Find first threshold crossing in numpy array

Question

Given the following artificially generated data:

t_steps = 30

data = np.array([
    np.arange(t_steps) * .05,
    np.arange(t_steps) * .1,
    np.arange(t_steps) * .2,
    np.arange(t_steps) * .3

])

I find the time-step the each line of data has passed a threshold. If it does not pass the given threshold, I assign a time-step of -1:

react_tms = []
thresh = 3.5

for dat in data:
    whr = np.where(dat > thresh)
    if len(whr[0]) == 0:
        react_tms.append(-1)
    else:
        react_tms.append(whr[0][0])

This gives:

[-1, -1, 18, 12]

Is there some way to do this without the for-loop? Even before the for-loop is removed, should I be using something other than np.where to find the threshold crossing?

Answer 1

In principle you can use numpy.argmax for this. The only problem is that if no value is above the threshold, the maximum is False, so it returns 0 as the index of the maximum. We therefore need to subtract 1 for those cases:

above_threshold = data > thresh
react_tms = np.argmax(above_threshold, axis=1)
react_tms = react_tms - (~np.any(data > thresh, axis=1)).astype(float)
print(react_tms)
# array([ -1.,  -1.,  18.,  12.])

Whether or not that is really more readable, I am not sure. It is, however slightly faster than using numpy.where (and probably also faster than a list comprehension): https://stackoverflow.com/q/16243955/4042267

In the end this does not really matter: