Question

I’ve been experimenting with different ways of implementing depth first search. I’ve found a few working ways, but they involved some rather tedious dictionary work. I’ve developed a new idea using lists, but the actions this implementation returns don’t match the desired result. I’ll try to comment the code as clearly as I can:

start = problem.getStartState()            ## returns an (x, y) tuple
children = problem.getSuccessors(start)    ##returns the children of a parent node in ((start                  
                                           state), action, cost) format. 
stack = Stack()                            ##creates a Stack (LIFO) data structure
visited = []                               ##list of visited nodes
visited.append(start)
for child in children:
    stack.push((child, [], [], 0))         ##push children to fringe in the format of (child,
    while stack:                           ##path taken, actions taken, cost) 
        parent = stack.pop()
        node = parent[0]
        if parent[0] in visited: continue
        visited.append(parent[0])
        path = parent[1] + [node[0]]           ##assigns previous path/actions/cost to new 
        actions = parent[2] + [node[1]]        ##node, creating a cumulative, ordered list of
        print actions                          ##the path/actions and a cumulative cost
        cost = parent[3] + node[2]
        if problem.isGoalState(node[0]):
            print parent[2]
            return parent[2]                    ## returns list of actions 
        children = problem.getSuccessors(node[0])
        if children != []:
            for child in children:
                stack.push((child, path, actions, cost))   ##assigns cumulative lists to child

Anyone see where my problems might lie in this implementation? BTW, I know DFS is an inefficient algorithm for most cases. But once I get this implementation right, it should be able to cross over to other search algorithms by simply changing the data structure that stores the children of the parent node.

Answer 1

CS188 pal 😀 It’s really hard to read your code here… all those indexes %)
Use more variables and it’ll be more clear.
My solution:

def depthFirstSearch(problem):
    fringe = util.Stack()
    expanded = set()
    fringe.push((problem.getStartState(),[],0))
    
    while not fringe.isEmpty():
        curState, curMoves, curCost = fringe.pop()
        
        if(curState in expanded):
            continue
        
        expanded.add(curState)
        
        if problem.isGoalState(curState):
            return curMoves
        
        for state, direction, cost in problem.getSuccessors(curState):
            fringe.push((state, curMoves+[direction], curCost))
    return []

I hope I don’t need to comment it. It’s easy to read 🙂
Have a good day 😉