ayhan - 2 years ago 101
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.

`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)
``````
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