(define (choose-n n lst)
;; operands: a number n and a list (of at least n elements)
;; result: evaluates to a list of all possible was of choosing n
elements from lst
(if (= n 0)
(list null)
(if (= (list-length lst) n)
(list lst) ; must use all elements
(list-append
(choose-n n (cdr lst)) ;; all possibilities not using first element
(list-map (lambda (clst) (cons (car lst) clst))
(choose-n (- n 1) (cdr lst)))))))
What is the list-map application expression doing?
(define (map2d f p)
(list-map
_________________________________________
p))
(define (list-map f p)
(if (null? p)
null
(cons (f (car p))
(list-map f (cdr p)))))
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(define (list-sum p)
(if (null? p)
0
(+ (car p) (list-sum (cdr p)))))
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(define (list-length p)
(if (null? p)
0
(+ 1 (list-length (cdr p)))))
|
(define (is-list? p)
(if (null? p)
true
(if (pair? p) (is-list? (cdr p)) false)))
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(define (list-cruncher baseres carproc combiner p)
(if (null? p) baseres
(combiner
(carproc (car p))
(list-cruncher baseres carproc combiner (cdr p)))))
(define (list-sum p)
(list-cruncher _____ ___________________ _____ p))
(define (list-map f p)
(list-cruncher _______ ______ ______ p))
(define (list-length p)
(list-cruncher ______ _____________ + p))
Is it possible to define is-list? using just list-cruncher?
How is list-cruncher similar and different from
list-accumulate? (Section 5.4.2)
(define (list-accumulate f base p)
(if (null? p) base
(f (car p) (list-accumulate f base (cdr p)))))
Gold star bonus: Is there any procedure that can be defined using list-cruncher that cannot be defined using list-accumulate?