Class_weights without known the class order?

Keras
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#1

Hi everyone!
I’m facing a problem using class_weight parameter in Keras.
I have a InterleaveDataset composed of images and their labels, which are integers (but the problem is still about classification).
Also, I have a dictionary with the weights for each class, i.e. {16: 6.0, 18: 8.0, 21: 5.0}.

If I tried to use that dictionary, I get the error: Expected class_weight to be a dict with keys from 0 to one less than the number of classes, found {16: 6.0, 18: 8.0, 21: 5.0}

I understand that error, but my problem is that I do not know the order of each class. I mean, I know that class 16 has a weight of 6, but if I try to write a dictionary in the way {0:weight, 1:weight…}, I do not know if the class 16 is the 0, or the 1…

Also, if I made a dictionary in that style, i.e. {0: 6.0, 1: 8.0, 2: 5.0} (that is bad because I do not know the order of each class), I get errors like: indices[0] = 18 is not in [0, 3)

How can I know the order of my dictionary, and not changing the numbers of my labels?

Thank you!.

#3

Hi @alexgczs, As far as i know, You need to send class weights for all the classes using the class weights dictionary. You can calculate for the class weights using sklearn

from sklearn.utils import class_weight
class_weights = class_weight.compute_class_weight('balanced', np.unique(y_train), y_train)

or you can also calculate the class weights manually. Let say we have 2 classes one is positive and other is negative you can calculate the weights

weight_for_0 = (1 / neg) * (total / 2.0)
weight_for_1 = (1 / pos) * (total / 2.0)

class_weight = {0: weight_for_0, 1: weight_for_1}

Thank You.

#4

Hi @Kiran_Sai_Ramineni , and thank you for your response. I already have the weights calculated, but my problem is another. Imagine I have 3 classes: class 16, class 18 and class 21. But I do not know the order that Keras will use classes, because I have a generator (from sample_from_datasets function).

So my problems are:

  1. I do not know the order in which keras will order the class indices, so I do not know if the “class 0” in the dictionary is my class 16, 18 or 21.
  2. Another problem that I had is that I needed to keep my labels as integers, but it seems that it causes some problems to have the labels as integers if I want to use class_weights

Thanks for your help!