Question

I have a list

 L=(1 j 3 k 4 h 5 n 6 w)

I need to do a function Verify that will verify if the first atom is before the 2nd.
I want to verify this:

> Verify(3 k)

result should return

> T

// because atom ‘3’ is before atom ‘k’

And in this case :

>Verify(h 4)

result should return

> NIL

// because atom ‘h’ is after atom ‘4’

I have to check position of each element and compare positions

Answer 1

What dialect of Lisp are you using? Here are some pointers on how to derive a solution, fill-in the blanks:

(define (verify lst a b)
        ; what happens if there's only one element left in the list?
  (cond ((null? (cdr lst)) <???>)
        ; how do we check if the current element is equal to the `a` parameter
        ; and the next element is equal to the `b` parameter?
        (<???> T)
        ; how do we continue traversing the rest of the list?
        (else (verify <???> a b))))

;;; tests

(define lst '(1 j 3 k 4 h 5 n 6 w))

(verify lst 3 'k)
> T
(verify lst 'h '4)
> F