Problem with homework scheme, need help

Question

Well, one of my problems with homework is to take the list of lists and return the car of each subscription to the list. I have where I can print the values, but this is not a list. Honestly, I do not know how to list. Here is what I got:

(define (car-print alist)
  (if (null? alist)
      (newline)
      (begin
         (write (car (car alist))) (display " ")
  (car-print(cdr alist)))))

This is a cheap way to do this, maybe some help in solving this problem would be much appreciated. Not necessarily a complete answer, but steps to get there. Thank.

Answer 1

My circuit is a little rusty. And I only have a dr schema translator on hand ...

You need to use your list and return only the first element of each sublist.

, cons, , ( ).

, .

, - , ( ) ( )

(cons 1) => error: cons need two parameters
(cons 1 null) => (list 1)
(cons 1 2) => error: need a value (first parameter) and a list (the second one)
(cons 1 (cons 2 null) => (list 1 2)

. , , ,

(define (car-list alist)
  (cond
    ((null? alist) null)
    (else (cons (car(car alist)) (car-list (cdr alist))))
  )
)
; tail recursion version usage: (car-acc-list alist null)
(define (car-acc-list alist acc)
  (cond
    ((null? alist) acc)
    (else (car-acc-list (cdr alist) (cons (car(car alist)) acc)))
  )
)

cond, if, , . : ( ) , cdr. ((null? Alist) null), . , .

, / .

, drscheme , (lgpl). , , .

Answer 2

map:

> (define lst '((1 2 3) (4 5 6) (7 8 9)))
;Value: lst
> (map car lst)
;Value: (1 4 7)

Answer 3

, , . :

cons : n l -> (n l)
Construct a list out of an element and a list, placing the element n
at the head of the list

car : l -> n
Return the head of the list

cdr : l -> l
Return the tail of the list

You want to write a firstl function that uses a list of lists and returns a list. Here are some examples:

(firstl (list (list 1 2 3) (list 9 2 3) (list 4 7 3))) -> (1 9 4)
(firstl (list (list 1 2 3))) -> (1)
(firstl '()) -> ()

The final example should give you your first key: if the argument is an empty list, return an empty list.

(define (firstl lol)     ; list-of-lists (no laughing!)
  (if (null? lol)
    '()
    ....    ; more stuff goes here for the recursive "else" clause. Hint: use cons
))