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Task/N-queens-problem/Yabasic/n-queens-problem.basic

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+DOCU The N Queens Problem:
+DOCU Place N Queens on an NxN chess board
+DOCU such that they don't threaten each other.
+
+N = 8 // try some other sizes
+
+sub threat(q1r, q1c, q2r, q2c)
+// do two queens threaten each other?
+
+   if q1c = q2c then    return true
+   elsif (q1r - q1c) = (q2r - q2c) then return true
+   elsif (q1r + q1c) = (q2r + q2c) then return true
+   elsif q1r = q2r then    return true
+   else return false
+   end if
+end sub
+
+sub conflict(r, c, queens$)
+// Would square p cause a conflict with other queens on board so far?
+   local r2, c2
+
+   for i = 1 to len(queens$) step 2
+       r2 = val(mid$(queens$,i,1))
+       c2 = val(mid$(queens$,i+1,1))
+       if threat(r, c, r2, c2) then
+            return true
+       end if
+    next i
+   return false
+end sub
+
+sub print_board(queens$)
+// print a solution, showing the Queens on the board
+   local k$
+
+   print at(1, 1);
+   print "Solution #", soln, "\n\n   ";
+   for c = asc("a") to (asc("a") + N - 1)
+        print chr$(c)," ";
+   next c
+   print
+   for r = 1 to N
+        print r using "##"," ";
+        for c = 1 to N
+            pos = instr(queens$, (str$(r)+str$(c)))
+            if pos and mod(pos, 2) then
+                queens$ = mid$(queens$,pos)
+                print "Q ";
+            else
+                print ". ";
+            end if
+        next c
+        print
+    next r
+    print "\nPress Enter. (q to quit) "
+    while(true)
+        k$ = inkey$
+        if lower$(k$) = "q" then
+           exit
+        elsif k$ = "enter" then
+           break
+        end if
+    wend
+end sub