Learn Algorithm Design by Building a Shortest Path Algorithm - Step 40

Python
1

Tell us what’s happening:

I created a nested if statement, and updated it after it gave a hint. I can’t see beyond that. Any hints?

Your code so far

my_graph = {
    'A': [('B', 3), ('D', 1)],
    'B': [('A', 3), ('C', 4)],
    'C': [('B', 4), ('D', 7)],
    'D': [('A', 1), ('C', 7)]
}

def shortest_path(graph, start):
    unvisited = list(graph)
    distances = {node: 0 if node == start else float('inf') for node in graph}
    paths = {node: [] for node in graph}
    paths[start].append(start)

/* User Editable Region */

    while unvisited:
        current = min(unvisited, key=distances.get)
        for node, distance in graph[current]:
            if distance + distances[current] < distances[node]:
                distances[node] = distance + distances[current]
                if paths[node][-1] = node:
                    pass

/* User Editable Region */

    print(f'Unvisited: {unvisited}\nDistances: {distances}\nPaths: {paths}')
    
#shortest_path(my_graph, 'A')

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Challenge Information:

Learn Algorithm Design by Building a Shortest Path Algorithm - Step 40

2

You appear to have created this post without editing the template. Please edit your post to Tell us what’s happening in your own words.

3

You are using assignment = instead of equality operator.

4

I re-read the step:

Blockquote
If the distance for the node in the processed tuple has been updated, the last item in its path is the node itself.

Then, inside the if distance + distances[current] < distances[node]: we update the distance: distances[node] = distance + distances[current].

So it makes sense to use the variable (distances[node]) from the previous if statement to check if it is equal to the path.

So I updated the nested if statement to this: if distances[node] == paths[node][-1]: pass

Is this line of thinking correct?

5

You were close to the answer before. Here your only mistake is the syntax:

Since = is to assign while you need to compare.

2 Likes
6

Thanks! Worked it out!

1 Like
7

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