sicp习题试解 (2.41)

; ======================================================================

;

; Structure and Interpretation of Computer Programs

; (trial answer to excercises)

;

; 计算机程序的构造和解释(习题试解)

;

; created: code17 08/19/05

; modified:

; (保持内容完整不变前提下，可以任意转载)

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;; SICP No.2.41

(define (sum-s-triple n s)

(define (sum-s? x) (= (accumulate + 0 x) s))

(filter sum-s?

(flatmap

(lambda (i) (flatmap

(lambda (j) (map

(lambda (k) (list i j k))

(enumerate-interval 1 (- j 1))))

(enumerate-interval 1 (- i 1))))

(enumerate-interval 1 n))))

;; Test-it:

;; Welcome to MzScheme version 209, Copyright (c) 2004 PLT Scheme, Inc.

;; > (sum-s-triple 7 10)

;; ((5 3 2) (5 4 1) (6 3 1) (7 2 1))

;; > (sum-s-triple 7 12)

;; ((5 4 3) (6 4 2) (6 5 1) (7 3 2) (7 4 1))