Extracting specific columns in numpy array

I assume you wanted columns 1 and 9?

To select multiple columns at once, use

X = data[:, [1, 9]]

To select one at a time, use

x, y = data[:, 1], data[:, 9]

With names:

data[:, ['Column Name1','Column Name2']]

You can get the names from data.dtype.names…

Assuming you want to get columns 1 and 9 with that code snippet, it should be:

extractedData = data[:,[1,9]]

if you want to extract only some columns:

idx_IN_columns = [1, 9]
extractedData = data[:,idx_IN_columns]

if you want to exclude specific columns:

idx_OUT_columns = [1, 9]
idx_IN_columns = [i for i in xrange(np.shape(data)[1]) if i not in idx_OUT_columns]
extractedData = data[:,idx_IN_columns]

Just:

>>> m = np.matrix(np.random.random((5, 5)))
>>> m
matrix([[0.91074101, 0.65999332, 0.69774588, 0.007355  , 0.33025395],
        [0.11078742, 0.67463754, 0.43158254, 0.95367876, 0.85926405],
        [0.98665185, 0.86431513, 0.12153138, 0.73006437, 0.13404811],
        [0.24602225, 0.66139215, 0.08400288, 0.56769924, 0.47974697],
        [0.25345299, 0.76385882, 0.11002419, 0.2509888 , 0.06312359]])
>>> m[:,[1, 2]]
matrix([[0.65999332, 0.69774588],
        [0.67463754, 0.43158254],
        [0.86431513, 0.12153138],
        [0.66139215, 0.08400288],
        [0.76385882, 0.11002419]])

The columns need not to be in order:

>>> m[:,[2, 1, 3]]
matrix([[0.69774588, 0.65999332, 0.007355  ],
        [0.43158254, 0.67463754, 0.95367876],
        [0.12153138, 0.86431513, 0.73006437],
        [0.08400288, 0.66139215, 0.56769924],
        [0.11002419, 0.76385882, 0.2509888 ]])

One thing I would like to point out is, if the number of columns you want to extract is 1 the resulting matrix would not be a Mx1 Matrix as you might expect but instead an array containing the elements of the column you extracted.

To convert it to Matrix the reshape(M,1) method should be used on the resulting array.