Edges either point one way or go both ways, and that choice is the whole distinction. In an undirected graph an edge means a mutual relationship, like two friends. In a directed graph an edge goes from one vertex to another, like a one-way street or a follow.
The practical effect shows up in reachability. Undirected edges let you travel from either end to the other. Directed edges only let you move along the arrow, so a path from A to B never guarantees a path back.
Pick based on whether the relationship is symmetric.
- Undirected for friendship, two-way roads and network cables
- Directed for follows, web links, and dependencies like task ordering
Getting this wrong breaks logic without an error. If you model "depends on" as undirected, a topological sort becomes meaningless, and cycle checks stop making sense. Match the edge type to the real relationship first.
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