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Best to give an example. Let us consider the function AdjacencyMatrix. When we pass, say a matrix to it, we get:

test = {{1,1,1},{1,2,2},{3,3,3}};
AdjacencyMatrix[%]

AdjacencyMatrix::graph: A graph object is expected at position 1 in AdjacencyMatrix[{{1,1,1},{1,2,2},{3,3,3}}].

AdjacencyMatrix[{{1, 1, 1}, {1, 2, 2}, {3, 3, 3}}]

Is there a way of modifying AdjacencyMatrix so that if the argument isn't a graph object it would simply return the argument itself?

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  • $\begingroup$ You want AdjacencyMatrix to return the argument itself only when it is a matrix, or whenever it is not a graph? $\endgroup$
    – AccidentalFourierTransform
    Commented Jul 12, 2019 at 16:50

2 Answers 2

Reset to default
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You can use Check in a user defined function:

adjacencyMatrix[g_] := Check[AdjacencyMatrix[g], g]

Then:

adjacencyMatrix[Graph[{1->2,2->3,3->4}]] //Normal
adjacencyMatrix[{1->2, 2->3, 3->4}] //Normal
adjacencyMatrix[{{1,2},{3,4}}]

{{0, 1, 0, 0}, {0, 0, 1, 0}, {0, 0, 0, 1}, {0, 0, 0, 0}}

{{0, 1, 0, 0}, {0, 0, 1, 0}, {0, 0, 0, 1}, {0, 0, 0, 0}}

AdjacencyMatrix::graph: A graph object is expected at position 1 in AdjacencyMatrix[{{1,2},{3,4}}].

{{1, 2}, {3, 4}}

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    $\begingroup$ This is much better than my approach. Note: OP may want to suppress the error message here: Quiet[Check[...], {AdjacencyMatrix::graph}]. $\endgroup$
    – AccidentalFourierTransform
    Commented Jul 13, 2019 at 1:09
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Bad answer:

Unprotect[AdjacencyMatrix];
AdjacencyMatrix[mat_List] := mat
Protect[AdjacencyMatrix];

Good answer:

adjacencyMatrix[graph_Graph] := AdjacencyMatrix[graph]
adjacencyMatrix[mat_List] := mat
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  • $\begingroup$ For the good answer, maybe remove the pattern head List on the second line, as this is the behavior requested for anything that's not a Graph. $\endgroup$
    – Roman
    Commented Jul 12, 2019 at 16:46
  • $\begingroup$ @Roman good point. Let me ask OP to be sure. (If they only want the identity for matrices, then it is safer to leave List in). $\endgroup$
    – AccidentalFourierTransform
    Commented Jul 12, 2019 at 16:49

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