【SICP练习】121 练习3.52
2015-03-28 21:55
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练习3-52
原文
Exercise 3.52. Consider the sequence of expressions(define sum 0) (define (accum x) (set! sum (+ x sum)) sum) (define seq (stream-map accum (stream-enumerate-interval 1 20))) (define y (stream-filter even? seq)) (define z (stream-filter (lambda (x) (= (remainder x 5) 0)) seq)) (stream-ref y 7) (display-stream z)
What is the value of sum after each of the above expressions is evaluated? What is the printed response to evaluating the stream-ref and display-stream expressions? Would these responses differ if we had implemented (delay ) simply as (lambda () ) without using the optimization provided by memo-proc ? Explain.
代码
(define seq (stream-map accum (stream-enumerate-interval 1 20))) sum ;Value: 1 (define y (stream-filter even? seq)) sum ;Value: 6 (define z (stream-filter (lambda (x) (= (remainder x 5) 0)) seq)) (stream-ref y 7) ;Value: 136
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