55 lines
1.4 KiB
Scheme
55 lines
1.4 KiB
Scheme
#lang sicp
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; a) Define unique-pairs
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(define (accumulate op initial sequence)
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(if (null? sequence)
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initial
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(op (car sequence)
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(accumulate op initial (cdr sequence)))))
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(define (flatmap proc seq)
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(accumulate append nil (map proc seq)))
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(define (enumerate-interval a b)
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(if (> a b)
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'()
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(cons a (enumerate-interval (+ a 1) b))))
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(define (unique-pairs n)
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(flatmap (lambda (i)
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(map (lambda (j) (list i j))
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(enumerate-interval 1 (- i 1))))
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(enumerate-interval 1 n)))
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; b) use it to define prime-sum-pairs
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(define (filter predicate sequence)
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(cond ((null? sequence) nil)
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((predicate (car sequence))
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(cons (car sequence)
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(filter predicate (cdr sequence))))
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(else (filter predicate (cdr sequence)))))
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(define (square x) (* x x))
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(define (smallest-divisor n)
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(find-divisor n 2))
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(define (find-divisor n test-divisor)
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(cond ((> (square test-divisor) n) n)
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((divides? test-divisor n) test-divisor)
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(else (find-divisor n (+ test-divisor 1)))))
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(define (divides? a b)
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(= (remainder b a) 0))
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(define (prime? n)
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(= n (smallest-divisor n)))
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(define (prime-sum? pair)
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(prime? (+ (car pair) (cadr pair))))
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(define (prime-sum-pairs n)
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(map (lambda (pair)
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(list (car pair) (cadr pair) (+ (car pair)
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(cadr pair))))
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(filter prime-sum?
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(unique-pairs n))))
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