A computational graph is a network of connected nodes, in which each node is a unit of computation and stores a variable. Each edge joining two nodes indicates a relationship between the corresponding variables. The graph may be either directed or undirected. In a directed graph, a node computes its associated variable as a function of the variables in the nodes that have edges incoming to it. In an undirected graph, the functional relationship works in both directions. Most practical computational graphs (e.g., conventional neural networks) are directed acyclic graphs, although many undirected probabilistic models in machine learning can be implicitly considered computational graphs with cycles. Similarly, the variables at the nodes might be continuous, discrete, or probabilistic, although most real-world computational graphs work with continuous variables.