How igraph functions handle attributes when the graph changesSource:
Many times, when the structure of a graph is modified, vertices/edges map of
the original graph map to vertices/edges in the newly created (modified)
graph. For example
simplify() maps multiple edges to single
edges. igraph provides a flexible mechanism to specify what to do with the
vertex/edge attributes in these cases.
The functions that support the combination of attributes have one or two
extra arguments called
that specify how to perform the mapping of the attributes. E.g.
contract() contracts many vertices into a single one, the
attributes of the vertices can be combined and stores as the vertex
attributes of the new graph.
The specification of the combination of (vertex or edge) attributes can be given as
a character scalar,
a function object or
a list of character scalars and/or function objects.
If it is a character scalar, then it refers to one of the predefined combinations, see their list below.
If it is a function, then the given function is expected to perform the combination. It will be called once for each new vertex/edge in the graph, with a single argument: the attribute values of the vertices that map to that single vertex.
The third option, a list can be used to specify different combination methods for different attributes. A named entry of the list corresponds to the attribute with the same name. An unnamed entry (i.e. if the name is the empty string) of the list specifies the default combination method. I.e.
specifies that the weight of the new edge should be sum of the weights of the corresponding edges in the old graph; and that the rest of the attributes should be ignored (=dropped).
Predefined combination functions
The following combination behaviors are predefined:
The attribute is ignored and dropped.
The sum of the attributes is calculated. This does not work for character attributes and works for complex attributes only if they have a
sumgeneric defined. (E.g. it works for sparse matrices from the
Matrixpackage, because they have a
The product of the attributes is calculated. This does not work for character attributes and works for complex attributes only if they have a
The minimum of the attributes is calculated and returned. For character and complex attributes the standard R
minfunction is used.
The maximum of the attributes is calculated and returned. For character and complex attributes the standard R
maxfunction is used.
Chooses one of the supplied attribute values, uniformly randomly. For character and complex attributes this is implemented by calling
Always chooses the first attribute value. It is implemented by calling the
Always chooses the last attribute value. It is implemented by calling the
The mean of the attributes is calculated and returned. For character and complex attributes this simply calls the
The median of the attributes is selected. Calls the R
medianfunction for all attribute types.
Concatenate the attributes, using the
cfunction. This results almost always a complex attribute.
edge_attr() on how to use graph/vertex/edge attributes in
igraph_options() on igraph parameters.
Vertex, edge and graph attributes
Gabor Csardi email@example.com
g <- make_graph(c(1, 2, 1, 2, 1, 2, 2, 3, 3, 4)) E(g)$weight <- 1:5 ## print attribute values with the graph igraph_options(print.graph.attributes = TRUE) igraph_options(print.vertex.attributes = TRUE) igraph_options(print.edge.attributes = TRUE) ## new attribute is the sum of the old ones simplify(g, edge.attr.comb = "sum") #> IGRAPH 0ce06d7 D-W- 4 3 -- #> + attr: weight (e/n) #> + edges from 0ce06d7: #>  1->2 2->3 3->4 ## collect attributes into a string simplify(g, edge.attr.comb = toString) #> IGRAPH 0b202c4 D-W- 4 3 -- #> + attr: weight (e/c) #> + edges from 0b202c4: #>  1->2 2->3 3->4 ## concatenate them into a vector, this creates a complex ## attribute simplify(g, edge.attr.comb = "concat") #> IGRAPH 1958647 D-W- 4 3 -- #> + attr: weight (e/x) #> + edges from 1958647: #>  1->2 2->3 3->4 E(g)$name <- letters[seq_len(ecount(g))] ## both attributes are collected into strings simplify(g, edge.attr.comb = toString) #> IGRAPH aa03b43 D-W- 4 3 -- #> + attr: weight (e/c), name (e/c) #> + edges from aa03b43: #>  1->2 2->3 3->4 ## harmonic average of weights, names are dropped simplify(g, edge.attr.comb = list( weight = function(x) length(x) / sum(1 / x), name = "ignore" )) #> IGRAPH 8833733 D-W- 4 3 -- #> + attr: weight (e/n) #> + edges from 8833733: #>  1->2 2->3 3->4