(define (getFirstFew lst)
(cond
((= (read) 0) '()) ;returns nothing
(else(cons (car lst)(getFirstFew (cdr lst)(- (read) 1))))))
That is my code above. So i'm trying to write a procedure that will get the first x elements (user gets to choose what x gets to be) from a list. For example, input 4 with (getFirstFew '(1 6 2 4 5)) will result in '(1 6 2 4).
My current problem with this is that with using read two times, it gets called twice and then breaks the program. Is there a way for me to store whatever the user inputs into a variable and then use that variable throughout the program? Or is there another solution to this problem?
Notice that you have to perform the read only once, and store the value for future reference. Normally we'd use a let for this, but given that we also have to iterate over the list and decrement x on each iteration, a named let will be more appropriate. Try this:
(define (getFirstFew lst)
(let loop ((lst lst) (x (read)))
(if (= x 0)
'()
(cons (car lst)
(loop (cdr lst) (- x 1))))))
It works as expected:
(getFirstFew '(1 6 2 4 5))
> 4
=> '(1 6 2 4)