ayhan ayhan - 7 months ago 15
Python Question

Generate a 2D boolean array from tuples

How can I generate a 2D boolean array using a list of tuples that shows the indices of the True values?

For example I have the following list of tuples:

lst = [(0,1), (0, 2), (1, 0), (1, 3), (2,1)]


What I do is, I first generate an array of False's:

arr = np.repeat(False, 12).reshape(3, 4)


Then, iterate over the list to assign True values:

for tup in lst:
arr[tup] = True
print(arr)
array([[False, True, True, False],
[ True, False, False, True],
[False, True, False, False]], dtype=bool)


It seems like a common use case to me so I was wondering if there is a built-in method for this, without the loops.

Answer

zip(*...) is a handy way of 'transposing' a list of lists (or tuples). And A[x,y] is the same as A[(x,y)].

In [397]: lst = [(0,1), (0, 2), (1, 0), (1, 3), (2,1)]

In [398]: tuple(zip(*lst))    # make a tuple of tuples (or lists)
Out[398]: ((0, 0, 1, 1, 2), (1, 2, 0, 3, 1))

In [399]: A=np.zeros((3,4),dtype=bool)  # make an array of False

In [400]: A[tuple(zip(*lst))] = True  # assign True to the 5 values

In [401]: A
Out[401]: 
array([[False,  True,  True, False],
       [ True, False, False,  True],
       [False,  True, False, False]], dtype=bool)