Use this if you are using igraph from R
decompose {igraph}  R Documentation 
Creates a separate graph for each component of a graph.
decompose(graph, mode = c("weak", "strong"), max.comps = NA, min.vertices = 0)
graph 
The original graph. 
mode 
Character constant giving the type of the components, wither

max.comps 
The maximum number of components to return. The first

min.vertices 
The minimum number of vertices a component should contain in order to place it in the result list. Eg. supply 2 here to ignore isolate vertices. 
A list of graph objects.
Gabor Csardi csardi.gabor@gmail.com
is_connected
to decide whether a graph is connected,
components
to calculate the connected components of a graph.
# the diameter of each component in a random graph
g < sample_gnp(1000, 1/1000)
components < decompose(g, min.vertices=2)
sapply(components, diameter)