How to store (write) an undirected graph?

Question

Python

There is a task in which it is necessary to check whether a path between two graph vertices is possible. The graph is written as a two-dimensional array:
[[a1,a2], [a2, a3], [a2, a4], [a1, a4], [a4, a5], [b1, b2], [b2, b3], [ b1, b3], [a3, b1]]
Next, using the breadth-first search algorithm, we look for a path, but the problem is that it finds a path only in one direction (for example, a1 b3 True, and b3 a1 False). This leads to the question, does writing a graph as a two-dimensional array correspond to a directed graph? If yes, then how to write this graph undirected? Or is there a different algorithm...

Answer 1

You just have a list of edges. Apparently, the algorithm interprets them as oriented. You can try adding edges to the list pointing in the opposite direction.
It is usually more convenient to use a slightly different representation for traversal: an adjacency list or an adjacency matrix (easily googled). Perhaps your algorithm converts the original list into one of these structures. It is possible to slip back edges at this conversion.