. If you have 2, 
they will be 1 and 2. And so on.

It's a good idea to get familiar with Annex A.

Second, Argument's return type is String. Assuming Number_Of_Elements is 
the same type as the value being assigned to it (String), assigning 
complete Strings is usually not a good idea, since source and target 
must be the same length.

Finally, assuming that Number_Of_Elements is, as its name suggests, an 
object of a numeric type, source and target have different types, and 
the statement will result in a compilation error.

-- 
Jeff Carter
"I fart in your general direction."
Monty Python & the Holy Grail
05