Passing a list of indices to another list in Python. Correct syntax?

Question

So I have the following code from sklearn:

>>> from sklearn import cross_validation
>>> X = np.array([[1, 2], [3, 4], [1, 2], [3, 4]])
>>> y = np.array([1, 2, 3, 4])
>>> kf = cross_validation.KFold(4, n_folds=2)
>>> len(kf)
2
>>> print(kf)  
sklearn.cross_validation.KFold(n=4, n_folds=2, shuffle=False,
                           random_state=None)
>>> for train_index, test_index in kf:
...    print("TRAIN:", train_index, "TEST:", test_index)
...    X_train, X_test = X[train_index], X[test_index]
...    y_train, y_test = y[train_index], y[test_index]
TRAIN: [2 3] TEST: [0 1]
TRAIN: [0 1] TEST: [2 3]
.. automethod:: __init__

It gives me an error when I pass on the train_index and the test_index in these lines of code (IndexError: indices are out-of-bounds):

...    X_train, X_test = X[train_index], X[test_index]
...    y_train, y_test = y[train_index], y[test_index]

Why can't I pass a list of indices to a list? What is the correct syntax to pass a list of indices to another list to get those elements of that list?

I am using Python 2.7.

Thanks.

Answer 1

Unlike Numpy arrays, python lists don't support accessing by multiple indexes.

It's easy to solve using list comprehensions, though:

l= range(10)
indexes= [1,3,5]
result= [l[i] for i in indexes]

Or the slighly less readable (but more useful in some occasions) map:

result= map(l.__getitem__, indexes)

However, as Ashwini Chaudhary noted, X and y are numpy arrays in your example, so you either entered the wrong example code or your particular indexes indeed are out of range.

Answer 2

you could also use :

res_list = list(itemgetter(*index_list)(test_list)) 

Edit: here is an example

>>> import operator
>>> indices = [1, 3, 4]
>>> list(operator.itemgetter(*indices)(range(10)))
[1, 3, 4]