CODEWITHSUNDEEP
listschemeracket
Don Johnson
Don Johnson

Reputation: 93

Rotate a list to the left in Scheme/Racket

Trying to rotate a list left in scheme/racket. For example: (rotate-left '(abc)) outputs (bca)

Here's ze code!

(define (rotate-left LIST)
    (if(null? LIST)'()
        (cons(car LIST)(cons(cdr LIST)'())))))

Unfortunately this just outputs the same list! Should be an easy fix I'm sure, and I know I'm close...Thanks for any help !

Upvotes: 7

Views: 4783

Answers (3)

Óscar López
Óscar López

Reputation: 235984

The answer given by @davepmiller doesn't work ... Here's a correct implementation - you have to replace the first cons with an append for adding the first element at the end of the list.

(define (rotate-left LIST)
  (if (null? LIST)
      '()
      (append (cdr LIST)
              (cons (car LIST)
                    '()))))


(rotate-left '(a b c))
> '(b c a)

Upvotes: 9

John Clements
John Clements

Reputation: 17203

Test cases! Test cases! A check-expect here would have helped you understand the problem, and others to understand what you were looking for.

I think I'm getting old.

Upvotes: 1

davepmiller
davepmiller

Reputation: 2708

You got car and cdr backwards ! Here you go :

(define (rotate-left LIST)
    (if(null? LIST)'()
        (cons(cdr LIST)(cons(car LIST)'())))))

Upvotes: -2

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