SICPゼミ第26回

練習問題3.50
(define (stream-map proc . argstreams)
  (if (null? (car argstreams ))
    the-empty-stream
    (stream-cons
      (apply proc (map stream-car argstreams))
      (apply stream-map
       (cons proc (map stream-cdr argstreams))))))

by dolicas

練習問題3.51
gosh> (define x
(stream-map show
(stream-enumerate-interval 0 10)))

0x
gosh> (stream-ref x 5)

1
2
3
4
55
gosh> (stream-ref x 7)

6
77
練習問題3.52
> (define sum 0)

>(define (accum x) (set! sum (+ x sum)) sum)
> sum
0

> (define seq
(stream-map accum
(stream-enumerate-interval 1 20)))
> sum
1

> (define y (stream-filter even? seq))
> sum
6

> (define z
(stream-filter (lambda (x) (= (remainder x 5) 0))
seq))
> sum
10

> (stream-ref y 7)
136
> sum
136

> (display-stream z)

10
15
45
55
105
120
190
210'done
> sum
210

メモ化していないと、毎回accumが呼ばれてsumに足されていってしまうのでへんなことになる。

by tube