Bellman-Ford Algorithm

Solution to Rosalind Algorithmic Heights Problem 022: Bellman-Ford
Algorithm

Algorithmic_022_BF.py

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+#!/usr/bin/env python
+'''
+A solution to a ROSALIND problem from the Algorithmic Heights problem area.
+Algorithmic Heights focuses on teaching algorithms and data structures commonly used in computer science.
+
+Problem Title: Bellman-Ford Algorithm
+Rosalind ID: BF
+Algorithmic Heights #: 022
+URL: http://rosalind.info/problems/bf/
+'''
+
+
+def bellman_ford(n, edges, source):
+    '''Uses the Bellman-Ford Algorithm to get the distances from a source node to all other nodes.'''
+    # Initialize distances.  Recall: Rosalind indicies start at 1.
+    dist = [float('inf') if i != source-1 else 0 for i in xrange(n)]
+
+    # Run the Bellman-Ford updated procedure |V|-1 times.
+    for i in xrange(n-1):
+        # Relaxation condition to break outer look early.
+        relaxed = True
+        for u, v, weight in edges:
+            if dist[u-1] + weight < dist[v-1]:
+                dist[v-1] = dist[u-1] + weight
+                relaxed = False
+        if relaxed:
+            break
+
+    # Return the distances in the desired format.
+    return [d if d != float('inf') else 'x' for d in dist]
+
+
+def main():
+    '''Main call. Reads, runs, and saves problem specific data.'''
+    # Read the input data.
+    with open('data/algorithmic/rosalind_bf.txt') as input_data:
+        n = map(int, input_data.readline().strip().split())[0]
+        edges = [map(int, line.strip().split()) for line in input_data]
+
+    # Get the minimum distances using the Bellman-Ford Algorithm.
+    distances = map(str, bellman_ford(n, edges, 1))
+
+    # Print and save the answer.
+    print ' '.join(distances)
+    with open('output/algorithmic/Algorithmic_022_BF.txt', 'w') as output_data:
+        output_data.write(' '.join(distances))
+
+if __name__ == '__main__':
+    main()