added a method to add edges one at a time

Fixes #11

src/dag_module.F90

@@ -71,6 +71,7 @@ module dag_module
         procedure,public :: set_vertices       => dag_set_vertices
         generic,public   :: set_edges          => dag_set_edges_no_atts, &
                                                   dag_set_edges_vector_atts
+        procedure,public :: add_edge           => dag_add_edge
         procedure,public :: remove_edge        => dag_remove_edge
         procedure,public :: remove_vertex      => dag_remove_node
         procedure,public :: set_vertex_info    => dag_set_vertex_info
@@ -393,6 +394,26 @@ function dag_get_vertex(me,i) result(v)
     end function dag_get_vertex
 !*******************************************************************************
 
+!*******************************************************************************
+!>
+!  Add an edge to a dag.
+
+    subroutine dag_add_edge(me,ivertex,iedge,label,attributes)
+
+    class(dag),intent(inout) :: me
+    integer(ip),intent(in)   :: ivertex !! vertex number
+    integer(ip),intent(in)   :: iedge   !! the vertex to connect to `ivertex`
+    character(len=*),intent(in),optional :: label !! edge label
+    character(len=*),intent(in),optional :: attributes !! other attributes when
+                                                       !! saving as a diagraph.
+
+    call me%vertices(ivertex)%set_edges(iedge,&
+                        label=label,&
+                        attributes=attributes)
+
+    end subroutine dag_add_edge
+!*******************************************************************************
+
 !*******************************************************************************
 !>
 !  set the edges for a vertex in a dag

test/dag_example.f90

@@ -69,6 +69,12 @@ program dag_example
     call d%remove_edge(5_ip,4_ip) ! the orignal node 6 is now 5
     call save_plot('test1_node-5-removed_6-4-edge-removed')
 
+    ! test adding an edge:
+    call d%add_edge(ivertex=5_ip,iedge=1_ip, &
+                    label='added',&
+                    attributes='penwidth=2,arrowhead=none,color=red')
+    call save_plot('test1_node-5-removed_6-4-edge-removed-edge-added')
+
     ! cleanup:
     call d%destroy()