SyntaxError with list comprehension on "not in" operator

Question

I'm trying to randomly split a list X with len(X) = 150 into two lists X_train and X_test with len(X_train) = 105 and len(X_test) = 45.

split = np.random.choice(150, 105)
X_train = [X[i] for i in split]
X_test = [X[i] for i not in split]

But in line 3 I get a SyntaxError.

How would I do this correctly?

`for i not in split` is not valid Python syntax - and what would it mean? If `i` is not in something, where should it be? — Thierry Lathuille, Apr 27 '20 at 13:49
@Anwarvic Of course you can. `for i in range(..)` is a basic construct. — Thierry Lathuille, Apr 27 '20 at 13:50
@ThierryLathuille, Sorry, I meant `not in` with `for` loop... thanks for the heads-up — Anwarvic, Apr 27 '20 at 13:51
@ThierryLathuille, I thought of the expression as either `i` is in the list `split` or not. — dYTe, Apr 27 '20 at 14:41
If `split = [42, -3.14, 777]`, `for i in split` means that `i` will take the values 42, -3.14, then 777. What could `for i not in split` mean? There's an infinity of things that aren't in `split`... By which one would you start? — Thierry Lathuille, Apr 27 '20 at 15:01

Leo Arad · Answer 1 · 2020-04-28T12:31:58.023

0

You can achieve that by

split = random.sample(range(len(X)), 105)
X_train = [X[i] for i in split]
X_test = [X[i] for i in [i for i in [i for i in range(len(X)) if i not in split]]]

That will create you a list of a random unique number of 105 items and will and then it will extract to X_train all the randoms number indexes and to X_test it will extract all the 45 number that not in the split list.

edited Apr 28 '20 at 12:31

answered Apr 27 '20 at 14:08

Leo Arad

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score 0 · Answer 2 · answered Apr 27 '20 at 14:10

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If you have a list to split you could just slice it

from random import randint

a = [1,2,3,4,5,6,7,8,9,10]
s = randint(0, len(a))
print(a[:s])
print(a[s:])

answered Apr 27 '20 at 14:10

Tony

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SyntaxError with list comprehension on "not in" operator

2 Answers2