Lesson 5 · Implementing compute_path() — Knowledge ChecksAnswer key

Why are all distances initialized to 999999 except the start?

1. A.It is the maximum grid size
2. B.It represents "infinity / no path yet" — any real distance is smaller, so the first path found always improves it✓
3. C.It is a random seed
4. D.To make the numbers line up

999999 is a stand-in for infinity. The start is 0 (zero steps from itself); everything else is 'unknown' until a real path is discovered.

What does this loop leave in the variable current?

1. A.The last node in the graph
2. B.The unvisited node with the smallest known distance✓
3. C.The destination
4. D.A node chosen at random

It scans all nodes, ignores visited ones, and keeps the one with the smallest distance — exactly the 'pick the nearest unvisited node' step from the paper trace.

Why the check `if new_distance < distances[neighbor]` before updating?

1. A.To skip blocked nodes
2. B.So a longer path never overwrites a shorter one already found✓
3. C.To count the neighbors
4. D.It is optional and does nothing

Only a strictly shorter distance should replace what's stored. Without the check, a later, longer route could clobber a shorter one — breaking the shortest-path guarantee.

What does the `if current is None` check protect against?

1. A.Running out of memory
2. B.An unreachable destination — no unvisited node has a known distance, so the loop would never end✓
3. C.A blocked start node
4. D.Too many neighbors

If obstacles seal off the destination, no reachable unvisited node remains. Without the check the loop would spin forever (or crash); with it, compute_path returns [] gracefully.

compute_path returns [(0,0), (0,1), (0,2), (1,2), (2,2)]. How many steps is that?

1. A.5
2. B.4✓
3. C.3
4. D.0

The path includes the start, so it lists 5 nodes but 4 moves between them. Steps = len(path) − 1 = 4.