Add 1.23

Author: shonfeder

problems/lists.org

@@ -314,3 +314,26 @@ Success:
 
     L = [4,5,6,7,8,9]
 #+end_src
+
+* 1.23 (**) Extract a given number of randomly selected elements from a list.
+
+The selected items shall be put into a result list.
+
+It should not include duplicates, unless there are duplicates items in the given
+list.
+
+Example:
+
+#+begin_src teyjus-edit
+
+> select-rnd [1,2,3,4,5,6,7,8] 3 L.
+
+Success:
+
+    L = [5,4,1]
+#+end_src
+
+Of course, your results will be random.
+
+Hint: Use the built-in random number generator random.int/2 and the result of
+problem 1.20.

solutions/lists.mod

@@ -251,6 +251,24 @@ range N N' _ :- N' < N, std.fatal-error "`range` with second index smaller than
 range N N  [N].
 range N N' [N|Ns] :- range {util.succ N} N' Ns.
 
+% 1.23
+% FIXME: Sometimes gets stuck at random.int. Report bug?
+pred select-rnd i:int, i:list A, o:list A.
+pred select-rnd.aux i:int, i:int, i:list A, o:list A.
+select-rnd N Ls Xs :-
+    random.self_init,
+    ( select-rnd.aux N _ _ Xs
+    & select-rnd.aux _ 0 _ Xs
+    & pi M M' Len Len' Opts Acc X Rest Idx\
+        select-rnd.aux M Len Opts Acc :-
+            random.int Len Idx,
+            select-nth Idx Opts X Rest,
+            util.succ M M',
+            util.succ Len' Len,
+            select-rnd.aux M' Len' Rest [X|Acc]
+    ) => select-rnd.aux 0 {len Ls} Ls []
+    .
+
 }
 
 
@@ -301,6 +319,8 @@ tests :- test (list.last [1, 2, 3, 4] (some 4))
       & test (list.select-nth 2 [1,2,3,4] 2 [1,3,4])
       & test (list.insert-at 10 2 [1,2,3,4] [1,2,10,3,4])
       & test (list.range 4 9 [4,5,6,7,8,9])
+      % FIXME
+      % & test (list.select-rnd 3 [1,2,3,4,5,6,7,8] [_, _, _])
       .
 
 pred main.