[英]How do I convert a list of pairs into a dictionary with each element as a key to a list of paired values?
我正在做涉及图表的课程。 我有边缘列表E = [('a','b'),('a','c'),('a','d'),('b','c')等等。我想要一个函数将它们转换成字典形式的邻接矩阵{'a':['b','c','d'],'b':['a'等}}所以我可以使用只输入这些词典的函数。
我的主要问题是我无法弄清楚如何使用循环来添加键:值而不只是覆盖列表。 我的函数的先前版本将输出[]作为所有值,因为'f'没有连接。
我试过这个:
V = ['a','b','c','d','e','f']
E=[('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd'), ('c', 'd')]
def EdgeListtoAdjMat(V,E):
GA={}
conneclist=[]
for v in V:
for i in range(len(V)):
conneclist.append([])
if (v,V[i]) in E:
conneclist[i].append(V[i])
for i in range(len(V)):
GA[V[i]]=conneclist[i]
return(GA)
EdgeListtoAdjMat(V,E)输出:
{'a': [], 'b': ['b'], 'c': ['c', 'c'], 'd': ['d', 'd', 'd'], 'e': [], 'f': []}
而它应该输出:
{'a':['b','c','d'],
'b':['a','c','d'],
'c':['a','b','d'],
'd':['a','b','c'],
'e':[],
'f':[]
}
你想要实现的逻辑实际上非常简单:
V = ['a','b','c','d','e','f']
E=[('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd'), ('c', 'd')]
result = {}
for elem in V:
tempList = []
for item in E:
if elem in item:
if elem == item[0]:
tempList.append(item[1])
else:
tempList.append(item[0])
result[elem] = tempList
tempList = []
print(result)
结果:
{'a': ['b', 'c', 'd'], 'b': ['a', 'c', 'd'], 'c': ['a', 'b', 'd'], 'd': ['a', 'b', 'c'], 'e': [], 'f': []}
对于V
每个元素,执行检查以查看该元素是否存在于E
中的任何元组中。 如果它存在,则将该元素组合在一起形成一对,并附加到临时列表。 检查E
每个元素后,更新result
字典并移至V
的下一个元素,直到完成为止。
要返回代码,您需要按以下方式修改它:
def EdgeListtoAdjMat(V,E):
GA={}
conneclist=[]
for i in range(len(V)):
for j in range(len(V)):
# Checking if a pair of two different elements exists in either format inside E.
if not i==j and ((V[i],V[j]) in E or (V[j],V[i]) in E):
conneclist.append(V[j])
GA[V[i]]=conneclist
conneclist = []
return(GA)
一种更有效的方法是迭代边缘并将列表的输出字典附加到两个方向上的顶点。 使用dict.setdefault
使用dict.setdefault
初始化每个新键。 当边缘上的迭代结束时,迭代尚未出现在输出dict中的其余顶点,为它们分配空列表:
def EdgeListtoAdjMat(V,E):
GA = {}
for a, b in E:
GA.setdefault(a, []).append(b)
GA.setdefault(b, []).append(a)
for v in V:
if v not in GA:
GA[v] = []
return GA
所以给出:
V = ['a', 'b', 'c', 'd', 'e', 'f']
E = [('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd'), ('c', 'd')]
EdgeListtoAdjMat(V, E))
将返回:
{'a': ['b', 'c', 'd'], 'b': ['a', 'c', 'd'], 'c': ['a', 'b', 'd'], 'd': ['a', 'b', 'c'], 'e': [], 'f': []}
由于您已经在V中有了顶点列表,因此很容易准备一个带有空连接列表的字典。 然后,只需浏览边缘列表并添加到每一侧的数组:
V = ['a','b','c','d','e','f']
E = [('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd'), ('c', 'd')]
GA = {v:[] for v in V}
for v1,v2 in E:
GA[v1].append(v2)
GA[v2].append(v1)
我认为你的代码不是非常pythonic,你可以编写一个更易读的代码,更简单的调试,也更快,因为你使用python的内置库和numpy的索引。
def EdgeListToAdjMat(V, E):
AdjMat = np.zeros((len(V), len(V))) # the shape of Adjancy Matrix
connectlist = {
# Mapping each character to its index
x: idx for idx, x in enumerate(V)
}
for e in E:
v1, v2 = e
idx_1, idx_2 = connectlist[v1], connectlist[v2]
AdjMat[idx_1, idx_2] = 1
AdjMat[idx_2, idx_1] = 1
return AdjMat
如果您考虑使用库, networkx
是针对这些类型的网络问题而设计的:
import networkx as nx
V = ['a','b','c','d','e','f']
E = [('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd'), ('c', 'd')]
G=nx.Graph(E)
G.add_nodes_from(V)
GA = nx.to_dict_of_lists(G)
print(GA)
# {'a': ['c', 'b', 'd'], 'c': ['a', 'b', 'd'], 'b': ['a', 'c', 'd'], 'e': [], 'd': ['a', 'c', 'b'], 'f': []}
您可以使用itertools.groupby
将边列表转换为地图
from itertools import groupby
from operator import itemgetter
V = ['a','b','c','d','e','f']
E = [('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd'), ('c', 'd')]
# add edge in the other direction. E.g., for a -> b, add b -> a
nondirected_edges = E + [tuple(reversed(pair)) for pair in E]
# extract start and end vertices from an edge
v_start = itemgetter(0)
v_end = itemgetter(1)
# group edges by their starting vertex
groups = groupby(sorted(nondirected_edges), key=v_start)
# make a map from each vertex -> adjacent vertices
mapping = {vertex: list(map(v_end, edges)) for vertex, edges in groups}
# if you don't need all the vertices to be present
# and just want to be able to lookup the connected
# list of vertices to a given vertex at some point
# you can use a defaultdict:
from collections import defaultdict
adj_matrix = defaultdict(list, mapping)
# if you need all vertices present immediately:
adj_matrix = dict(mapping)
adj_matrix.update({vertex: [] for vertex in V if vertex not in mapping})
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