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Here is a simple list:

l={1, 0, 2, 1, 0, 1, 0, 1, 2, 0, 2, 0, 1, 0, 1, 0, 0}

To get the position of non-zero elements I use Position with Except:

Position[l,Except[0]]

Mathematica 9.0.1 returns

{{0}, {1}, {3}, {4}, {6}, {8}, {9}, {11}, {13}, {15}, {}}

I can't figure out why the first element is {0} and the last one is {}. If I use Cases it works as expected:

Cases[l,Except[0]]

{1, 2, 1, 1, 1, 2, 2, 1, 1}

What am I missing here?

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The {0} represents the head of the expression. For example,

l[[0]]

(* ==> List *)

About {}, the docs say

A part specification {} returned by Position represents the whole of expr.

You get these results because Position searches the expression at all levels. To get what you were looking for, use

Position[l, Except[0], {1}, Heads -> False]

to search only at level 1 and to ignore heads.

To see what the results returned by Position mean, you can use Extract. Extract supports all part specifications returned by Position, including {}:

Mathematica graphics

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    $\begingroup$ It may be worth adding that the traversal is depth-first, head before elements, but the entire expression is considered last, not first. This would explain the ordering of the results. $\endgroup$
    – Leonid Shifrin
    Mar 20, 2013 at 15:01
  • $\begingroup$ Or, perhaps, Position[l, Except[0, _?NumericQ]] == Position[l, Except[0, _Integer]] $\endgroup$
    – user1066
    Mar 20, 2013 at 18:12

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