How to find the index of a value in 2d array in Python?

Question

I need to figure out how I can find all the index of a value in a 2d numpy array.

For example, I have the following 2d array:

([[1 1 0 0],   [0 0 1 1],   [0 0 0 0]]) 

I need to find the index of all the 1's and 0's.

1: [(0, 0), (0, 1), (1, 2), (1, 3)] 0: [(0, 2), (0, 3), (1, 0), (1, 1), (the entire all row)] 

I tried this but it doesn't give me all the indexes:

t = [(index, row.index(1)) for index, row in enumerate(x) if 1 in row] 

Basically, it gives me only one of the index in each row [(0, 0), (1, 2)].

Answer 1

You can use np.where to return a tuple of arrays of x and y indices where a given condition holds in an array.

If a is the name of your array:

>>> np.where(a == 1) (array([0, 0, 1, 1]), array([0, 1, 2, 3])) 

If you want a list of (x, y) pairs, you could zip the two arrays:

>>> zip(*np.where(a == 1)) [(0, 0), (0, 1), (1, 2), (1, 3)] 

Or, even better, @jme points out that np.asarray(x).T can be a more efficient way to generate the pairs.

Answer 2

The problem with the list comprehension you provided is that it only goes one level deep, you need a nested list comprehension:

a = [[1,0,1],[0,0,1], [1,1,0]]  >>> [(ix,iy) for ix, row in enumerate(a) for iy, i in enumerate(row) if i == 0] [(0, 1), (1, 0), (1, 1), (2, 2)] 

That being said, if you are working with a numpy array, it's better to use the built in functions as suggested by ajcr.