September 2017 Update

Task/N-queens-problem/Zkl/n-queens-problem-1.zkl

@@ -0,0 +1,9 @@
+fcn isAttacked([(x,y)],a,b) // explode list (x,y) into args x & y
+   { (x==a or y==b or x+y==a+b or x-y==a-b) }
+fcn isSafe(a,b,qs){(not qs.filter1(isAttacked,a,b))} // stop at first attack
+fcn queensN(N=8,row=1,queens=T){  // T is read only list
+   qs := [1..N].filter(isSafe.fpM("101",row,queens))  // fpM makes r&q first & third args
+               .apply(fcn(c,r,qs){qs+T(r,c)},row,queens);
+   if (row == N) return(qs);
+   return(qs.apply(self.fcn.fp(N,row+1)).flatten());  // recurse
+}

Task/N-queens-problem/Zkl/n-queens-problem-2.zkl

@@ -0,0 +1,6 @@
+queens := queensN(4);
+println(queens.len()," solution(s):");
+queens.apply2(Console.println);
+--> 2 solution(s):
+L(L(1,2),L(2,4),L(3,1),L(4,3))
+L(L(1,3),L(2,1),L(3,4),L(4,2))