Question

have got a problem with do this kind of code , can’t figure how to search for a element (a) and replace i by ( b) , how to do it? Thx in advance

Answer 1

Try this function:

(define subst
  (lambda (new old l)
    (cond
     ((null? l) (quote ()))
     ((atom? (car l))
      (cond
       ((eq? (car l) old) (cons new
                                (subst new old (cdr l))))
       (else (cons (car l)
                   (subst new old (cdr l))))))
     (else (cons (subst new old (car l))
                 (subst new old (cdr l)))))))

This will search through a list of S expressions and substitute every occurrence of old with an occurrence of new.