Add append-lists.

scribblings/utils.scrbl

@@ -392,6 +392,17 @@ a couple conversions.
 (lists-transpose '((a b) (1 2 3) (#t)))
 ]}
 
+@defproc[(append-lists [lsts (Listof (List (Listof a) (Listof a)))])
+		       (Listof (Listof a))]{
+
+@racket[lsts] is a list of rows, in which each row is split in two halves.
+The function returns the list of the same rows, with the two halves appended.
+
+@examples[#:eval utils-evaluator
+(append-lists '(((1 2) (a b))
+                ((3 4) (c d))))
+]}
+
 @section{Randomness}
 
 @defproc*[([(in-random) (Sequenceof Flonum)]

utils.rkt

@@ -13,7 +13,7 @@
  list-sets->list-strings pretty-print-set pretty-print-set-sets
  update-vertices/unweighted update-graph dotit collect-by-key
  collect-by-key/sets ht-values/list->set hash->list/ordered
- multi-split-at lists-transpose in-random cartesian-product-2/stream
+ multi-split-at lists-transpose append-lists in-random cartesian-product-2/stream
  cartesian-product/stream boolean-power boolean-power/stream any->01
  01->boolean
 
@@ -443,6 +443,18 @@
   (test-case "lists-transpose"
     (check-equal? (lists-transpose '((1 2) (a b))) '((1 a) (2 b)))))
 
+(: append-lists (All (a) (-> (Listof (List (Listof a) (Listof a))) (Listof (Listof a)))))
+(define (append-lists lsts)
+  (for/list ([pr lsts])
+    (append (car pr) (cadr pr))))
+
+(module+ test
+  (test-case "append-lists"
+    (check-equal? (append-lists '(((1 2) (a b))
+                                  ((3 4) (c d))))
+                  '((1 2 a b)
+                    (3 4 c d)))))
+
 (: in-random (case->
               (-> (Sequenceof Flonum))
               (-> Integer (Sequenceof Nonnegative-Fixnum))