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I have a series of circles in space and I am trying to find the intersections of a given line and the circles and storing that in an array using Table. As the line will not intersect all circles the array has a number of empty entries

eg

int = {{}, {1, 2}, {}, {3, 4}, {}, {}, {}}

I am aware of Position and Cases and have had some success with something like:

Cases[int, Except[{}]]

But I cant figure out how to combine that with Position to find the position of the non empty entries in the int matrix.

Any help would be greatly appreciated.

Thanks

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Is this all you need?:

Position[int, {__}, 1]

{{2}, {4}}
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  • $\begingroup$ Yes this is exactly the thing, excellent, you are in fact a wizard. Thanks. I can't vote you up unfortunately because "Vote Up requires 15 reputation" $\endgroup$
    – Whose
    Mar 13, 2015 at 22:20
  • $\begingroup$ @Whose I am glad I could help. Nevertheless this is probably too simple a question I will likely be closed. Please understand that I don't mean that to be insulting; Mathematica takes some getting used to and the pattern behavior can be subtle and complicated. $\endgroup$
    – Mr.Wizard
    Mar 13, 2015 at 22:34
  • $\begingroup$ Just for the record, the OP's pattern Except@{} is actually a shade faster than {__}. $\endgroup$
    – Martin Ender
    Mar 13, 2015 at 23:54
  • $\begingroup$ @MartinBüttner and Transpose@{Pick[Range@Length@int, Unitize[Length /@ int], 1]} will be faster yet... $\endgroup$
    – ciao
    Mar 14, 2015 at 1:08
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    $\begingroup$ @rasher I would use SparseArray[Length /@ int]["NonzeroPositions"] myself for that goal. ;-) $\endgroup$
    – Mr.Wizard
    Mar 14, 2015 at 7:23

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