tests/testthat/test-graph.eigen.R

In igraph: Network Analysis and Visualization

test_that("spectrum works for symmetric matrices", {
  set.seed(42)

  std <- function(x) {
    x <- zapsmall(x)
    apply(x, 2, function(col) {
      if (any(col < 0) && col[which(col != 0)[1]] < 0) {
        -col
      } else {
        col
      }
    })
  }

  g <- sample_gnp(50, 5 / 50)
  e0 <- eigen(as_adj(g, sparse = FALSE))

  e1 <- spectrum(g, which = list(howmany = 4, pos = "LA"))
  expect_that(e0$values[1:4], equals(e1$values))
  expect_that(std(e0$vectors[, 1:4]), equals(std(e1$vectors)))

  e2 <- spectrum(g, which = list(howmany = 4, pos = "SA"))
  expect_that(e0$values[50:47], equals(e2$values))
  expect_that(std(e0$vectors[, 50:47]), equals(std(e2$vectors)))
})