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

This replaces all the occurrences of fsym in the list lst to the symbol tsym in DrRacket

(define (subst fsym tsym lst)

(cond

[(null? lst) lst]

[eq? (first lst) fsym)(cons tsym (subst fsym tsym (rest lst)))]

[else (cons (first lst)(subst fsym tsym (rest lst)))]))

(subst 'a 'b '(f g a h a))

;the results will be as follows.

'(f g b h b)

Answer 2

Use map with a function which returns the replacement item when its argument is equal to the item you want to replace and the argument otherwise.

Answer 3

; 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))))))

Answer 4

; 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) (cons a (cdr ls)))
    (else (cons (car ls) (replace a b (cdr ls))))))