Better equivalent of this crazy nested python for loop

问题

for a in map:
    for b in map[a]:
        for c in map[b]:
            for d in map[c]:
                for e in map[d]:
                    print a+b+c+d+e

The above code is used to create all paths of certain length in a graph. map[a] represents the points you can reach from point a.

How can I change it to simulate having an arbitrary number of loops?

This is like a cartesian product (itertools.product) where at each iteration your choice for the next element is limited to those in map[current_point].

回答1:

map = {
    'a': ['b', 'c'],
    'b': ['c', 'd'],
    'c': ['d', 'a'],
    'd': []
}

def print_paths(map, start, length, prefix = ''):
    if length == 0:
        print prefix
    else:
        for a in map[start]:
            print_paths(map, a, length - 1, prefix + start)

for a in map.keys():
    print_paths(map, a, 5)

回答2:

This is a classical recursive problem. Your function should returns concatenation of node value with all results of your function result for each child node. As you can see in sentence, function behavior is explained in a recursive way:

myGraph = { 1:[2,3], 2:[3,4] }

def recorre( node_list, p = '' ):    
    for node in node_list:
        if node in myGraph and myGraph[node]: 
            recorre(myGraph[node], p+unicode( node ))
        else:
            print p+unicode( node )

recorre( myGraph )

Result:

>>> recorre( myGraph )
123
124
13
23
24

This code is a start point. You can store all paths in a list, use yield generator, etc. Don't forget to prevent circles.

Also, take a look to igraph solution. Sure this library can helps to you, see this example: Finds all shortest paths (geodesics) from a vertex to all other vertices.

Regards.

回答3:

Just like other suggested, with recursion:

    distances = []        

    def compute_distance(map, depth, sum):
         if depth == 0 or len(map) == 0:
            distances.append[sum]
         else:
            for a in map:
               compute_distance(map[a], depth - 1, sum + a)

   compute_distance(map, x, 0)

来源：https://stackoverflow.com/questions/8906351/better-equivalent-of-this-crazy-nested-python-for-loop

标签

python

recursion

generator

nested-loops