commit 3bee7f468475b890395720d0d6b1946b1d71be10 parent bcc1047690bc8a69c974320a35fbfbe4bc5ba20e Author: Ed van Bruggen <edvb@uw.edu> Date: Sun, 19 Jan 2020 13:00:47 -0800 Add head and tail list functions Diffstat:
| tibs/lib.tsp | | | 28 | ++++++++++++++++++++++------ |
1 file changed, 22 insertions(+), 6 deletions(-)
diff --git a/tibs/lib.tsp b/tibs/lib.tsp @@ -158,12 +158,28 @@ (car lst))) ; TODO make nth generic for list str vec, made up of list-ref vec-ref str-ref -(define (nth lst i) - (if (pair? lst) - (if (<= i 0) - (car lst) - (nth (cdr lst) (- i 1))) - (error 'nth "index of list out bounds"))) +(define (nth lst n) + (cond + ((atom? lst) + (error 'nth "index of list out of bounds")) + ((<= n 0) (car lst)) + (else (nth (cdr lst) (- n 1))))) + +; TODO diff name head/tail since conflicts w/ unix +; TODO support negative numers like unix tail/head to count from end backwards +(define (head lst n) + (cond + ((<= n 0) nil) + ((atom? lst) + (error 'name "index of list out of bounds")) + (else (cons (car lst) (head (cdr lst) (- n 1)))))) + +(define (tail lst n) + (cond + ((<= n 0) lst) + ((atom? lst) + (error 'tail "index of list out of bounds")) + (else (tail (cdr lst) (- n 1))))) (define (count x lst) (cond ((nil? lst) 0)