python - How to split a numpy array based on a column?

Question

I have an array of the form :

[[ 1. ,    2.,     3.,     1.,     3.,     3.,     4.   ],
 [ 1.3,    2.3,    3.3,    3.,     3.3,    3.3,    4.3  ],
 [ 1.2,    2.2,    3.2,    2.,     3.2,    3.2,    4.2  ],
 [ 1.1,    2.1,    1.,     1.,     3.,     3.,     4.   ],
 [ 1.3,    2.3,    3.5,    3.,     3.3,    3.3,    4.3  ],
 [ 1.2,    2.7,    3.2,    2.,     3.2,    3.2,    4.2  ],
 [ 1.3,    2.2,    1.,     1.,     3.,     3.,     4.   ],
 [ 1.3,    2.3,    3.6,    3.,     3.3,    3.3,    4.3  ],
 [ 1.2,    2.8,    3.2,    2.,     3.2,    3.2,    4.2  ],
 [ 1.4,    2.3,    1.,     1.,     3.,     3.,     4.   ],
 [ 1.3,    2.3,    3.7,    3.,     3.3,    3.3,    4.3  ],
 [ 1.2,    2.9,    3.2,    2.,     3.2,    3.2,    4.2  ],
 [ 1.5,    2.1,    1.,     1.,     3.,     3.,     4.   ],
 [ 1.89,   2.3,    3.5,    3.,     3.3,    3.3,    4.3  ],
 [ 1.2,    2.7,    3.2,    2.,     3.2,    3.231,  4.2  ],
 [ 1.9,    2.2,    1.,     1.,     3.,     3.,     4.   ],
 [ 1.3,    2.22,   3.6,    3.,     3.3,    3.3,    4.3  ],
 [ 1.2,    2.8,    3.2,    2.,     3.66,   3.2,    4.2  ],
 [ 1.89,   2.3,    1.,     1.,     3.,     3.,     4.   ],
 [ 1.3,    2.99,   3.7,    3.,     3.3,    3.3,    4.3  ],
 [ 1.2,    2.9,    3.2,    2.,     3.34,   3.2,    4.2  ]]

I want to split this array into a number of subarrays based on the fourth column. I.e. I want one subarray whose fourth column is equal to 1, another one where the fourth column is equal to 2, etc. I do not know in advance what all possible values are there in fourth column.

For instance, the subarray corresponding to fourth column being 1 is :

[[ 1.     2.     3.     1.     3.     3.     4.   ],
 [ 1.1    2.1    1.     1.     3.     3.     4.   ],
 [ 1.3    2.2    1.     1.     3.     3.     4.   ],
 [ 1.4    2.3    1.     1.     3.     3.     4.   ],
 [ 1.5    2.1    1.     1.     3.     3.     4.   ],
 [ 1.9    2.2    1.     1.     3.     3.     4.   ],
 [ 1.89   2.3    1.     1.     3.     3.     4.   ]]

Answer 1

To make a list of arrays:

y = [x[x[:,3]==k] for k in np.unique(x[:,3])]