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Given an adjacency matrix, what is an algorithm/pseudo-code to convert a directed graph to an undirected graph without adding additional vertices (does not have to be reversable)?

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Ibrahim
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1 Answers1

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Note that for an undirected graph, the adjacency matrix is symmetric, i.e. A[i,j] == A[j,i].

From this, we can see that we can simply compute the new value of A[i,j] = A[j,i] depending if A[i,j] or A[j,i] is set. Assuming the graph is unweighted, we can do:

for i from 0 to n-1
    for j from 0 to i
        if A[i,j] == 1 OR A[j,i] == 1
            A[i,j] = A[j,i] = 1
        else 
            A[i,j] = A[j,i] = 0

Note that we only have to consider 1 + 2 + 3 + ... + n-1 entries since the resultant adjacency matrix is symmetric.

If we have a weighted graph, we now have the problem of which edge weight to take as the new undirected graph edge weight. For example, if w(2,5) = 5 but w(5,2) = 10, the resultant edge weight is ambiguous. However, this is enough for you to figure out what else you need from here.

Irvin Lim
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