Indexing numpy arrays

Question

I'm trying to perform pairs bootstrap and I have some problems when indexing the bootstrap samples. I will show a brief piece of code that matches the problem I am facing. How should I index the sample to get the bootstrap sample?

This is the data:

Y = [1,4,2,5,3,2,4,6]
X = [1,2,3,4,3,1,5,2]

First of all I create the index:

ind = np.arange(len(X))

Secondly I perform bootstrap on the index:

ind_b = np.random.choice(ind, len(ind))

And finally I try to filter both variables with the bootstrap index:

Y_b = Y[ind_b] 
X_b = X[ind_b]

Doing this I get an error message:

TypeError: only integer scalar arrays can be converted to a scalar index

Could someone explain how I can do it correctly?

Y and X are only a Python list, they do not know how to index based off np.array. You can instead change Y = np.array(Y) and then you can index like that — Chrispresso
– Chrispresso, Commented Apr 3, 2019 at 20:52

Chrispresso · Accepted Answer · 2019-04-03 20:57:22Z

1

The problem is that X and Y are only Python list's in this case. If you were to do type(Y) you would get list. Since you are indexing into a list with np.array, Python has no idea what that is and throws the error because you are doing invalid operations on a list. Instead, you need X and Y to be type np.array as well

>>> ind_b
array([6, 2, 7, 4, 0, 5, 7, 0])
>>> np.array(Y)[ind_b]
array([4, 2, 6, 3, 1, 2, 6, 1])
>>> Y = np.array(Y)
>>> X = np.array(X)
>>> Y_b = Y[ind_b]
>>> Y_b
array([4, 2, 6, 3, 1, 2, 6, 1])
>>> X_b = X[ind_b]
>>> X_b
array([5, 3, 2, 3, 1, 1, 2, 1])

A quick fix is to change it so the assignments are:

Y = np.array([1,4,2,5,3,2,4,6])
X = np.array([1,2,3,4,3,1,5,2])

answered Apr 3, 2019 at 20:57

Chrispresso

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2 Comments

Scott Boston Over a year ago

Great oberservation. +1

Alberto Soriano Gómez Over a year ago

Thank you for your time and help. It works perfectly.

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