Re: How do I test for existence of a list element? Clarified

*To*: mathgroup at smc.vnet.net*Subject*: [mg112696] Re: How do I test for existence of a list element? Clarified*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>*Date*: Mon, 27 Sep 2010 05:47:24 -0400 (EDT)

The function Check is (probably) what you are looking for. list == {a, b, c, d}; x == Check[list[[60]], {}] During evaluation of In[69]:== Part::partw:Part 60 of {a,b,c,d} does not exist. >> {} You can use whatever you like instead of {}. If you don't want to see the error message you don't have to: x==Quiet[Check[list[[60]],{}]] {} Andrzej Kozlowski On 26 Sep 2010, at 08:42, Joseph Gwinn wrote: > I have an application where I am bouncing around in a list, and may > accidentally ask for an element beyond the end of the list, which causes > Mathematica to complain and balk, preventing completion. > > Is there any way to test for the existence of a list element without > provoking complaint or balking should the list item fail to exist? > > > > Many of the suggestions made offline in response to the above imply that > I wasn't clear enough, so here is some expansion: > > All the list operations like MemberQ tell you if a member of the list > has some property or not. But it does not solve the problem of telling > if a slot (part) exists or not. > > For instance: > > list=={a, b, c, d}; x==list[[60]] > > Mathematica will complain that "list[[60]]" does not exist, which is true. > > Mathematica's specific complaint is "Part::partw: Part 60 of {a,b,c,d} does not > exist." Turning the error message off does not solve the problem. > > The question is how to test if list[[60]] exists without losing control > if it does not. > > Other than by doing the usual arithmetic and test involving Length[list]. > > > It may be that no such test exists, which would also be useful to know. > > > Thanks, > > Joe Gwinn >