Topologically Sorting DAGS def naive topsort(G,S=None): if S is None: S=set(G) # Default: All nodes if len(S)==1: return list(S)# Base case, single node # Reduction: Remove a node # Recursion (assumption), n=1 v=S.pop () seq=naive topsort (G,S) min i=0 for i,u in enumerate(seq): if v in G[u]: min i=i+1 seq.insert (min i,v) # After all dependencies return seq Using adjacency sets or dicts to represent a DAG.

Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
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# Topologically Sorting DAGs

Below is a Python function `naive_topsort` demonstrating how to perform a topological sort on a Directed Acyclic Graph (DAG). This implementation assumes a set-based representation of the graph nodes.

```python
def naive_topsort(G, S=None):
    if S is None: S = set(G)             # Default: All nodes
    if len(S) == 1: return list(S)       # Base case, single node
    v = S.pop()                          # Reduction: Remove a node
    seq = naive_topsort(G, S)            # Recursion (assumption), n=1
    min_i = 0
    for i, u in enumerate(seq):
        if v in G[u]: min_i = i + 1       # After all dependencies
    seq.insert(min_i, v)
    return seq
```

- **Explanation**
  - **Line 2-3**: If the set `S` is not provided, initialize it with all nodes in `G`.
  - **Line 4-5**: Base case for recursion. If there's only one node, return it as a list.
  - **Line 6**: Remove a node `v` from the set `S`.
  - **Line 7**: Recursively call `naive_topsort` for the reduced graph.
  - **Lines 9-13**: Determine the position `min_i` to insert `v` after all dependencies.
  - **Line 14**: Insert `v` into the sequence at the appropriate position.
  - **Line 15**: Return the updated sequence.

- **Notes**
  - The graph `G` is represented using adjacency sets or dictionaries.
  - This script poses the question, "What’s the expected runtime?," prompting a discussion on the algorithm’s efficiency.

The function provides a foundational approach for educational purposes, illustrating the recursive nature of topological sorting and dependency management in DAGs.
Transcribed Image Text:# Topologically Sorting DAGs Below is a Python function `naive_topsort` demonstrating how to perform a topological sort on a Directed Acyclic Graph (DAG). This implementation assumes a set-based representation of the graph nodes. ```python def naive_topsort(G, S=None): if S is None: S = set(G) # Default: All nodes if len(S) == 1: return list(S) # Base case, single node v = S.pop() # Reduction: Remove a node seq = naive_topsort(G, S) # Recursion (assumption), n=1 min_i = 0 for i, u in enumerate(seq): if v in G[u]: min_i = i + 1 # After all dependencies seq.insert(min_i, v) return seq ``` - **Explanation** - **Line 2-3**: If the set `S` is not provided, initialize it with all nodes in `G`. - **Line 4-5**: Base case for recursion. If there's only one node, return it as a list. - **Line 6**: Remove a node `v` from the set `S`. - **Line 7**: Recursively call `naive_topsort` for the reduced graph. - **Lines 9-13**: Determine the position `min_i` to insert `v` after all dependencies. - **Line 14**: Insert `v` into the sequence at the appropriate position. - **Line 15**: Return the updated sequence. - **Notes** - The graph `G` is represented using adjacency sets or dictionaries. - This script poses the question, "What’s the expected runtime?," prompting a discussion on the algorithm’s efficiency. The function provides a foundational approach for educational purposes, illustrating the recursive nature of topological sorting and dependency management in DAGs.
Expert Solution
Recurrence Analysis :
  1. The given algorithm recursively finds the solution of the topological sort of the graph.
  2. Let T(n) be the time complexity for finding the topological sort of the graph having n number of vertices :
    • T(n) =  T(n - c) + O(1) where c = number of the neightbour of nth vetrex
    • Here, c=  O(V) i.e the maximum number of neighbors can be at a maximum of a total number of vertices.
    • This recurrence equation is used for finding the solution to the time complexity of the given equation.
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