SICP Exercise 2.22

Exercise 2.22.  Louis Reasoner tries to rewrite the first square-list procedure of exercise 2.21 so that it evolves an iterative process:


(define (square-list items)
  (define (iter things answer)
    (if (null? things)
        answer
        (iter (cdr things)
              (cons (square (car things))
                    answer))))
  (iter items nil))

Unfortunately, defining square-list this way produces the answer list in the reverse order of the one desired. Why?
Louis then tries to fix his bug by interchanging the arguments to cons:


(define (square-list items)
  (define (iter things answer)
    (if (null? things)
        answer
        (iter (cdr things)
              (cons answer
                    (square (car things))))))
  (iter items nil))

This doesn't work either. Explain.

SOLUTION


Comments

Post a Comment

Popular posts from this blog

SICP Exercise 4.18 a alternative strategy for interpreting internal definitions

Exercise 4.18.   Consider an alternative strategy for scanning out definitions that translates the example in the text to (lambda < vars >   (let ((u '*unassigned*)         (v '*unassigned*))     (let ((a < e1 >)           (b < e2 >))       (set! u a)       (set! v b))     < e3 >)) Here  a  and  b  are meant to represent new variable names, created by the interpreter, that do not appear in the user's program. Consider the  solve  procedure from section  3.5.4 : (define (solve f y0 dt)   (define y (integral (delay dy) y0 dt))   (define dy (stream-map f y))   y) Will this ...
Read more

SICP Exercise 3.11 make-account internal definitions with local state

Image
Exercise 3.11.    In section  3.2.3  we saw how the environment model described the behavior of procedures with local state. Now we have seen how internal definitions work. A typical message-passing procedure contains both of these aspects. Consider the bank account procedure of section  3.1.1 : (define (make-account balance)   (define (withdraw amount)     (if (>= balance amount)         (begin (set! balance (- balance amount))                balance)         "Insufficient funds"))   (define (deposit amount)     (set! balance (+ balance amount))     balance)   (define (dispatch m)     (cond...
Read more

SICP Exercise 3.13 make-cycle

Image
Exercise 3.13.    Consider the following  make-cycle  procedure, which uses the  last-pair  procedure defined in exercise  3.12 : (define (make-cycle x)   (set-cdr! (last-pair x) x)   x) Draw a box-and-pointer diagram that shows the structure  z  created by (define z (make-cycle (list 'a 'b 'c))) What happens if we try to compute  (last-pair z) ? SOLUTION The code is here: Exercise 3.13 make-cycle The box-and-pointer diagrams are here:
Read more