http://stackoverflow.com/questions/10475488/calculating-crossing-intercept-points-of-a-series-or-dataframe
def cross(series, cross=0, direction='cross'):
"""
Given a Series returns all the index values where the data values equal
the 'cross' value.
Direction can be 'rising' (for rising edge), 'falling' (for only falling
edge), or 'cross' for both edges
"""
# Find if values are above or bellow yvalue crossing:
above=series.values > cross
below=np.logical_not(above)
left_shifted_above = above[1:]
left_shifted_below = below[1:]
x_crossings = []
# Find indexes on left side of crossing point
if direction == 'rising':
idxs = (left_shifted_above & below[0:-1]).nonzero()[0]
elif direction == 'falling':
idxs = (left_shifted_below & above[0:-1]).nonzero()[0]
else:
rising = left_shifted_above & below[0:-1]
falling = left_shifted_below & above[0:-1]
idxs = (rising | falling).nonzero()[0]
# Calculate x crossings with interpolation using formula for a line:
x1 = series.index.values[idxs]
x2 = series.index.values[idxs+1]
y1 = series.values[idxs]
y2 = series.values[idxs+1]
x_crossings = (cross-y1)*(x2-x1)/(y2-y1) + x1
return x_crossings
# Test it out:
time = [0, 0.1, 0.21, 0.31, 0.40, 0.49, 0.51, 0.6, 0.71, 0.82, 0.93]
voltage = [1, -1, 1.1, -0.9, 1, -1, 0.9,-1.2, 0.95, -1.1, 1.11]
df = DataFrame(data=voltage, index=time, columns=['voltage'])
x_crossings = cross(df['voltage'])
y_crossings = np.zeros(x_crossings.shape)
plt.plot(time, voltage, '-ob', x_crossings, y_crossings, 'or')
plt.grid(True)
Comment From: WarrenWeckesser
Looks like this question has come up more than once. My numpy answer to basically the same question: http://stackoverflow.com/questions/15112964/digitizing-an-analog-signal/15114952#15114952
Comment From: ehsteve
Looking forward to having this capability in pandas.
Comment From: jreback
you can certainly make your own function for this i am not sure that this is general enough to be a mainline pandas function could be in the cookbook of course
Comment From: MarcoGorelli
closing for now as there's been no activity on this for over a decade