Enforce 1st column to be smaller than the 2nd

row-wise.py

# Construct 2-column ndarray
>>> a = numpy.random.randint(0, 100, (10, 2))
>>> a
array([[29,  1],
       [33, 36],
       [79, 32],
       [72, 52],
       [30,  9],
       [33, 85],
       [72, 29],
       [52,  1],
       [60, 89],
       [12, 49]])
 
# Swap column if 1st value is larget than the 2nd.
>>> a[a[:, 1] < a[:, 0]] = a[a[:, 1] < a[:, 0]][:, ::-1]
>>> a
array([[ 1, 29],
       [33, 36],
       [32, 79],
       [52, 72],
       [ 9, 30],
       [33, 85],
       [29, 72],
       [ 1, 52],
       [60, 89],
       [12, 49]])