What is the difference between feedforward and recurrent neural networks?

Feedforward neural networks (FNNs) and recurrent neural networks (RNNs) differ primarily in how they process data and handle sequential information. FNNs are the simplest type of neural network, where data flows in one direction—from input nodes through hidden layers to output nodes, with no cycles or loops. Each layer is fully connected to the next, and the network processes each input independently. For example, in an image classification task using an FNN, each pixel’s data is fed into the network once, transformed through layers, and mapped to a class label like “cat” or “dog.” This makes FNNs efficient for static data where past inputs don’t affect future outputs. However, they lack memory, meaning they can’t model dependencies over time or sequences.

RNNs, in contrast, are designed for sequential data by introducing cycles in their architecture. These cycles allow information to persist across time steps, enabling the network to maintain a “hidden state” that captures context from previous inputs. For instance, in a text prediction task, an RNN processes each word in a sentence while updating its hidden state to remember earlier words, which helps predict the next word accurately. This makes RNNs suitable for tasks like time-series forecasting, speech recognition, or machine translation, where the order of inputs matters. However, traditional RNNs struggle with long-term dependencies due to issues like vanishing gradients, which newer variants like LSTMs or GRUs address by using gating mechanisms to control information flow.

The key distinction lies in their handling of temporal or sequential data. FNNs treat each input as independent, making them fast and straightforward but limited to tasks without time-based relationships. RNNs explicitly model sequences by retaining memory of prior inputs, enabling them to learn patterns over time. For developers, choosing between them depends on the problem: use FNNs for image classification or tabular data, and RNNs (or their variants) for text, audio, or sensor data with temporal structure. Architecturally, FNNs are simpler to implement and train, while RNNs require careful handling of states and gradients. Tools like TensorFlow or PyTorch abstract much of this complexity, but understanding the core differences helps in selecting the right model for the task.