Question

Write a Scheme predicate function that tests for the structural equality of two given lists. Two lists are structurally equal if they have the same list structure, although their atoms may be different.

(123) (456) is ok
(1(23))((12)3) is not ok

I have no idea how to do this. Any help would be appreciated.

Answer 1

Here are some hints. This one is a bit repetitive to write, because the question looks like homework I’ll let you fill-in the details:

(define (structurally-equal l1 l2)
  (cond ( ? ; if both lists are null
         #t)
        ( ? ; if one of the lists is null but the other is not
         #f)
        ( ? ; if the `car` part of both lists is an atom
         (structurally-equal (cdr l1) (cdr l2)))
        ( ? ; if the `car` part of one of the lists is an atom but the other is not
         #f)
        (else
         (and (structurally-equal ? ?)     ; recur over the `car` of each list
              (structurally-equal ? ?))))) ; recur over the `cdr` of each list