Key Concepts on Deep Neural Networks

1. What is the “cache” used for in our implementation of forward propagation and backward propagation?

• We use it to pass $Z$ computed during forward propagation to the corresponding backward propagation step. It contains useful values for backward propagation to compute derivatives.
• It is used to cache the intermediate values of the cost function during training.
• It is used to keep track of the hyperparameters that we are searching over, to speed up computation.
• We use it to pass variables computed during backward propagation to the corresponding forward propagation step. It contains useful values for forward propagation to compute activations.