ayhan - 9 months ago 27

Python Question

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)
```