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From: Jeff Carter <spam.carter.not@spam.innocon.com>
Subject: Re: Constraint Error - Please Help
Date: 1998/12/03
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Date: 1998-12-03T00:00:00+00:00
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Fanni Kolchina wrote:
> 
> Hi, All:
> 
> I am writing a generic sorting program in Ada using heap sort.  I am
> new to Ada, and have no experience with this. When I run my program, I
> get a Constraint Error: range error. I have spent lots of time
> thinking what is wrong, but I just cannot comprehend it.
> If someone could help me, I will greatly appreciate it.  Below is my
> code, and the line where the exception is thrown is marked by comment
> lines above and below it.  Thanks a lot!
> 
> with ada.text_io;                       use ada.text_io;
> with ada.integer_text_io;           use ada.integer_text_io;
> with ada.float_text_io;             use ada.float_text_io;
> 
> procedure test_sort is
> 
>    generic
>       type ITEM is private;
>       type SORT_ARRAY is array(Positive range <>) of ITEM;
>       with function "<" (u,v: ITEM) return boolean is <>;
> 
>       function sort_generic (x: SORT_ARRAY) return SORT_ARRAY;
> 
>       subtype index is positive range 1..10;
>       type floatArray is array (positive range <>) of float;
>     fa, hfa, shfa: floatArray(index);
> 
>     procedure printFloatArray (fa: floatArray) is
>     begin
>        for count in fa'first..fa'last loop

This is a good place to use 'range:

   for Count in Fa'range loop

>          put(fa(count), fore=>3, aft=>1, exp =>0);
>        end loop;
>        new_line;
>     end;
> 
>     function sort_generic (x: SORT_ARRAY) return SORT_ARRAY is
>        copy: SORT_ARRAY(x'range);

This is a good place to initialize Copy:

   Copy : Sort_Array := X;

>        last, left, right, max_l_r, current, this, parent: integer;
>        end_node, contents : ITEM;
>     begin
> 
>     for i in x'first..x'last loop
>        copy(i):=x(i);
>     end loop;

If you initialized Copy in its declaration, as I suggested above, then
this loop may be deleted.

> 
>     for i in x'first+1..x'last loop
>        this:=i;
>        parent:= (i-1)/2;
>        contents:=copy(this);
> ----------------------------------------------------------------
> -- here's the line with the exception ---
>        while this /= x'first and then copy(parent)<contents loop
> ----------------------------------------------------------------
> ...

We see later in your code that this is called with X'First = 1. So the
first time through this loop, I = 2, which means Parent = (2 - 1) / 2 =
1 / 2 = 0 (integer division truncates towards zero). So the condition of
the while loop is

2 /= 1 and then Copy (0) < Contents

Obviously, 2 is never equal to 1, so the comparison of Copy (0) to
Contents is performed. Zero is not a valid index for Copy, and a
run-time check detects this constraint violation and raises
Constraint_Error.

-- 
Jeff Carter
E-mail: carter commercial-at innocon [period | full stop] com
"Perfidious English mouse-dropping hoarders."
Monty Python & the Holy Grail