Fix merging of networks without vertices

The function 'merge.networks' is not able to handle networks that have
not vertices and also (!) no vertex attributes: The final call to
'igraph::graph.data.frame' results in an error.
To fix this, the special case is identified and the list of vertices is
adjusted accordingly. A corresponding test is added to the test suite.

Additionally, the default case of merging only one network is handled by
returning the only network right away.

Signed-off-by: Claus Hunsen <hunsen@fim.uni-passau.de>

tests/test-networks.R

@@ -86,6 +86,16 @@ test_that("Merge networks", {
     ## Merge two empty networks (i.e., no vertices at all)
     ##
 
+    network.empty = igraph::graph.empty(0, directed = TRUE)
+        #create.empty.network(directed = FALSE)
+    expect_error(merge.networks(list(network.empty, network.empty)), NA) # expect that no error occurs
+    expect_true(igraph::vcount(merge.networks(list(network.empty, network.empty))) == 0) # vertices
+    expect_true(igraph::ecount(merge.networks(list(network.empty, network.empty))) == 0) # edges
+
+    ##
+    ## Merge two empty networks (i.e., no vertices at all, but with vertex attributes)
+    ##
+
     network.empty = create.empty.network(directed = FALSE)
     expect_error(merge.networks(list(network.empty, network.empty)), NA) # expect that no error occurs
     expect_true(igraph::vcount(merge.networks(list(network.empty, network.empty))) == 0) # vertices

util-networks.R

@@ -1125,6 +1125,11 @@ merge.network.data = function(vertex.data, edge.data) {
 merge.networks = function(networks) {
     logging::logdebug("merge.networks: starting.")
 
+    if (length(networks) == 1) {
+        logging::logdebug("Only one network given, so a merge is not necessary.")
+        return(networks[[1]])
+    }
+
     ## list with all vertex data frames
     vertex.data = lapply(networks, function(network) {
         return(igraph::as_data_frame(network, what = "vertices"))
@@ -1138,6 +1143,11 @@ merge.networks = function(networks) {
     ## merge all edge and vertex data frames
     new.network.data = merge.network.data(vertex.data, edge.data)
 
+    ## catch case where no vertices (and no vertex attributes) are given
+    if (ncol(new.network.data[["vertices"]]) == 0) {
+        new.network.data[["vertices"]] = NULL # igraph::graph.data.frame can handle this
+    }
+
     ## build whole network form edge and vertex data frame
     whole.network = igraph::graph.data.frame(
         new.network.data[["edges"]],