python edge list to adjacency matrix

Question

Given an edge list, I need to convert the list to an adjacency matrix in Python. I am very, very close, but I cannot figure out what I am doing incorrectly. Where is my thinking incorrect?

E= [[0, 0], [0, 1], [1, 0], [1, 1]]

nmax = max(E)
nmax2 =max(nmax)

m = []
for i in range(nmax2+1):
    row = []
    for j in range(nmax2+1):
         if [i,j]== E[i]:
               row.append(1)
         else:
               row.append(0)
    m.append(row)

 print(m)

I would expect the result to be: 1 1 1 1

But my code produces: 1 0 0 0

E[i] goes maximum up to E[1] because i goes only from 0 to 1. So you never reach [1, 0] and [1, 1] — Sheldore
– Sheldore, Commented Mar 30, 2019 at 22:45

modesitt · Accepted Answer · 2019-03-30 22:49:24Z

3

As the comment suggests, you are only checking edges for as many rows as you have in your adjacency matrix, so you fail to reach many edges in the general case. Consider the following instead:

E = [[0, 0], [0, 1], [1, 0], [1, 1]]

# nodes must be numbers in a sequential range starting at 0 - so this is the
# number of nodes. you can assert this is the case as well if desired 
size = len(set([n for e in E for n in e])) 
# make an empty adjacency list  
adjacency = [[0]*size for _ in range(size)]
# populate the list for each edge
for sink, source in E:
    adjacency[sink][source] = 1

>>> print(adjacency)
>>> [[1, 1], [1, 1]]

If you want to be brief at the cost of clarity:

adjacency = [[1 if [i, j] in set(map(tuple, E)) else 0 for j in range(size)] for i in range(size)]

size representing the number of nodes - as before.

answered Mar 30, 2019 at 22:49

modesitt

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Comments

kcp · Accepted Answer · 2019-03-30 23:11:04Z

0

i blv below is cleaner and does the job

    E= [[0, 0], [0, 1], [1, 0], [1, 1]]
    size = max(max(E))+1
    r = [[0 for i in range(size)] for j in range(size)]
    for row,col in E:
        r[row][col] = 1
    print(r)

edited Mar 30, 2019 at 23:11

answered Mar 30, 2019 at 22:54

kcp

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1 Comment

modesitt Over a year ago

in an adjacency matrix, the dimmension is always square - this is not correct generally

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