Finding duplicate elements in a Scheme List

Question

I'm trying to become more familiar with recursion in Scheme. I have searched the question bank and see many "built-in" functions for finding duplicate entries in a scheme list, but am trying to design my own. I have not learned about "lambda" yet either. My concern is when I call the recursion function, the car element will be updated. I wish to keep it the same, but pass a new cdr each time, so the original car element can be compared against each subsequent element in the list. I want to return a #t if there is a match, and #f if there is no match or the cdr is empty (base case). Any help/advice would be greatly appreciated.

(define (findDuplicates list)
    (if (null? list)
       #f
       (if (null? (cdr list))
           #f
             (if (= (car list) (getCarOfCdr list)) 
              #t
              (findDuplicates (cdr list))    //trying to use recursion
             )
        )
    )
)


(define (getCarOfCdr list)          //Helper function
  (car (cdr list))
)

Óscar López Óscar López · Accepted Answer · 2015-10-14T01:11:57

You should use at least one built-in procedure to make things simpler: member (which is standard). Some general suggestions:

You should not call a parameter list, that clashes with a built-in function with the same name.
Don't nest ifs like that, use cond instead.
We only need two base cases for this problem, not three as in your code.
getCarOfCdr is not necessary: it can be replaced by a call to cadr and besides, that won't work: you have to check each element against all the rest of the list - that's why you should use member.

Taking all of the above suggestions into consideration, here's my proposed solution:

(define (findDuplicates lst)
  (cond ((null? lst) #f)
        ((member (car lst) (cdr lst)) #t)
        (else (findDuplicates (cdr lst)))))

If using member doesn't satisfy your requirements, then it's easy to implement, and simpler than findDuplicates; you should try to write your own version, just for fun.

Finding duplicate elements in a Scheme List

1 Answers