Find whether 1D sub-array is in 2D array

Question

I have a data-array like:

all = [[1,-1], [1,0], [1,1], [2,-1], [2,0], [2,1]]  etc.

It has about 4000 pairs in it, but typically fewer on the order of a few hundred.

I need to find whether a two-valued array already exists in this large data-array, eg. does [1,1] exist in the array?

So the function I need should act something like this:

isValid( all, [1,1] )
>>> True

isValid( all, [1,100] )
>>> False

I couldn't get the numpy functions isin() or in1d() to do this for me. The one function I did find works, for lists, is:

all.index( [1,1] )
>> True

but when the arg is not in the all array, I have to try/catch a ValueError and then return False - acceptable for now, but not ideal.

Maybe [1,1] in lst? By the way, don't name your variable all, which is the name of a built-in function. I've used lst instead of all. — j1-lee
– j1-lee, Commented Jun 2, 2022 at 1:14
What does this have the do with numpy? What you've shown is a list not an array — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jun 2, 2022 at 1:49
nothing to with with numpy, apart from numpy being one module that might help - converting to array would be acceptable, as the 2nd answer does. — Demis
– Demis, Commented Jun 2, 2022 at 4:14

Jupri · Accepted Answer · 2022-06-02 01:25:54Z

1

You can use simple array lookup like this:

a = [[1,-1], [1,0], [1,1], [2,-1], [2,0], [2,1]]

[2,0] in a # True
[2,3] in a # False

or

a.index([2,0]) # result: 4
a.index([3,5]) # throw error, use try catch

answered Jun 2, 2022 at 1:25

Jupri

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vht981230 · Accepted Answer · 2022-06-02 01:18:42Z

1

If you have numpy installed, you can use np.where to find the indices of 1d array in a 2d numpy array and check if the return result

import numpy as np

def isValid(arr, val):
   return len(np.where(np.prod(arr == val, axis = -1))[0]) != 0

all_items = np.array([[1,-1], [1,0], [1,1], [2,-1], [2,0], [2,1]] )
search1 = isValid(all_items, [1,1] ) # True
search2 = isValid(all_items, [1,100] ) # False

answered Jun 2, 2022 at 1:18

vht981230

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