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Create dfs #7

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59 changes: 59 additions & 0 deletions dfs.py
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from collections import defaultdict

# This class represents a directed graph using
# adjacency list representation
class Graph:
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medium

According to Python's style guide PEP 257, it's standard practice to use docstrings ("""...""") for documenting classes, functions, and modules. This is more idiomatic than using a comment block above the class definition and allows tools to automatically generate documentation and users to get help via the help() function.

    """Represents a directed graph using an adjacency list."""


# Constructor
def __init__(self):

# Default dictionary to store graph
self.graph = defaultdict(list)


# Function to add an edge to graph
def addEdge(self, u, v):
self.graph[u].append(v)
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medium

Per Python's style guide PEP 8, method names should be in snake_case. Additionally, PEP 257 recommends using docstrings for documenting functions instead of comments above them.

    def add_edge(self, u, v):
        """Adds an edge from vertex u to v."""



# A function used by DFS
def DFSUtil(self, v, visited):

# Mark the current node as visited
# and print it
visited.add(v)
print(v, end=' ')

# Recur for all the vertices
# adjacent to this vertex
for neighbour in self.graph[v]:
if neighbour not in visited:
self.DFSUtil(neighbour, visited)
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medium

This method can be significantly improved in terms of reusability and Python conventions:

  1. Reusability: Printing directly to the console couples the traversal logic with a specific action. A more flexible design is to make this a generator that yields each node. The caller can then decide what to do with the nodes (e.g., print them, collect them into a list).
  2. Naming (PEP 8): Helper methods intended for internal use should be prefixed with an underscore (e.g., _dfs_util) and use snake_case.
  3. Documentation (PEP 257): The comment should be converted to a docstring.
    def _dfs_util(self, v, visited):
        """A recursive generator for DFS traversal from a given vertex."""
        visited.add(v)
        yield v

        # Recur for all the vertices adjacent to this vertex
        for neighbour in self.graph[v]:
            if neighbour not in visited:
                yield from self._dfs_util(neighbour, visited)



# The function to do DFS traversal. It uses
# recursive DFSUtil()
def DFS(self, v):

# Create a set to store visited vertices
visited = set()

# Call the recursive helper function
# to print DFS traversal
self.DFSUtil(v, visited)
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high

This DFS implementation has two main issues:

  1. Correctness/Clarity: It only traverses the graph from a specific starting vertex v. If the graph has multiple disconnected components, it will only visit the component containing v. The method name DFS is misleading as it implies a full graph traversal. Renaming it to dfs_from_vertex makes its behavior explicit and the API clearer.
  2. API Design: To work with the suggested generator-based _dfs_util, this method should return the generator to the caller.
    def dfs_from_vertex(self, v):
        """Performs DFS traversal starting from a given vertex.

        Returns:
            A generator that yields nodes in DFS order.
        """
        # Create a set to store visited vertices for this traversal
        visited = set()

        # Return the generator from the helper function
        return self._dfs_util(v, visited)



# Driver's code
if __name__ == "__main__":
g = Graph()
g.addEdge(0, 1)
g.addEdge(0, 2)
g.addEdge(1, 2)
g.addEdge(2, 0)
g.addEdge(2, 3)
g.addEdge(3, 3)

print("Following is Depth First Traversal (starting from vertex 2)")

# Function call
g.DFS(2)
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medium

This driver code needs to be updated to reflect the suggested changes to method names (add_edge, dfs_from_vertex) and to correctly handle the generator returned by the new DFS implementation.

if __name__ == "__main__":
    g = Graph()
    g.add_edge(0, 1)
    g.add_edge(0, 2)
    g.add_edge(1, 2)
    g.add_edge(2, 0)
    g.add_edge(2, 3)
    g.add_edge(3, 3)

    print("Following is Depth First Traversal (starting from vertex 2)")

    # The dfs_from_vertex method returns a generator.
    # We can unpack it with * to print all items.
    traversal_generator = g.dfs_from_vertex(2)
    print(*traversal_generator)