CODEWITHSUNDEEP
pythonarraysnumpy
Fomite
Fomite

Reputation: 2273

Finding the Last Time a Particular Numpy Array has a Value

I have some data, expressed as a Numpy array, that has a sequence that looks faintly like this:

np.array([1, 0, 2, 5, 10, 6, 2, 0, 4, 1, 0, 1, 2, 3, 4, 2, 0, 0, 0, 0, 0, 0, 0])

I would like to know what position the last non-zero value of this array - so in this case, the "2" that occurs at position 15.

What's the fastest and easiest way to do this? I'm possibly generating a large number of these, so my initial plan, which was to iterate over the array, remove the last element if it = 0, then take the length of the resulting array, which seems potentially too slow to be useful.

Upvotes: 0

Views: 310

Answers (2)

Alex Riley
Alex Riley

Reputation: 176820

With a as your array, you could use np.where to specify the condition you are looking for:

>>> np.where(a != 0)[0][-1]
15

This function returns the indexes of array for which a given condition holds (in this case the indexes of all elements which are not 0). (Actually np.where(a != 0) returns a one-tuple containing the array so we access it with [0].)

The [-1] is the familiar Python notation for accessing the last element of an iterable object.

Upvotes: 4

Shashank Agarwal
Shashank Agarwal

Reputation: 2804

You can use the nonzero function - 

>>> arr1.nonzero()[0][-1]
15

Upvotes: 5

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