SICP exercise 2.25

From Drewiki

Problem

Give combinations of cars and cdrs that will pick 7 from each of the following lists:

(1 3 (5 7) 9) 
((7)) 
(1 (2 (3 (4 (5 (6 7))))))

Solution

For testing purposes, let's name each list:

(define list1 (list 1 3 (list 5 7) 9))
 
(define list2 (list (list 7)))
 
(define list3 (list 1 (list 2 (list 3 (list 4 (list 5 (list 6 7)))))))

A simple method for constructing these car/cdr sequences is to find the 7 in the list, start with a car, and then work backwards towards the beginning of the list. For each integer you encounter, use a cdr. For the beginning of each list encountered, use a car until you reach the outer-most list.

For each combination given below, the output from the interpreter follows.

(car (cdr (car (cdr (cdr list1)))))

Output:

7

(car (car list2))

Output:

7

 
(car (cdr (car (cdr (car (cdr (car (cdr (car (cdr (car (cdr list3))))))))))))

Output:

7