check if numpy array is subset of another array

Question

Similar questions have already been asked on SO, but they have more specific constraints and their answers don\'t apply to my question.

Generally speaking, what is t

Answer 1

starting with:

master=[12,155,179,234,670,981,1054,1209,1526,1667,1853] #some indices of interest

triangles=np.random.randint(2000,size=(20000,3)) #some data

What's the most pythonic way to find indices of triplets contained in master? try using np.in1d with a list comprehension:

inds = [j for j in range(len(triangles)) if all(np.in1d(triangles[j], master))]

%timeit says ~0.5 s = half a second

--> MUCH faster way (factor of 1000!) that avoids python's slow looping? Try using np.isin with np.sum to get a boolean mask for np.arange:

inds = np.where(
 np.sum(np.isin(triangles, master), axis=-1) == triangles.shape[-1])

%timeit says ~0.0005 s = half a millisecond!

Advice: avoid looping over lists whenever possible, because for the same price as a single iteration of a python loop containing one arithmetic operation, you can call a numpy function that does thousands of that same arithmetic operation

Conclusion

It seems that np.isin(arr1=triangles, arr2=master) is the function you were looking for, which gives a boolean mask of the same shape as arr1 telling whether each element of arr1 is also an element of arr2; from here, requiring that the sum of a mask row is 3 (i.e., the full length of a row in triangles) gives a 1d mask for the desired rows (or indices, using np.arange) of triangles.