Finding mode of unique array combination in the rows of 2d numpy array

Question

I have a 2d numpy array which I'm trying to return the mode array along axis = 0 (rows). However, I would like to return the most frequent unique row combination. And not the three modes for all three columns which is what scipy stats mode does. The desired output in the example below would be [9,9,9], because thats the most common unique row. Thanks

from scipy import stats

arr1 = np.array([[2,3,4],[2,1,5],[1,2,3],[2,4,4],[2,8,2],[2,3,1],[9,9,9],[9,9,9]])

stats.mode(arr1, axis = 0)

output:

ModeResult(mode=array([[2, 3, 4]]), count=array([[5, 2, 2]]))

amirhm · Accepted Answer · 2022-11-15 12:27:08Z

1

you could use the numpy unique funtion and return counts.

unique_arr1, count = np.unique(arr1,axis=0, return_counts=True)
unique_arr1[np.argmax(count)]

output:

array([9, 9, 9])

np.unique return the unique array in sorted order, which means it is guranteed that last one is the maximum. you could simply do:

out = np.unique(arr1,axis=0)[-1]

however, I do not know for what purpose you want to use this but just to mention that you could have all counts just in case you want to verify or account for multiple rows with same counts as well.

Update given additional information that this is for images (which could be big) and most importantly second dim could fit in a int (either each values is uin8 or 16) could fir in int32 or 64. (considering of values of each pixel in uint8):

pixel, count = np.unique(np.dot(arr, np.array([2**16,2**8,1])),  return_counts=True)
pixel = pixel[np.argmax(count)]
b,g, r, = np.ndarray((3,), buffer=pixel, dtype=np.uint8)

This could result in a big speedup.

edited Nov 15, 2022 at 12:27

answered Nov 15, 2022 at 9:48

amirhm

1,23911 silver badges15 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

lingyau lee Over a year ago

thank you! it does exactly what i want it to. just fyi, i'm using it to find the most frequent RGB values. These red , green , blue values are extracted from an image and i'm trying to find the most common color among it. cheers

amirhm Over a year ago

you are welcome, if you are using this for big image, since every pixel could fit in a int just creat a array of int then look at the frequency of each pixel, that could be a lot faster. I will edit my answer to include this

lingyau lee Over a year ago

ok thx, useful for the images. altho i'll need to go throught it later to understand properly.

Collectives™ on Stack Overflow

Finding mode of unique array combination in the rows of 2d numpy array

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related