Question

scheme:

Write a recursive Scheme function (subst x y L), which returns a list identical to L except that every occurrence of x has been replaced with y. The following example illustrates the use of this function: > (subst 'c 'k '(c o c o n u t)) (k o k o n u t)

Write a recursive Scheme function (all-different? L), which determines whether all elements of list L are distinct (that is, not equal?). The following example illustrates the use of this function: > (all-different? '(3 7 2 7)) #f

Answer 1

car gets the first pointer

cdr gets the second pointer

cpr and ctr are not required for this task

One method of doing it:

(define (subst k c list)     #; define the substitute procedure

       (cond   #; conditional computation

#; Syntax: cond < phrase 1 > < phrase 2 > etc ……..

#; where phrase = (<test> <expression> …….. )

#; that is, cond (<test> <expression> <test> <expression> …….)

                ((null? list) ‘())   #; make sure that the list is not null

#; cons will construct objects of memory to hold a couple of values

#; it may also hold couple of pointers to those values

#; ( cons c o c o n u t ) will construct (c . o . c . o . n . u . t)

#; can also be written as (c.(o. (c. (o. (n. (u. (t. nil)))))))

                ((list? (car list)) (cons (subst k c (car list)) (subst k c (cdr list))))

#; car list will get the first element in our case car (cococnut) returns c

#; cdr list returns the second or last pointer

                ((eq? (car list) k) (cons c (subst k c (cdr list))))

#; check for equality, if equal construct by replacing c with k

#; hence coconut will become kokonut, cat become kat, circuit becomes kirkuit etc

                (else

#; in the else case construct by getting the first element of the list c and calling the subst method again

                (cons (car list) (subst k c (cdr list))))))

#’ end of the scheme method

#; this is a comment line

#; (caar ‘((c o) (n u t))) is equivalent to

#; (car (car ‘((c o) (n u t))))

#; the result is c

#; because ( car ‘((c o) (n u t))) will return (c n )

#; then the outer car written as

#; ( car ( c n) ) will return c

#;

#|     this is a multi line comments block

spanning over few lines of commenting

Scheme is similar to LISP the language of Artificial Intelligence, Humanoids, Matrix Agents and Robotics

up to this line is a block of comments in scheme |#