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Count the number of zero crossings!
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Write a function that returns the number of zero crossings of the sequence of elements in vector.

Syntax: n = zerocrossings(v)
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where 	v is a vector representing the sequence of numbers,
	n is the number of zero crossings of the sequence in the vector v.

Basic functionality:

>> v = [1 2 -1 -3 9 -2 12]

 v =

     1   2  -1  -3   9  -2  12

>> n = zerocrossings(v)

 n =

     4

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