Numpy advanced indexing on a matrix given a condition

Question

How do I select rows of a matrix with a specific condition using np indexing?

My matrix is

n = np.array([[1,2],[4,5], [1,22]])

and I would like to select the rows whose first element is greater than one. Something similar to:

n[lambda x: x[0] > 1]

jayelm · Accepted Answer · 2015-11-10 00:39:23Z

3

Edit: np.where is optional, thanks @user2357112.

n[n[:, 0] > 1]

Try
n[np.where(n[:, 0] > 1)]
where np.where returns an array of row indices that satisfy the given condition.

edited Nov 10, 2015 at 0:39

answered Nov 10, 2015 at 0:27

jayelm

7,7586 gold badges46 silver badges62 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user2357112 Over a year ago

Don't use np.where. Any indexing of the form x[np.where(whatever)] can be replaced with x[whatever], and the code will be equivalent, more concise, and more efficient.

jayelm Over a year ago

@user2357112 funny, I thought that returned an error. Thanks!

gauteh · Accepted Answer · 2015-11-11 17:08:20Z

2

You can use:

n[n[:,0] > 0, :]

edited Nov 11, 2015 at 17:08

answered Nov 10, 2015 at 0:28

gauteh

17.4k4 gold badges32 silver badges38 bronze badges

Collectives™ on Stack Overflow

Numpy advanced indexing on a matrix given a condition

2 Answers 2

2 Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Related