I'm trying to create a Scheme program (Language R5RS) which applies an operation to a set of lists, depending on how big a number is on a list.
So the function looks like
(apply-function f g)
The condition is, if f(a b) where a < 5, then apply operation f(a b).
But if a is equal or greater than 5, apply g(a b). (The second operation)
This sounds confusing but a visual example should clear it up: So, an example would be
((apply-function '* '+) '((2 3) (8 6)))
Would return:
'(6 14)
Another example would be
((apply-function '* '+) '((5 7) (2 3) (3 3))
Returns
'(12 6 9)
I've tackled operations on Scheme before, but the condition part is throwing me off and I'm not sure where to start. Any help is appreciated!
Version 1: If you use procedures + and * instead of symbols '+ and '*:
#lang r5rs
(define apply-function
(lambda (f g)
(lambda (lop)
(map (lambda (p)
(if (< (car p) 5)
(f (car p) (cadr p))
(g (car p) (cadr p))))
lop))))
(display ((apply-function * +) '((2 3) (8 6))))
(newline)
(display ((apply-function * +) '((5 7) (2 3) (3 3))))
Version 2: You can make an association list matching symbols to procedures
#lang r5rs
(define proc-list `((* . ,*) (+ . ,+)))
(define (get-proc s)
(let ((p (assq s proc-list)))
(if p (cdr p) s)))
(define apply-function
(lambda (f g)
(lambda (lop)
(map (lambda (p)
(if (< (car p) 5)
((get-proc f) (car p) (cadr p))
((get-proc g) (car p) (cadr p))))
lop))))
(display ((apply-function '* '+) '((2 3) (8 6))))
(newline)
(display ((apply-function '* '+) '((5 7) (2 3) (3 3))))
Version 3: Uses eval
#lang r5rs
(define (my-eval e)
(eval e (scheme-report-environment 5)))
(define apply-function
(lambda (f g)
(lambda (lop)
(map (lambda (p) (if (< (car p) 5)
(my-eval (list f (car p) (cadr p)))
(my-eval (list g (car p) (cadr p)))))
lop))))
(display ((apply-function '* '+) '((2 3) (8 6))))
(newline)
(display ((apply-function '* '+) '((5 7) (2 3) (3 3))))