in a nested list how to test whether a given indexing sequence is valid in mathematica

I have a highly irregular nested list myList in mma, whenever I am given a integer sequence, such as 1,1,2,3,1 of any length, I want to kn开发者_如何学编程ow if

myList[[1,1,2,3,1]]

is valid; because if it is not, then I will get an error saying

Part::partw: part... does not exist

Thanks.

   Quiet[
       Check[mylist[[1, 1, 2, 3, 1]], Print[False], Part::partd], 
   Part::partd];  

You can replace Print[False] for any other action ...

Edit

To check for both partd and partw messages the syntax is:

Quiet[Check[{{{1, 3}}, {2}}[[1, 4, 2, 3, 1]], 
    Print[False], {Part::partd, Part::partw}], 
   {Part::partd, Part::partw}];

HTH!

Coincidently this came up on MathGroup a few weeks ago. Below is a URL to what I believe was the last and best response (from Ray Koopman).

http://forums.wolfram.com/mathgroup/archive/2011/Jan/msg00326.html

It also has links to earlier posts in the same thread.

Daniel Lichtblau

Wolfram Research

Here is another way of doing it.

Quiet[MemberQ[#, #[[1, 4, 2, 3, 1]], Infinity] &[{{{1, 3}}, {2}}]]

Returns False.

It will essentially return true or false after checking to see whether the element at that position (if it exists) is in the list.

Quiet[MemberQ[#, #[[1, 1, 2]], Infinity] &[{{{1, 3}}, {2}}]]

Returns True.

belisarius's answer will give more low level control though.