Adds the solution of exercise 2.40(uniqure-pairs)

veselinn · Apr 16, 2012 · 0a0ebcf · 0a0ebcf
1 parent d1bbd39
commit 0a0ebcf
Showing 1 changed file with 16 additions and 0 deletions.
diff --git a/chapter2/40.scm b/chapter2/40.scm
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+; Exercise 2.40: 
+; Define a procedure unique-pairs that, given an in-
+; teger n , generates the sequence of pairs (i , j ) with 1 ≤ j < i ≤ n .
+; Use unique-pairs to simplify the definition of prime-sum-pairs
+; given above.
+
+(define (enumerate-interval low high)
+  (if (> low high)
+    null
+    (cons low (enumerate-interval (+ low 1) high))))
+
+(define (unique-pairs low high)
+  (if (>= low high)
+    null
+    (append (map (lambda (x) (cons high x)) (enumerate-interval low (- high 1)))
+      (unique-pairs low (- high 1)))))