[sort matched arrays pairs] #python #numpy #argsort #unique #timeseries #monotonic #strictly_increasing

sort_matched_arrays.py

# Sometimes timeseries need to be concatenated and then sorted so that the time vector is monotonic. 

# ---- Monotonic in x, but not strictly increasing
# ********************************************************************************
x1 = np.array([1, 2, 3, 4, 4])  # Not all of the values are unique
y1 = np.array([10, 20, 30, 42, 40])
x2 = np.array([1.5, 2.5, 3.5, 4.5])
y2 = np.array([15, 25, 35, 45])

x_ = np.concatenate((x1, x2))
# [1.  2.  3.  4.  4.  1.5 2.5 3.5 4.5]
y_ = np.concatenate((y1, y2))
# [10 20 30 42 40 15 25 35 45]

sort_index = np.argsort(x_)
# [0 5 1 6 2 7 3 4 8]

x = x_[sort_index]
# [1.  1.5 2.  2.5 3.  3.5 4.  4.  4.5]

y = y_[sort_index]
# [10 15 20 25 30 35 42 40 45]

# ---- Strictly increasing in x
# *******************************************************************************
sort_index = np.unique(x_, return_index=True)[1]
# [0 5 1 6 2 7 3 8]

x = x_[sort_index]
# [1.  1.5 2.  2.5 3.  3.5  4.  4.5]

y = y_[sort_index]
# [10 15 20 25 30 35 42 45]