sicp每日一题[2.40]

Exercise2.40

Define a procedure unique-pairs that, given an integer 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.

这道题还是挺简单的，最难的部分书上已经实现了，我们只要理解了书上这部分的代码，然后再加个大小的判断即可。

(define (unique-pairs n)
  (flatmap (lambda (i) (map (lambda (j) (cond ((> i j) (list i j))))
                            (enumerate-interval 1 (- i 1))))
           (enumerate-interval 1 n)))

(unique-pairs 5)

(define (prime-sum-pairs n)
  (map make-pair-sum
       (filter prime-sum? (unique-pairs n))))

(prime-sum-pairs 10)

; 执行结果
'((2 1) (3 1) (3 2) (4 1) (4 2) (4 3) (5 1) (5 2) (5 3) (5 4))
'((2 1 3) (3 2 5) (4 1 5) (4 3 7) (5 2 7) (6 1 7) (6 5 11) (7 4 11) (7 6 13) (8 3 11) (8 5 13) (9 2 11) (9 4 13) (9 8 17) (10 1 11) (10 3 13) (10 7 17) (10 9 19))