Python Sklearn train_test_split（）：如何设置要训练的数据？

Question

For the following scikit-learn function: train_test_split() :

Is it possible to tell the function where to set the split of the data?

Or in other words:

Can I tell the function that X_train, X_test should be on the left or right side from the split point and that y_train, y_test should be on the right side?

(and does the splitting really work this way - or are just arbitrary rows of the input data taken until the split ratio is obeyed?)

If it is not possible to tell the function which data should be taken for training and testing: is there any equivalent alternative that is usable for this use case?

Answer 1

From Scikit Learn documentation: Split arrays or matrices into random train and test subsets..

>>> import numpy as np
>>> from sklearn.model_selection import train_test_split
>>> X, y = np.arange(10).reshape((5, 2)), range(5)
>>> X
array([[0, 1],
       [2, 3],
       [4, 5],
       [6, 7],
       [8, 9]])
>>> list(y)
[0, 1, 2, 3, 4]

>>> X_train, X_test, y_train, y_test = train_test_split(
...     X, y, test_size=0.33, random_state=42)
...
>>> X_train
array([[4, 5],
       [0, 1],
       [6, 7]])
>>> y_train
[2, 0, 3]
>>> X_test
array([[2, 3],
       [8, 9]])
>>> y_test
[1, 4]

also you can turn off shuffling:

>>> train_test_split(y, shuffle=False)
[[0, 1, 2], [3, 4]]

Answer 2

The solution using KFold would look like:

import numpy as np
from sklearn.model_selection import KFold

X = np.arange(20).reshape((10, 2))
y = np.arange(20)

print(X)
print(y)
kf = KFold(n_splits=10)
for train_index, test_index in kf.split(X):
    print("TRAIN size: {0:5d} from: {1:5d} to: {2:5d}".format(train_index.size, train_index[0], train_index[train_index.size - 1]))
    print("TEST  size: {0:5d} from: {1:5d} to: {2:5d}".format(test_index.size, test_index[0], test_index[test_index.size - 1]))
    X_train, X_test = X[train_index], X[test_index]
    y_train, y_test = y[train_index], y[test_index]

results in:

 [[ 0  1]
 [ 2  3]
 [ 4  5]
 [ 6  7]
 [ 8  9]
 [10 11]
 [12 13]
 [14 15]
 [16 17]
 [18 19]]
[ 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19]
--
TRAIN size:     9 from:     1 to:     9
TEST  size:     1 from:     0 to:     0
--
TRAIN size:     9 from:     0 to:     9
TEST  size:     1 from:     1 to:     1
--
TRAIN size:     9 from:     0 to:     9
TEST  size:     1 from:     2 to:     2
--
TRAIN size:     9 from:     0 to:     9
TEST  size:     1 from:     3 to:     3
--
TRAIN size:     9 from:     0 to:     9
TEST  size:     1 from:     4 to:     4
--
TRAIN size:     9 from:     0 to:     9
TEST  size:     1 from:     5 to:     5
--
TRAIN size:     9 from:     0 to:     9
TEST  size:     1 from:     6 to:     6
--
TRAIN size:     9 from:     0 to:     9
TEST  size:     1 from:     7 to:     7
--
TRAIN size:     9 from:     0 to:     9
TEST  size:     1 from:     8 to:     8
--
TRAIN size:     9 from:     0 to:     8
TEST  size:     1 from:     9 to:     9