June 2018 Update

Task/N-queens-problem/Stata/n-queens-problem-1.stata

@@ -9,34 +9,32 @@ real matrix queens(real scalar n) {
 	s = J(1, n, 0)
 	u = J(1, 2*n-1, 1)
 	v = J(1, 2*n-1, 1)
-	i = j = 1
-L1:	k = a[j]
-	a[j] = a[i]
-	a[i] = k
+	i = 1
+L1:	if (i > n) {
+		m = m\a
+		goto L4
+	}
+	j=i
+L2:	k = a[j]
 	p = i-k+n
 	q = i+k-1
 	if (u[p] & v[q]) {
-		s[i++] = j
 		u[p] = v[q] = 0
-		if (i > n) {
-			m = m\a
-			goto L3
-		}
-		j = i
+		a[j] = a[i]
+		a[i] = k
+		s[i++] = j
 		goto L1
 	}
-L2:	if (++j <= n) goto L1
-	while (--j > i) {
-		k = a[i]
-		a[i] = a[j]
-		a[j] = k
-	}
-L3:	if (--i == 0) return(m)
-	p = i-a[i]+n
-	q = i+a[i]-1
+L3:	if (++j <= n) goto L2
+L4:	if (--i == 0) return(m)
 	j = s[i]
+	k = a[i]
+	a[i] = a[j]
+	a[j] = k
+	p = i-k+n
+	q = i+k-1
 	u[p] = v[q] = 1
-	goto L2
+	goto L3
 }
 
 a = queens(8)

Task/N-queens-problem/Stata/n-queens-problem-3.stata

@@ -1,5 +1,5 @@
 mata
-real matrix queens(real scalar n) {
+real matrix queens_rec(real scalar n) {
 	real rowvector a, u, v
 	real matrix m
 	
@@ -13,46 +13,25 @@ real matrix queens(real scalar n) {
 
 void queens_aux(real scalar n, real scalar i, real rowvector a,
 	real rowvector u, real rowvector v, real matrix m) {
-	
-	real scalar j, k, s
+	real scalar j, k
 	
 	if (i > n) {
 		m = m\a
 	} else {
-		for (k = i; k <= n; k++) {
-			j = a[k]
-			p = i+j-1
-			q = i-j+n
+		for (j = i; j <= n; j++) {
+			k = a[j]
+			p = i-k+n
+			q = i+k-1
 			if (u[p] & v[q]) {
 				u[p] = v[q] = 0
-				s = a[i]
-				a[i] = a[k]
-				a[k] = s
+				a[j] = a[i]
+				a[i] = k
 				queens_aux(n, i+1, a, u, v, m)
 				u[p] = v[q] = 1
-				s = a[i]
-				a[i] = a[k]
-				a[k] = s
+				a[i] = a[j]
+				a[j] = k
 			}
 		}
 	}
 }
-
-a = queens(8)
-e = I(8)
-1:/e[a[1,.],.]
-       1   2   3   4   5   6   7   8
-    +---------------------------------+
-  1 |  1   .   .   .   .   .   .   .  |
-  2 |  .   .   .   .   1   .   .   .  |
-  3 |  .   .   .   .   .   .   .   1  |
-  4 |  .   .   .   .   .   1   .   .  |
-  5 |  .   .   1   .   .   .   .   .  |
-  6 |  .   .   .   .   .   .   1   .  |
-  7 |  .   1   .   .   .   .   .   .  |
-  8 |  .   .   .   1   .   .   .   .  |
-    +---------------------------------+
-
-rows(a)
-  92
 end

Task/N-queens-problem/Stata/n-queens-problem-4.stata

@@ -0,0 +1,2 @@
+queens(8) == queens_rec(8)
+  1