Question

Please use the Scheme programming language with Dr. Racket to solve a and b below. (Use Scheme and not Python)

Write before–in–list?, which takes a list and two elements of the list. It should return #t if the second argument appears in the list argument before the third argument:

> (before–in–list? '(back in the ussr) 'in 'ussr)
#T

> (before–in–list? '(back in the ussr) 'the 'back)
#F

The procedure should also return #f if either of the supposed elements doesn't appear at all.

Answer 1

Code:

(define (before-in-last? list x y)
(cond
((null? list) #f) ;; if list empty returns false
((equal? (car list) x) (member? y (cdr list))) ;;if second arg = 1st element of list, check for third arg in rest of the list
((equal? (car list) y) #f) ;;if third argument is 1st element of the list return true as it has to appear after second argument
(else (before-in-last? (cdr list) x y)))) ;;otherwise recursively traverse rest of the list

(define (member? a list) ;;checks an element is member of list or not
(cond ((null? list ) #f) ;;if list is empty returns false
((equal? (car list)a) #t) ;;if searching element is equal to first member of list returns true
(else (member? a (cdr list))))) ;;otherwise check in remaining list

Snapshot of Code and Output: