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


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.



1 Answer 1


Is this all you need?:

Position[int, {__}, 1]
{{2}, {4}}
  • $\begingroup$ 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 {__}. $\endgroup$ 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
  • 1
    $\begingroup$ @rasher I would use SparseArray[Length /@ int]["NonzeroPositions"] myself for that goal. ;-) $\endgroup$
    – Mr.Wizard
    Mar 14, 2015 at 7:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.