【SICP练习】142 练习3.77

练习3-77

原文

Exercise 3.77. The integral procedure used above was analogous to the “implicit” definition of the infinite stream of integers in section 3.5.2. Alternatively, we can give a definition of integral that is more like integers-starting-from (also in section 3.5.2):

(define (integral integrand initial-value dt)   (cons-stream initial-value                            (if (stream-null? integrand)                             the-empty-stream                               (integral (stream-cdr integrand)                                                           (+ (* dt (stream-car integrand))                                                                 initial-value)                                                           dt))))

When used in systems with loops, this procedure has the same problem as does our original version of integral. Modify the procedure so that it expects the integrand as a delayed argument and hence can be used in the solve procedure shown above.

代码

 (define (integral delayed-integrand initial-value dt)          (cons-stream initial-value                  (let ((integrand (force delayed-integrand)))                          (if (stream-null? integrand)                                  the-empty-stream                                  (integral (delay (stream-cdr integrand))                                            (+ (* dt (stream-car integrand))                                                       initial-value)                                                 dt))))) 

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