neural network - What's the difference between sparse_softmax_cross_entropy_with_logits and softmax_cross_entropy_with_logits?

Question

I recently came across tf.nn.sparse_softmax_cross_entropy_with_logits and I can not figure out what the difference is compared to tf.nn.softmax_cross_entropy_with_logits.

Is the only difference that training vectors y have to be one-hot encoded when using sparse_softmax_cross_entropy_with_logits?

Reading the API, I was unable to find any other difference compared to softmax_cross_entropy_with_logits. But why do we need the extra function then?

Shouldn't softmax_cross_entropy_with_logits produce the same results as sparse_softmax_cross_entropy_with_logits, if it is supplied with one-hot encoded training data/vectors?

Answer 1

Having two different functions is a convenience, as they produce the same result.

The difference is simple:

• For sparse_softmax_cross_entropy_with_logits, labels must have the shape [batch_size] and the dtype int32 or int64. Each label is an int in range [0, num_classes-1].
• For softmax_cross_entropy_with_logits, labels must have the shape [batch_size, num_classes] and dtype float32 or float64.

Labels used in softmax_cross_entropy_with_logits are the one hot version of labels used in sparse_softmax_cross_entropy_with_logits.

Another tiny difference is that with sparse_softmax_cross_entropy_with_logits, you can give -1 as a label to have loss 0 on this label.