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Can you place eight  
queens on a chess    
board so that no two 
attack each other?   
Almost!
 Press the spacebar to continue! 
Positions Tested: 
      Queens Set: 
 END OF DEMO. ! 
return.chn
DEMO of QUEENS.PAS
)Enter a speed from 1 to 9 (1=slowest),   
!or 0 to single step thru program 
*Choose a display mode: Full or Overview.  
Enter F or O 
 Press spacebar to continue... 
0SPACE=continue, ESC=quit, 0-9 to change speed,  
+ to change mode,  
 to scroll
InRow
InDiag1
InDiag2
QueenPos
Success
"{QUEENS.PAS = program to solve the
+ 8-queens problem  by John Sigle  1/21/88 }
%PROGRAM EightQueens( Input, Output );
VAR K: INTEGER;
    Success: Boolean;
"    InRow: ARRAY[1..8] OF BOOLEAN;
*    InDiag1: ARRAY[2..16] OF BOOLEAN;  {/}
*    InDiag2: ARRAY[-7..7] OF BOOLEAN;  {\}
%    QueenPos: ARRAY[1..8] OF INTEGER;
PROCEDURE Try( Col: INTEGER );
VAR Row: INTEGER;
,BEGIN { Try to place a queen in column Col }
  Row := 0;
  REPEAT   { Try next row }
    Row := Row+1;
    Success := FALSE;
    { Test position }
)    IF NOT(InRow[Row] OR InDiag1[Col+Row]
#        OR InDiag2[Col-Row])   THEN
      BEGIN
        { Set the queen }
        QueenPos[Col] := Row;
        InRow[Row] := TRUE;
!        InDiag1[Col+Row] := TRUE;
!        InDiag2[Col-Row] := TRUE;
        IF Col<8 THEN
(          BEGIN { Try to place another }
            Try( Col+1 );
            IF NOT Success THEN
*              BEGIN  { Remove this queen }
#                QueenPos[Col] := 0;
$                InRow[Row] := FALSE;
*                InDiag1[Col+Row] := FALSE;
)                InDiag2[Col-Row] := FALSE
              END
          END
        ELSE Success := TRUE
	      END
  UNTIL Success OR (Row=8)
END;   { of Try }
BEGIN    { Main program }
  { Initialize indicators }
)  FOR K := 1 TO 8 DO   InRow[K] := FALSE;
+  FOR K := 2 TO 16 DO  InDiag1[K] := FALSE;
+  FOR K := -7 TO 7 DO  InDiag2[K] := FALSE;
(  FOR K := 1 TO 8 DO   QueenPos[K] := 0;
  { Try to find a solution }
  TRY( 1 );
END.  { of EightQueens }
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