add `slicef-after`

list.rkt

@@ -9,6 +9,7 @@
 
 (define (list-of-lists? xs) (and (list? xs) (andmap list? xs)))
 
+
 (define+provide/contract (slicef-at xs pred [force? #f])
   ((list? procedure?) (boolean?) . ->* . list-of-lists?)
   (define-values (last-list list-of-lists)
@@ -25,6 +26,19 @@
         (cdr list-of-lists)
         list-of-lists)))
 
+
+(define+provide/contract (slicef-after xs pred)
+  (list? procedure? . -> . list-of-lists?)
+  (define-values (last-list list-of-lists)
+    (for/fold ([current-list empty][list-of-lists empty])([x (in-list xs)])
+      (if (pred x)
+          (values empty (cons (reverse (cons x current-list)) list-of-lists))
+          (values (cons x current-list) list-of-lists))))
+  (reverse (if (empty? last-list)
+               list-of-lists
+               (cons (reverse last-list) list-of-lists))))
+
+
 (define+provide/contract (slice-at xs len [force? #f])
   ((list? (and/c integer? positive?)) (boolean?) . ->* . list-of-lists?)
   (define-values (last-list list-of-lists)
@@ -36,10 +50,7 @@
                list-of-lists
                (cons (reverse last-list) list-of-lists))))
 
-;; this function could possibly be implemented as a post-processing step on slicef-at:
-;; every list after the first list starts with an element matching pred.
-;; so drop the first element from all lists except the first.
-;; (cons (car xs) (map cdr (cdr xs)))
+
 (define+provide/contract (filter-split xs pred)
   (list? predicate/c . -> . list-of-lists?)
   (define-values (last-list list-of-lists)
@@ -72,7 +83,6 @@
     [else (error (format "members-unique? cannot be determined for ~a" x))]))
 
 
-
 (define+provide/contract (members-unique?/error x)
   (any/c . -> . boolean?)
   (define result (members-unique? x))

scribblings/list.scrbl

@@ -65,9 +65,22 @@ Divide @racket[_lst] into sublists starting with elements matching @racket[_pred
 @examples[#:eval my-eval
 (slicef-at (range 5) even?)
 (slicef-at '(1 2 2 1 2) even?)
+(slicef-at '(1 2 2 1 2) even? #t)
 (slicef-at (range 5) odd?)
 (slicef-at (range 5) odd? #t)]
 
+@defproc[
+(slicef-after
+[lst list?]
+[pred procedure?])
+(listof list?)]
+Divide @racket[_lst] into sublists ending with elements matching @racket[_pred]. (Distinct from @racket[slicef-at], which gives you sublists that @italic{start} with elements matching @racket[_pred].) If none of the elements match @racket[_pred], there is no slice to be made, and the result is the whole input list.
+
+@examples[#:eval my-eval
+(slicef-after '(1 2 2 1 2) even?)
+(slicef-after (range 5) odd?)
+(slicef-after (range 5) string?)]
+
 
 @defproc[
 (frequency-hash
@@ -132,6 +145,8 @@ Convert @racket[_values] to a simple list.
 list?]
 Return a sublist of the @racket[_lst] starting with item @racket[_start-idx] and ending one item @bold{before} item @racket[_end-idx]. (Similar to how list slices are denominated in Python.) Thus the maximum value for @racket[_end-idx] is @racketfont{(length @racket[_lst])}. Errors will be triggered by nonsensical values for @racket[_end-idx].
 
+Bear in mind that @racket[sublist] is built for convenience, not performance. If you need to do a lot of random access into the middle of an ordered sequence of items, you'd be better off putting them into a @racket[vector] and using @racket[vector-copy].
+
 @examples[#:eval my-eval
 (sublist '(0 1 2 3 4 5 6 7 8) 0 8)
 (sublist '(0 1 2 3 4 5 6 7 8) 8 9)