Chapter 2, Building Abstractions with Data
Section - 2.2 - Hierarchical Data and the Closure Property
Exercise 2.40
Code for generating unique-pairs:
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#lang sicp
(define (fold-right op initial sequence)
(if (null? sequence)
initial
(op (car sequence)
(fold-right op initial (cdr sequence))
)
)
)
(define (fold-left op initial sequence)
(define (iter result rest)
(if (null? rest)
result
(iter (op result (car rest))
(cdr rest)
)
)
)
(iter initial sequence)
)
(define (flatmap proc seq)
(fold-right append nil (map proc seq))
)
(define (enumerate-interval i j)
(if (> i j)
nil
(cons i (enumerate-interval (+ i 1) j))
)
)
(define (unique-pairs n)
(flatmap
(lambda (i)
(map
(lambda (j)
(list j i)
)
(enumerate-interval 1 (- i 1))
)
)
(enumerate-interval 1 n)
)
)
Example/Output:
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> (display (unique-pairs 4))
((2 1) (3 1) (3 2) (4 1) (4 2) (4 3))
> (display (unique-pairs 8))
((2 1) (3 1) (3 2) (4 1) (4 2) (4 3) (5 1) (5 2) (5 3) (5 4) (6 1) (6 2) (6 3) (6 4) (6 5)
(7 1) (7 2) (7 3) (7 4) (7 5) (7 6) (8 1) (8 2) (8 3) (8 4) (8 5) (8 6) (8 7))
Using unique-pairs in procedure prime-sum-pairs:
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(define (prime-sum-pairs n)
(map make-pair-sum
(filter prime-sum?
(unique-pairs n)
)
)
)