How to replace an item by another in a list in DrScheme when given paramters are two items and a list?

Question

How to replace an item by another in a list in DrScheme when given paramters are two items and a list?

Answer 1

; replace first occurrence of b with a in list ls, q? is used for comparison
(define (replace q? a b ls)
  (cond ((null? ls) '()) 
        ((q? (car ls) b) (replace q? a b (cons a (cdr ls))))
        (else (cons (car ls) (replace a b (cdr ls))))))