使用np.where（）[0]

Question

My code detects all the points under a threshold, then locates the start and end points.

below = np.where(self.data < self.threshold)[0]


startandend = np.diff(below)
startpoints = np.insert(startandend, 0, 2)
endpoints = np.insert(startandend, -1, 2)
startpoints = np.where(startpoints>1)[0]
endpoints = np.where(endpoints>1)[0]
startpoints = below[startpoints]
endpoints = below[endpoints]

I don't really get the use of [0] after np.where() function here

Answer 1

below = np.where(self.data < self.threshold)[0]

means:

take the first element from the tuple of ndarrays returned by np.where() and assign it to below .

Answer 2

np.where is tricky. It returns an array of lists of indices where the conditions are met, even if the condition is never satisfied. In the case of np.where(my_numpy_array==some_value)[0] specifically, this means that you want the first value in the array, which is a list, and which contains the list of indices of condition-meeting cells.

Quite a mouthful. In simple terms, np.where(array==x)[0] returns a list of indices where the conditions have been met. I'm guessing this is a result of designing numpy for extensively broad applications.

Keep in mind that no matches still returns an empty list ; errors like only size-1 arrays can be converted to python (some type) may be attributed to that.