tasks a-s

Task/N-queens-problem/Ada/n-queens-problem.ada

@@ -0,0 +1,50 @@
+with Ada.Text_IO;  use Ada.Text_IO;
+
+procedure Queens is
+   Board : array (1..8, 1..8) of Boolean := (others => (others => False));
+   function Test (Row, Column : Integer) return Boolean is
+   begin
+      for J in 1..Column - 1 loop
+         if (  Board (Row, J)
+            or else
+               (Row > J and then Board (Row - J, Column - J))
+            or else
+               (Row + J <= 8 and then Board (Row + J, Column - J))
+            )  then
+            return False;
+         end if;
+      end loop;
+      return True;
+   end Test;
+   function Fill (Column : Integer) return Boolean is
+   begin
+      for Row in Board'Range (1) loop
+         if Test (Row, Column) then
+            Board (Row, Column) := True;
+            if Column = 8 or else Fill (Column + 1) then
+               return True;
+            end if;
+            Board (Row, Column) := False;
+         end if;
+      end loop;
+      return False;
+   end Fill;
+begin
+   if not Fill (1) then
+      raise Program_Error;
+   end if;
+   for I in Board'Range (1) loop
+      Put (Integer'Image (9 - I));
+      for J in Board'Range (2) loop
+         if Board (I, J) then
+            Put ("|Q");
+         elsif (I + J) mod 2 = 1 then
+            Put ("|/");
+         else
+            Put ("| ");
+         end if;
+      end loop;
+      Put_Line ("|");
+   end loop;
+   Put_Line ("   A B C D E F G H");
+end Queens;