[igraph] A query about finding singleton cycles in a graph

[Simon Tavare]

In response to a query on Stack Overflow on March 10 2019 by Ankit, the following  function, FindCycles, was given:

## More efficient version

FindCycles = function(g) {

  Cycles = NULL

  for(v1 in V(g)) {

    if(degree(g, v1, mode="in") == 0) { next }

    GoodNeighbors = neighbors(g, v1, mode="out")

    GoodNeighbors = GoodNeighbors[GoodNeighbors > v1]

    for(v2 in GoodNeighbors) {

      TempCyc = lapply(all_simple_paths(g, v2,v1, mode="out"), function(p) c(v1,p))

      TempCyc = TempCyc[which(sapply(TempCyc, length) > 3)]                                 ###

      TempCyc = TempCyc[sapply(TempCyc, min) == sapply(TempCyc, `[`, 1)]

      Cycles  = c(Cycles, TempCyc)                   }

  }

  Cycles

}

I am interested in finding cycles of any length, so that the line marked ### should be removed.

The resultant code works for cycles of size at least two, but does not catch singleton cycles.

Is there a simple fix?

(I wrote an obvious modification of the code above to search for just singleton cycles, but it seems that there must be a simple tweak to the code above.)

Many thanks,

Simon