# Graph from Sparse Adjacency Matrix

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I spent a decent chunk of my morning trying to figure out how to construct a sparse adjacency matrix for use with `graph.adjacency()`

. I’d have thought that this would be rather straight forward, but I tripped over a few subtle issues with the Matrix package. My biggest problem (which in retrospect seems rather trivial) was that elements in my adjacency matrix were occupied by the pipe symbol.

> adjacency[1:10,1:10] 10 x 10 sparse Matrix of class 'ngCMatrix' [1,] . . . . . | . . . . [2,] . . . . . . . | . . [3,] . . . . . . . . . . [4,] . . . . . . . . . . [5,] . . . . | . . . . . [6,] . . . . . . . . . . [7,] . . . . . . . . . . [8,] . . . . . . . . . . [9,] . . . . . . . . . . [10,] . | . . . . . . . .

Of course, the error message I was encountering didn’t point me to this fact. No, that would have been far too simple! The solution is highlighted in the sample code below: you need to specify the symbol used for the occupied sites in the sparse matrix.

> library(Matrix) > > set.seed(1) > > edges = data.frame(i = 1:20, j = sample(1:20, 20, replace = TRUE)) > > adjacency = sparseMatrix(i = as.integer(edges$i), + j = as.integer(edges$j), + x = 1, + dims = rep(20, 2), + use.last.ij = TRUE + )

The resulting adjacency matrix then looks like this:

> adjacency[1:10,1:10] 10 x 10 sparse Matrix of class 'dgCMatrix' [1,] . . . . . 1 . . . . [2,] . . . . . . . 1 . . [3,] . . . . . . . . . . [4,] . . . . . . . . . . [5,] . . . . 1 . . . . . [6,] . . . . . . . . . . [7,] . . . . . . . . . . [8,] . . . . . . . . . . [9,] . . . . . . . . . . [10,] . 1 . . . . . . . .

And can be passed into `graph.adjacency()`

without any further issues.

> library(igraph) > graph = graph.adjacency(adjacency, mode = "undirected")

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