Find the empty element in a factor array

Hi all,

I have a factor array, and some of the elements are empty. How would I
return the index number of the empty elements. For example,

test<-factor(c('A','','B','C','E'))

test

[1] A   B C E
Levels:  A B C E

I would like the result equal to 2.

Thank you,
Wendy

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