Search All of the Math Forum:
Views expressed in these public forums are not endorsed by
NCTM or The Math Forum.


Daniel
Posts:
59
Registered:
7/24/12


Re: Rather simple function returns curious result. Explanation
Posted:
Jul 3, 2013 4:49 AM


You probably intended to write (and it works as expected) > f [run : { ___List } ] which means that you expect a list of zero or more lists
instead of > f [run : { List___ } ] in which you expect a list of expressions, the Sequence of which you name List (and that is a bad idea)
> For reasons that escape me, the simple function below > fails to return > an empty List when n==2; Is this a bug? If not, what > is the > explanation? If the return value for n==2 is changed > to an integer or > a string, the function behaves as expected.(Of > course, this bizarre > function is the result of simplifying a more > reasonable one.) > > Clear[ f ]; > f [run : { List___ } ] := Module [ { n }, > n = run // Length; > If [ n != 2, Return [ n ] ]; > Module [ { } , > { } (* Return Empty List if n==2 *) > ] > ] ; > f [ { } ] > f [ { { } } ] > f [ { { }, { } } ] > f [ { { }, { }, { } } ] > f [ { { }, { }, { }, { } } ] > > When I run the above, I get these four outputs: > 0 > 1 > Sequence[{}, {}][] > 3 > 4 >



