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import torch | ||
import torch.utils.data | ||
import torchvision | ||
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class ImbalancedDatasetSampler(torch.utils.data.sampler.Sampler): | ||
"""Samples elements randomly from a given list of indices for imbalanced dataset | ||
Arguments: | ||
indices (list, optional): a list of indices | ||
num_samples (int, optional): number of samples to draw | ||
""" | ||
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def __init__(self, dataset, indices=None, num_samples=None): | ||
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# if indices is not provided, | ||
# all elements in the dataset will be considered | ||
self.indices = list(range(len(dataset))) \ | ||
if indices is None else indices | ||
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# if num_samples is not provided, | ||
# draw `len(indices)` samples in each iteration | ||
self.num_samples = len(self.indices) \ | ||
if num_samples is None else num_samples | ||
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# distribution of classes in the dataset | ||
label_to_count = {} | ||
for idx in self.indices: | ||
label = self._get_label(dataset, idx) | ||
if label in label_to_count: | ||
label_to_count[label] += 1 | ||
else: | ||
label_to_count[label] = 1 | ||
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# weight for each sample | ||
weights = [1.0 / label_to_count[self._get_label(dataset, idx)] | ||
for idx in self.indices] | ||
self.weights = torch.DoubleTensor(weights) | ||
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def _get_label(self, dataset, idx): | ||
dataset_type = type(dataset) | ||
if dataset_type is torchvision.datasets.MNIST: | ||
return dataset.train_labels[idx].item() | ||
elif dataset_type is torchvision.datasets.ImageFolder: | ||
return dataset.imgs[idx][1] | ||
else: | ||
raise NotImplementedError | ||
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def __iter__(self): | ||
return (self.indices[i] for i in torch.multinomial( | ||
self.weights, self.num_samples, replacement=True)) | ||
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def __len__(self): | ||
return self.num_samples |
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