Shift list elements to left by 3: Difference between revisions

=={{header|Scheme}}==

{{works with|Chez Scheme}}

<lang scheme>; Rotate a list by the given number of elements.

; Positive = Rotate left; Negative = Rotate right.

(define list-rotate

(lambda (lst n)

(cond

((or (not (pair? lst)) (= n 0))

lst)

((< n 0)

(let ((end (1- (length lst))))

(list-rotate (append (list (list-ref lst end)) (list-head lst end)) (1+ n))))

(else

(list-rotate (cdr (append lst (list (car lst)))) (1- n))))))</lang>

{{out}}

<pre>(list-rotate '(1 2 3 4 5 6 7 8 9) 3) --> (4 5 6 7 8 9 1 2 3)

(list-rotate '(1 2 3 4 5 6 7 8 9) -3) --> (7 8 9 1 2 3 4 5 6)</pre>