How can I map each value of an integer NumPy array to a list according to a lambda function?
I have the following code:
x = [0, 1, 2, 0, 0, 2, 1, 1]
colours = [[0, 0, 0], [255, 0, 0], [0, 255, 0], [0, 0, 255]]
coloured = lambda v: colours[int(v)]
vcoloured = np.vectorize(coloured)
x_color = vcoloured(x) # throws ValueError: setting an array element with a sequence.
# would like to get x_color = [[0, 0, 0], [255, 0, 0], [0, 255, 0], [0, 0, 0], [0, 0, 0], [0, 255, 0], [255, 0, 0], [255, 0, 0]]
Don't bother with lists at all. If you have numpy arrays, it's much faster to use the power of numpy:
>>> x = np.array([0, 1, 2, 0, 0, 2, 1, 1])
>>> colors = np.array([[0, 0, 0], [255, 0, 0], [0, 255, 0], [0, 0, 255]])
>>> x_color = colors[x]
>>> x_color
array([[ 0, 0, 0],
[255, 0, 0],
[ 0, 255, 0],
[ 0, 0, 0],
[ 0, 0, 0],
[ 0, 255, 0],
[255, 0, 0],
[255, 0, 0]])
Yeah, I went ahead and Americanized your spelling, but functionally, the array x_colored will behave just like a list for all practical purposes. If you absolutely need a list, just call tolist on the result:
x_color = colors[x].tolist()
Your use of vectorized has problems because it has to guess as to the returned array dtype. Your function returns a list, not a number. So we need to specify the return dtype - as object:
In [25]: x = [0, 1, 2, 0, 0, 2, 1, 1]
...: colours = [[0, 0, 0], [255, 0, 0], [0, 255, 0], [0, 0, 255]]
...:
...: coloured = lambda v: colours[int(v)]
...: vcoloured = np.vectorize(coloured, otypes=[object])
In [26]: vcoloured(x)
Out[26]:
array([list([0, 0, 0]), list([255, 0, 0]), list([0, 255, 0]),
list([0, 0, 0]), list([0, 0, 0]), list([0, 255, 0]),
list([255, 0, 0]), list([255, 0, 0])], dtype=object)
That could be turned into a 2d array with:
In [27]: np.stack(_)
Out[27]:
array([[ 0, 0, 0],
[255, 0, 0],
[ 0, 255, 0],
[ 0, 0, 0],
[ 0, 0, 0],
[ 0, 255, 0],
[255, 0, 0],
[255, 0, 0]])
But vectorize is not a performance tool (read its docs). It's faster to use a list comprehension:
In [28]: np.array([colours[v] for v in x])
Out[28]:
array([[ 0, 0, 0],
[255, 0, 0],
[ 0, 255, 0],
[ 0, 0, 0],
[ 0, 0, 0],
[ 0, 255, 0],
[255, 0, 0],
[255, 0, 0]])
The direct array indexing of @Mad's answer is better.
