提取numpy数组中的特定列

Question

This is an easy question but say I have an MxN matrix. All I want to do is extract specific columns and store them in another numpy array but I get invalid syntax errors. Here is the code:

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

It seems like the above line should suffice but I guess not. I looked around but couldn't find anything syntax wise regarding this specific scenario.

Answer 1

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 …

Answer 2

假设您想使用该代码片段获取第 1 列和第 9 列，它应该是：

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

Answer 3

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]

Answer 4

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 ]])

Answer 5

One thing I would like to point out is, if the number of columns you want to extract is 1 the resulting 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.

Answer 6

One more thing you should pay attention to when selecting columns from ND array using a list like this:

data[:,:,[1,9]]

If you are removing a dimension (by selecting only one row, for example), the resulting array will be (for some reason) permuted . So:

print data.shape            # gives [10,20,30]
selection = data[1,:,[1,9]]
print selection.shape       # gives [2,20] instead of [20,2]!!

Answer 7

您可以使用以下内容：

extracted_data = data.ix[:,['Column1','Column2']]

Answer 8

I think the solution here is not working with an update of the python version anymore, one way to do it with a new python function for it is:

extracted_data = data[['Column Name1','Column Name2']].to_numpy()

which gives you the desired outcome.

The documentation you can find here: https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.to_numpy.html#pandas.DataFrame.to_numpy

Answer 9

I could not edit the chosen answer so I'm adding an answer to clarify that using an integer to index seems to be returning a view (not a copy) while using a list returns a copy

>>> x = np.zeros(shape=[2, 3])
>>> y = x[:, [0, 1]]
>>> z1, z2 = x[:, 0], x[:, 1]

>>> y[0, 0] = 1
>>> print(y)
[[1. 0.]
 [0. 0.]]
>>> print(x)
[[0. 0. 0.]
 [0. 0. 0.]]

>>> z1[0] = 2
>>> print(z1)
[2. 0.]
>>> print(x)
[[2. 0. 0.]
 [0. 0. 0.]]

Answer 10

你也可以使用extractData=data([:,1],[:,9])