Custom dataset - CIFAR10 - Label shift

Nima: <p>Hi, I am trying to simulate the label shift problem. To do so, I need to make custom datasets (in this case CIFAR10) and give the number of images in each class. For example, [5000, 3000, 1500,…], which has a length of 10 because there are 10 classes. I have attached my code below. The code seems to work well but the problem is that when I set all of the elements of the vector to 5000, which means the whole training set, I get an accuracy of 81% on ResNet18, while I get 93% accuracy on the original dataset with the same parameters of the neural network. I also checked the tensor values of images in both cases and they look alike.</p>
<p>Any help would be appreciated.</p>
<pre><code class="lang-auto">class MyDataset(Dataset):
    
    def __init__(self, data, target, domain, transform=None):
        self.data = data 
        self.target = target 
        self.transform = transform
        self.domain = domain
        
    def __getitem__(self, index):
        x = self.data[index]
        y = self.target[index]
        
        if self.transform:
            x = self.transform(x)
        
        return x, y, index, self.domain
    
    def __len__(self):
        return len(self.data)
    
def Custom_Dataset(custom_labels, train):
    
    target_labels = np.zeros(sum(custom_labels)) 
    images = []
    labels = []
    
    if train == True:
        for i in range(50000):
            img, label  = trainset.__getitem__(i)
            images.append(np.array(img))
            labels.append(label)
        images = np.array(images)
        labels = np.array(labels)

    if train == False:
        for i in range(10000):
            img, label = testset.__getitem__(i)
            images.append(np.array(img))
            labels.append(label)
        images = np.array(images)
        labels = np.array(labels)
        
    # data has 10 lists. each of them consisting of indexes of each category
    data = np.empty(10, dtype=np.object)  
    for i in range(data.shape[0]):
        data[i] = []

    for j in range(10):
        indexes=[]
        for i in range(len(labels)):
            if labels[i] == j:
                indexes.append(i)
        data[j]=indexes

    temp=[]
    category=0
    for label in custom_labels:     
        for i in range(label):
            temp.append(data[category][i])
        category = category+1
    random.shuffle(temp)

    cut_images = images[0:sum(custom_labels)]
    target_images = np.zeros_like(cut_images)

    count=0
    for i in range(len(temp)):
        target_images[i] = images[temp[i]]
        for j in range(10):
            if temp[i] in data[j]:
                target_labels[count] = j
        count = count+1
    return target_images, target_labels

custom_labels_train = [5000,5000,500,500,500,500,500,500,5000,5000]

target_images, target_labels = Custom_Dataset(custom_labels_train, train=True)
MyTrainSet = MyDataset(data = target_images, target = target_labels, domain = 1)
trainloader = torch.utils.data.DataLoader(MyTrainSet, batch_size=128, shuffle=True, num_workers=0)                                     
</code></pre>

Nima: <p>If there is a better way to make the custom datasets, please let me know. Thanks.</p>

eqy: <p>Since you have access to the original dataset, you might want to consider a sanity check where you put a hash of all images into a set for each of the classes, and compare this to the corresponding sets for the original dataset. You can either hash the filenames of the images (I think you should be able to get this by modifying the underlying <code>DatasetFolder</code> if that is what you are using) or by using some hash function on the image data itself.</p>