Lesson 4 · The Dijkstra Class — Knowledge ChecksAnswer key

When is self.graph built?

1. A.The first time you call compute_path
2. B.Automatically in __init__, when the object is created✓
3. C.You must call build_graph yourself after creating the object
4. D.Every time you access it

The constructor calls self.build_graph() once, so the graph is stored in self.graph and ready to use as soon as the object exists.

What does `if (row, col) in self.blocked: continue` do?

1. A.Adds the node with an empty neighbor list
2. B.Skips a blocked node so it never becomes a key in the graph✓
3. C.Stops the whole loop
4. D.Marks the node as visited

continue jumps to the next loop iteration, so a blocked node is never added as a key. Combined with the neighbor checks, blocked nodes are fully absent.

Why leave compute_path as a placeholder for now?

1. A.Because the algorithm is impossible
2. B.To test the class structure (constructor and graph) first, before adding the harder algorithm next lesson✓
3. C.To save memory
4. D.It will never be implemented

Building in stages keeps the load manageable — verify the graph is correct now, then focus entirely on the algorithm in Lesson 5.

On a 4×4 grid, how many nodes does Dijkstra((0,0), [(1,0),(2,1)]).graph contain?

1. A.16
2. B.15
3. C.14✓
4. D.2

16 total minus 2 blocked = 14. Both blocked nodes are skipped as keys, and removed from all neighbor lists too.

Why does it matter that Dijkstra's compute_path returns the same type as Manhattan's?

1. A.It makes the code run faster
2. B.The Navigator can use either class interchangeably, since it just calls compute_path and gets a list of tuples back✓
3. C.It saves disk space
4. D.It doesn't matter

A shared interface — same method name, same return type — is what makes Dijkstra a drop-in replacement for Manhattan in the Navigator.