如何获取元组列表中的元素

Question

I have a set of nodes

nodes = ["uno","dos","tres","cuatro","cinco","seis"]

and a set of edges

edges = [("uno", "dos"),
     ("uno", "tres"), 
     ("dos", "tres"), 
     ("dos", "cuatro"), 
     ("tres", "cuatro"), 
     ("tres", "cinco"), 
     ("cuatro", "cinco"), 
     ("cuatro", "seis"), 
     ("cinco", "seis")]

I have defined the inverse of these edges

inverses =[(j,i) for (i,j) in edges]

And now I have my set of arcs

arcs = edges + inverses

My problem is that I want to get all the elements in the set of arcs which have a determined first element. That is

ArcsOut[i in nodes] = [(i,j) for (i,j) in arcs for i in nodes for j in nodes].

I want to obtain something like this. If I order to get all arcs that have their fist component "uno", I would like to have:

ArcsOut["uno"]= [("uno", "dos"),("uno", "tres"),("dos", "uno"),("tres", "uno")] 

But I do not know how I can order that. Thanks in advance!

Answer 1

Use the following syntax to create a dictionary:

ArcsOut = dict(((i,[(k,l) for k,l in arcs if k == i]) for (i,j) in arcs))

Then ArcsOut['unos'] = desired output.

/* EDIT */

Performancewise consider using a defaultdict :

ArcsOut = defaultdict(list)
for k, v in arcs:
    ArcsOut[k].append((k,v)) 

Answer 2

You can do this with a list comprehension. The nice part is you can use the tuple unpacking together with the condition to make it very short and to the point, at the same time readable.

arc_out = {node: [(i, j) for i, j in arcs if i == node] for node in nodes]

Instead, if you want to get all edges where either i or j matches a specific node name, you can do this

[edge for edge in arcs if 'uno' in edge]