Re: Dropping some misfeatures in for-loops and `absolute' variables

29 Jun 2005

      One can handle nested for loops with REPEAT and WHILE loops, without
going into FOR loop contortions:

 From a Delphi program I wrote long ago.  By using dynamic arrays, you 
can get around a fixed
dimension.

unit ForLoopUnit;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs;

type
  TForm1 = class(TForm)
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation
{$R *.DFM}

const  Dim = 3;

type  TIndex = array[1..Dim]  of  Integer;

var  F: TextFile;
     Index, LowerBound, UpperBound: TIndex;

procedure  Initialize;

  var  K: Integer;

  begin
  for  K := 1  to  Dim  do
    begin
    LowerBound[K] := 0;
    UpperBound[K] := 5;    //Random(6) + 1;
    end;
  end;  // Initialize

procedure  P(const  Index: TIndex);   // for testing

  var  K: Integer;
       S: string;

  begin
  S := 'Index = [';
  for  K := 1  to  Dim - 1  do  S := S + IntToStr(Index[K]) + ', ';
  S := S + IntToStr(Index[Dim]) + ']';
  Writeln(F, S);
  end;  // P

procedure  MultiForLoop;

  var  K: Integer;

  begin
  Index := LowerBound;
  K := Dim;
  repeat
    P(Index);
    while  (K >= 1) and (Index[K] = UpperBound[K])  do  Dec(K);
    if  K = 0  then  Break;  // repeat loop
    // K >= 1, Index[K] < UpperBound[K],
    // and Index[J] < UpperBound[J] for all J > K
    Inc(Index[K]);  Inc(K);
    while  K <= Dim  do
      begin  Index[K] := LowerBound[K];  Inc(K)  end;
  until  False;
  end;

initialization
AssignFile(F, 'For.Pas');
Rewrite(F);
Initialize;
MultiForLoop;
CloseFile(F);
end.

Harley Flanders

====================================

Frank D. Engel, Jr. wrote:
...
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On Jun 29, 2005, at 2:19 AM, Frank Heckenbach wrote:
...
GPC accepts the following program, but produces quite silly results:
program Foo;
var
  a: array [1 .. 2] of Integer value (0, 0);
  i: Integer;
begin
  i := 1;
  for a[i] := 1 to 10 do
    begin
      WriteLn (i, ' ', a[i]);
      if a[i] = 5 then i := 2
    end
end.
The problem is that it evaluates the index i each time, not once
before the loop. It would be possible to fix it, but do we really
want that? Neither BP nor CP/EP allow it, they allow only
identifiers in for-loops.
Waldek and I see no need for such a feature. (If it made it possible
to nest a compile-time-unknown number of `for' loops, I could see
some point, but of course, it doesn't.)
Actually, I think it does (untested, though):
CONST
    max_depth = 100;
FUNCTION Multi_For(depth, limit : INTEGER) : LONGINT;
VAR
    i : INTEGER;
    a : ARRAY[1..max_depth] OF INTEGER;
    v : LONGINT;
BEGIN
    { setup }
    i := 1;
    v := 0;
{ the loops }
    FOR a[i] := 1 TO limit DO BEGIN
        IF (i < depth) THEN BEGIN
            INC(i);
            a[i] := 1
        END ELSE BEGIN
            { actual loop stuff }
            v := v + a[i];
{ handle reduction in depth at end of loop }
            IF (a[i] = limit) AND (i > 1) THEN
                DEC(i)
        END
    END;
Multi_For := v
END;
- -----------------------------------------------------------
Frank D. Engel, Jr.  <fde101@fjrhome.net>
$ ln -s /usr/share/kjvbible /usr/manual
$ true | cat /usr/manual | grep "John 3:16"
John 3:16 For God so loved the world, that he gave his only begotten 
Son, that whosoever believeth in him should not perish, but have 
everlasting life.
$
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===========================================
Prof. Harley Flanders
3533 Windemere Court
Ann Arbor, MI 48105-2867
Home: 734 668 1546
harley@umich.edu
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