Question

6.00 Pset 11 - Help with constraints?

Hi all,
I've been stuck for days on PS11. I have been building off the code presented in the lectures but am totally stuck when it comes to adding constraints.

Can anyone help? I'm posting my code in a comment below.

Answer 1

The code that I'm specifically referring to is a function I've named called "shortestPathWeightedConstraintsGabi". The previous three functions were iterations to get to this point.

Answer 2

Constraints:
If you are doing a brute-force search, all you need to do is to check that a particular path satisfies \(both\) given constraints of total distance and outdoor distance.
If you find one (feasible solution), then put it aside.
If you find another feasible solution, compare with the previous and keep only the one with a lesser total distance.
At the end of the search, the retained feasible solution will be the feasible path with minimum total distance as required.

Answer 3

Thanks - I think conceptually I understand it, I'm just having trouble with the implementation. Here's the code that (I thought) goes through that process:

def shortestPathWeightedConstraintsGabi(graph, start, end, maxTotalDist, maxDistOutdoors, visited = []):
start = Node(start)
end = Node(end)
if not (graph.hasNode(start) and graph.hasNode(end)):
raise ValueError('Start or end not in graph.')
path = [str(start)]
if start == end:
return path
shortest = None
for node in graph.childrenOf(start):
if (str(node) not in visited):
visited = visited + [str(node)] #new list
newPath = shortestPathWeightedConstraintsGabi(graph, node, end, maxTotalDist, maxDistOutdoors, visited)
if newPath == None:
continue
if (shortest == None or calcTotalDist(newPath, graph) < calcTotalDist(shortest, graph)):
if calcTotalDist(newPath, graph) < maxTotalDist and calcDistOutdoors(newPath, graph) < maxDistOutdoors:
shortest = newPath
if shortest != None:
path = path + shortest
else:
path = None
return path


I've tested my custom functions (calcTotalDist and calcDistOutdoors) and I know they return the correct integers. Is there something I'm missing/not seeing here?

Answer 4

Here's a different implementation I've also been testing that breaks when I put in the constraints:

def shortestPathWeightedConstraints2(graph, start, end, path, shortest, maxTotalDist, maxDistOutdoors):
start = Node(start)
end = Node(end)
if not (graph.hasNode(start) and graph.hasNode(end)):
raise ValueError('Start or end not in graph.')
path = path + [str(start)]
#print 'Current DFS path:', path
#print 'Total Dist Outdoors:', calcDistOutdoors(path, graph)
if start == end:
return path
#shortest = None
for node in graph.childrenOf(start):
if (str(node) not in path):
newPath = shortestPathWeightedConstraints2(graph, node, end, path, shortest, maxTotalDist, maxDistOutdoors)
if shortest == None or calcTotalDist(newPath, graph) < calcTotalDist(shortest,graph):
if newPath != None:
if calcTotalDist(newPath, graph) < maxTotalDist and calcDistOutdoors(newPath, graph) < maxDistOutdoors:
print 'FOUND BETTER PATH'
shortest = newPath

return shortest