CODEWITHSUNDEEP
rustgraph-algorithmpetgraph
fuji
fuji

Reputation: 1203

Random walk using petgraph

I am trying to implement a random walk on a directed graph using the petgraph crate.

So far, I have defined a RandomWalk struct which implements the Walker trait:

extern crate petgraph; // 0.4.13
use petgraph::visit::{GraphBase, Walker};
use petgraph::Direction;

pub struct RandomWalk<G> 
    where G: GraphBase
{
   next: G::NodeId, 
}

impl<G> Walker<G> for RandomWalk<G>
    where G: GraphBase
{
   type Item = G::NodeId; 

   fn walk_next(&mut self, graph: G) -> Option<Self::Item> {
       // Even this deterministic walk does not work:
       graph.neighbors_directed(self.next, Direction::Incoming).next()
   }
}

However, I get the error:

error[E0599]: no method named `neighbors_directed` found for type `G` in the current scope
  --> src/lib.rs:50:11
   |
50 |     graph.neighbors_directed(self.next, Direction::Incoming).next()
   |           ^^^^^^^^^^^^^^^^^^
   |
   = note: the method `neighbors_directed` exists but the following trait bounds were not satisfied:
           `&G : petgraph::visit::IntoNeighborsDirected`
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `neighbors_directed`, perhaps you need to implement it:
           candidate #1: `petgraph::visit::IntoNeighborsDirected`

I don't really understand how the petgraph API works, is GraphBase not the correct type?

Upvotes: 0

Views: 543

Answers (1)

hellow
hellow

Reputation: 13420

The solution is quiet clear, if you understand the compiler.
Rust does not assume anything about the type, unless you specify it, e.g. you can't add two types T together, unless the implement the Add trait. Then you can write T + T.
Your problem is very similar.

You are trying to use the function neighbors_directed which is not implemented for G (which is bound to GraphBase in your example). Instead you have to specify that G also must implement the trait IntoNeighborsDirected by adding that to your impl block.

impl<G> Walker<G> for RandomWalk<G> where G: GraphBase + IntoNeighborsDirected

This will tell the compiler, that G has the method neighbors_directed implemented and you can use it (playground)

Upvotes: 2

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