Epoch vs Iteration when training neural networks

Question

What is the difference between epoch and iteration when training a multi-layer perceptron?

Answer 1

In the neural network terminology:

• one epoch = one forward pass and one backward pass of all the training examples
• batch size = the number of training examples in one forward/backward pass. The higher the batch size, the more memory space you'll need.
• number of iterations = number of passes, each pass using [batch size] number of examples. To be clear, one pass = one forward pass + one backward pass (we do not count the forward pass and backward pass as two different passes).

Example: if you have 1000 training examples, and your batch size is 500, then it will take 2 iterations to complete 1 epoch.

FYI: Tradeoff batch size vs. number of iterations to train a neural network

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The term "batch" is ambiguous: some people use it to designate the entire training set, and some people use it to refer to the number of training examples in one forward/backward pass (as I did in this answer). To avoid that ambiguity and make clear that batch corresponds to the number of training examples in one forward/backward pass, one can use the term mini-batch.