Making a new list with recursion from scheme -


this question has answer here:

i picking scheme way learn recursive , found me:d now, have question. how make function called thirds picks 1 element , skips 2 , repeats process over. returns new list first element from, every triple of elements example (thirds '(a b c d e f g h)) should return (a d g)

(define (thirds lst)  (cond    ((not(list? lst)) (newline) "usage: (thirds [list])")    ((null? lst) lst)    ((null? (cdr lst)) (car lst))    (else (cons (car lst) (thirds (cdr(cdr(cdr lst)))) )) ))

thats code have tried not real luck.. help?

there's way this, create helper function , pass along index.

(define (thirds l)  (thirds-helper l 0))  (define (thirds-helper l i)  (cond ((null? l) '())        ((= 0 (modulo 3))          (cons (car l)                (thirds-helper (cdr l) (+ 1))))        (else (thirds-helper (cdr l) (+ 1)))))

exercise reader. can modify pick out given nth values of list?


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