Sort rows of a 2D array, based on the first column

Question

I want to sort the rows of a 2D array based on the elements of the first column, in Python 3. For example, if

    x = array([[ 5. ,  9. ,  2. ,  6. ],
               [ 7. , 12. ,  3.5,  8. ],
               [ 2. ,  6. ,  7. ,  9. ]])

then I need the sorted array to be

    x = array([[ 2. ,  6. ,  7. ,  9. ],
               [ 5. ,  9. ,  2. ,  6. ],
               [ 7. , 12. ,  3.5,  8. ]])

How can I do that? A similar question was asked and answered here , but it does not work for me.

Answer 1

The following should work:

import numpy as np
x = np.array([[ 5. ,  9. ,  2. ,  6. ],
               [ 7. , 12. ,  3.5,  8. ],
               [ 2. ,  6. ,  7. ,  9. ]])

x[x[:, 0].argsort()]

Out[2]:
array([[ 2. ,  6. ,  7. ,  9. ],
       [ 5. ,  9. ,  2. ,  6. ],
       [ 7. , 12. ,  3.5,  8. ]])

Documentation : numpy.argsort

Answer 2

#using sorted    
x = ([[5.,9.,2.,6. ], [7.,12.,3.5,8.], [2.,6.,7.,9.]])
x = sorted(x, key=lambda i: i[0]) #1st col 
print(x)