Re: Constraint Error - Please Help

[Richard D Riehle <LaoXhai@ix.netcom.com>]

In article <7445iv$gog$1@news.nyu.edu>,
	kolc7797@PROBLEM_WITH_YOUR_MAIL_GATEWAY_FILE.nyu.edu (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!

 You have an error in your algorithm.  The first four iterations work
 fine and the index will be incremented as 2, 3, 4, 5.  Then, because
 of the way you compute parent and later assign it to this, the whole
 algorithm wobbles out of phase and the variable, this, returns to 
 the value 2.  This is inconsistent with the rest of the behavior 
 expected of the while loop.  

 If you handle and raise exceptions for the while loop and put in a
 line at the end of the for loop to print the successive values of
 your variables you will see at once the problem associated with 
 your calculation on the variable parent and its subsequent assignment
 to the variable this.

>
>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
>         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);
>       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;
>    
>    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
>----------------------------------------------------------------
>         copy(this):= copy(parent);
>	   this:= parent;
>	     parent:=(parent-1)/2;
>       end loop;
>       copy(this):=contents;
>    end loop;
>    
>    for last in reverse x'first+1..x'last loop
>       --swap first position with last
>       --end_node needs to be filtered down into heap
>       end_node:= copy(last);
>       copy(last):=copy(x'first);
>       
>       --start at the root node
>       current:=x'first;
>       --traverse the heap
>       loop
>         --look left
>	   left:=2*current+1;
>	     --if no left node, you've found the right slot
>	       exit when left>last-1;
>	         -- if there is a left node then find whether left or 
>		   -- right node (if any) is max_l_r
>		     right:=left+1;
>		       if right>last-1 or copy(right)<copy(left) then
>		            max_l_r:= left;
>			      else
>			           max_l_r:= right;
>				     end if;
>				       -- if end_node is bigger than
>       both left and right children, then it
>         -- must go here, so stop traversing
>	   exit when copy(max_l_r) < end_node;
>	     -- otherwise, move down
>	       copy(current):=copy(max_l_r);
>	         current:=max_l_r;
>       end loop;
>       -- after the loop, current points to the insertion spot
>       copy(current):= end_node;
>    end loop;
>    return copy;
>end;
>
>function sortFloat is new sort_generic (float, floatArray, "<");
>      
>begin
>   fa:=(7.2, 8.4, 3.5, 0.3, -5.4, 9.9, -0.5, 8.0, 1.1, 3.8);
>   new_line;
>   put("Testing sort for float vectors");
>   new_line;
>   put("Before: ");
>   new_line;
>   printFloatArray (fa);
>   
>   shfa:=sortFloat(fa);
>   put("After: ");
>   new_line;
>   printFloatArray(fa);
>   new_line;
>end;