6 files changed, 130 insertions, 1 deletions
diff --git a/coding-exercises/2/40.rkt b/coding-exercises/2/40.rkt
new file mode 100644
index 0000000..cf71b5d
--- /dev/null
+++ b/coding-exercises/2/40.rkt
@@ -0,0 +1,42 @@
+#lang racket
+(require "../../shared/lists.rkt")
+(require "../../shared/chapter1.rkt")
+
+(define (enumerate-interval j k)
+  (define (iter i interval)
+    (if (< i j) 
+      interval
+      (iter (- i 1) (cons i interval))))
+  (iter k '()))
+      
+((lambda ()
+   (newline)
+   (display (enumerate-interval 2 5))))
+
+(define (unique-pairs n) 
+  (flatmap
+    (lambda (i) 
+      (map (lambda (j) (list j i)) 
+           (enumerate-interval 1 (- i 1))))
+    (enumerate-interval 1 n)))
+
+(define (prime? n)
+  (fermat? 2 n))
+(define (prime-sum? pair)
+  (prime? (+ (car pair) (cadr pair))))
+(define (make-pair-sum pair)
+  (list (car pair) (cadr pair)
+        (+ (car pair) (cadr pair))))
+(define (prime-sum-pairs n)
+  (map make-pair-sum
+       (filter prime-sum? (unique-pairs n))))
+
+((lambda ()
+   (display "unique-pairs")
+   (newline)
+   (display (unique-pairs 10))
+   (newline)
+   (display "prime-sum-pairs")
+   (newline)
+   (display (prime-sum-pairs 10))
+   (newline)))
diff --git a/coding-exercises/2/41.rkt b/coding-exercises/2/41.rkt
new file mode 100644
index 0000000..0871313
--- /dev/null
+++ b/coding-exercises/2/41.rkt
@@ -0,0 +1,19 @@
+#lang racket
+(require "../../shared/lists.rkt")
+
+(define (ordered-triples-sum n s)
+  (if (< n 3)
+    '()
+    (filter (lambda (triplet)
+              (= (fold-right + 0 triplet) s))
+            (flatmap (lambda (i)
+                       (flatmap (lambda (pair)
+                                 (permutations (cons i pair)))
+                        (unique-pairs (- i 1))))
+              (enumerate-interval 3 n)))))
+
+((lambda ()
+   (newline)
+   (display (enumerate-interval 3 3))
+   (newline)
+   (display (ordered-triples-sum 4 8))))
diff --git a/coding-exercises/2/42.rkt b/coding-exercises/2/42.rkt
new file mode 100644
index 0000000..eb1a5fc
--- /dev/null
+++ b/coding-exercises/2/42.rkt
@@ -0,0 +1,44 @@
+#lang racket
+(require "../../shared/lists.rkt")
+
+(define (adjoin-position new-row k rest-of-queens)
+  (cons (list new-row k) rest-of-queens))
+
+(define (safe? k positions)
+  (let ((maybe-new-queen (car positions))
+        (rest-of-queens (cdr positions)))
+     (fold-right (lambda (queen safe)
+                   (let ((dx (abs (- (car queen) (car maybe-new-queen)))))
+                     (and
+                       safe
+                       (not (or (= dx 0) (= dx (- k (cadr queen))))))))
+                true
+                rest-of-queens)))
+
+((lambda ()
+   (newline)
+   (display "safe?-test")
+   (newline)
+   (display (safe? 2 (list (list 1 2) (list 1 1) (list 0 0))))
+   (display (safe? 2 (list (list 1 2) (list 1 1) (list 1 0))))
+   (display (safe? 2 (list (list 1 2) (list 1 1) (list 2 0))))
+   (display (safe? 2 (list (list 5 2) (list 1 1) (list 3 0))))
+   (display (safe? 6 (list (list 6 6) (list 5 5) (list 4 4))))))
+
+(define empty-board '())
+
+(define (queens board-size)
+  (define (queen-cols k)
+    (if (= k 0)
+      (list empty-board)
+      (filter
+        (lambda (positions) (safe? k positions))
+        (flatmap
+          (lambda (rest-of-queens)
+            (map (lambda (new-row)
+                   (adjoin-position new-row k rest-of-queens))
+                 (enumerate-interval 1 board-size)))
+          (queen-cols (- k 1))))))
+  (queen-cols board-size))
+
+(length (queens 8))
diff --git a/coding-exercises/2/43.rkt b/coding-exercises/2/43.rkt
new file mode 100644
index 0000000..96b9373
--- /dev/null
+++ b/coding-exercises/2/43.rkt
@@ -0,0 +1,2 @@
+#lang racket
+(require "../../shared/lists.rkt")
diff --git a/coding-exercises/2/44.rkt b/coding-exercises/2/44.rkt
new file mode 100644
index 0000000..ae76481
--- /dev/null
+++ b/coding-exercises/2/44.rkt
@@ -0,0 +1,2 @@
+#lang racket
+(require sicp-pic)
diff --git a/shared/lists.rkt b/shared/lists.rkt
index 4f2184d..b721623 100644
--- a/shared/lists.rkt
+++ b/shared/lists.rkt
@@ -2,7 +2,9 @@
 (provide accumulate
          accumulate-n
          fold-right
-         fold-left)
+         fold-left
+         flatmap
+         enumerate-interval)
 (define (append list1 list2)
   (if (null? list1)
     list2
@@ -48,3 +50,21 @@
       (iter (op result (car rest))
             (cdr rest))))
   (iter initial sequence))
+
+(define (flatmap proc seq)
+  (accumulate append '() (map proc seq)))
+
+(define (enumerate-interval j k)
+  (define (iter i interval)
+    (if (< i j)
+      interval
+      (iter (- i 1) (cons i interval))))
+  (iter k '()))
+
+(define (unique-pairs n)
+  (flatmap
+    (lambda (i)
+      (map (lambda (j) (list j i))
+           (enumerate-interval 1 (- i 1))))
+    (enumerate-interval 1 n)))
+