I have created the following program. It takes an input G, which consists of the vertices of a graph, along with the edges and the corresponding edge weights. The aim of the program is to extract just the edges
def edges(G):
E =[]
for i in G[1]:
E.append(i[0])
return E
print (edges(G))
On the following input
G = [({'a', 'b'}, 4), ({'a', 'c'}, 6), ({'a', 'd'}, 8), ({'b', 'e'}, 1) ,
({'b', 'f'}, 9), ({'c', 'f'}, 3), ({'d', 'g'}, 7), ({'d', 'h'}, 0)]
The following output is produced:
[{'a', 'b'}, {'a', 'c'}, {'a', 'd'}, {'e', 'b'}, {'f', 'b'}, {'f', 'c'}, {'g', 'd'}, {'h', 'd'}]
The output I am trying to get is:
[{'a', 'b'}, {'a', 'c'}, {'a', 'd'}, {'b', 'e'}, {'b', 'f'}, {'c', 'f'}, {'d', 'g'}, {'d', 'h'}]
Can anyone explain why the tuples I have extracted are reordered?
A set is an unordered collection. What you are requesting is not possible.
The best you can do is use an ordered collection, such as list or tuple. Below is an example.
res = [tuple(sorted(x[0])) for x in G]
print(res)
[('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'e'),
('b', 'f'), ('c', 'f'), ('d', 'g'), ('d', 'h')]
This is also possible functionally, but messy since Python has no native function composition. For composition, you can use 3rd party library toolz.
from operator import itemgetter
from toolz import compose
res = list(map(compose(tuple, sorted, itemgetter(0)), G))