#!/usr/bin/env guile -s
!#


; * Rick van der Zwet
; * 0433373
; * Scheme programming assigment 1a
; * Licence: BSD
; * $Id: exercise1a.scm 374 2007-12-10 20:18:45Z rick $

; replace find and replace
(define sub_atom
    (lambda (atom sub)
        ;write down replacement else procede to next of the list
        (cond
            ((null? sub) atom)
            ((eq? (caar sub) atom) (cadar sub))
            (else (sub_atom atom (cdr sub)))
        )
    )
)

(define substitute
    (lambda (lijst sub)
        ;if element a return sub if found else element using adding pair (a
        ; a) to replace sub
        (if (list? lijst)
            ;if list and both head and tail recurse both
            ;else recurse single
            (if (null? (cdr lijst))
                (list (substitute (car lijst) sub))
                (cons (substitute (car lijst) sub)
                       (substitute (cdr lijst) sub))
            )
            ; try to do the replacement
            (sub_atom lijst sub)
        )
    )
)




(display
    (equal?
        (substitute
            '((a (b c) d) a (f b))
            '((a z) (b y))
        )
        '((z (y c) d) z (f y))
    )
)