将向量的numpy数组映射到字典

Question

I'm trying to map a color histogram where each pixel also as another (float) property, alpha, from a similar size array. I want eventually to have a dictionary of (color) -> (count, sum) where count is actually the histogram count for that color, and sum is the sum of alpha values that correspond to a certain color.

here's a simple python code that makes what i want (c and d are the same length, and are very long):

for i in range(len(c)):
        if str(c[i]) in dict:
            dict[str(c[i])][0] += 1
            dict[str(c[i])][1] += alpha[i]
        else:
            dict[str(c[i])] = [0, alpha[i]]

but naturally that takes a lot of time. Any ideas for a numpy equivalent?

Thanks

Answer 1

Okay, so i eventually found a very nice solution using this answer using only numpy: https://stackoverflow.com/a/8732260/1752591

Which is a function that sums up vector according to another vector of indices. So all I had to do is to give an id for each color, and make the dictionary:

d = alpha.reshape((-1))
id = color_code_image(colormap)

v, g = sum_by_group(d, id)
count, g = sum_by_group(np.ones(len(d)), id)
avg = v/count

return dict(np.array([g, avg]).T)