Enforce 1st row to be smaller than the 2nd

column-wise.py

# Construct 2-row ndarray
>>> a = numpy.random.randint(0, 100, (2, 10))
>>> a
array([[95, 31, 25, 68, 88, 25, 71, 55, 16, 14],
       [57, 59, 38, 30, 78, 92, 30, 47, 89, 41]])

# Extract columns whose 1st row is larger than the 2nd.
>>> a[:, a[1] < a[0]] 
array([[95, 68, 88, 71, 55],
       [57, 30, 78, 30, 47]])

# Swap 1st and 2nd row of such columns
>>> a[:, a[1] < a[0]] = a[:, a[1] < a[0]][::-1]
>>> a
array([[57, 31, 25, 30, 78, 25, 30, 47, 16, 14],
       [95, 59, 38, 68, 88, 92, 71, 55, 89, 41]])