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\name{boundary}
\alias{boundary}
\title{ Returns the Boundary between a Graph and a SubGraph }
\description{
The boundary of a subgraph is the set of nodes in the original graph
that have edges to nodes in the subgraph. The function \code{boundary}
computes the boundary and returns it as a list whose length is the same
length as the number of nodes in the subgraph.
}
\usage{
boundary(subgraph, graph)
}
\arguments{
\item{graph}{ the original graph from which the boundary will
be created }
\item{subgraph}{ can either be the vector of the node labels or the
subgraph itself.}
}
\details{
The \emph{boundary} of a \emph{subgraph} is the set of nodes in the
graph which have an edge that connects them to the specified subgraph
but which are themselves not elements of the subgraph.
For convenience users can specify the subgraph as either a graph or a
vector of node labels.
}
\value{
This function returns a named list of length equal to the number of
nodes in \code{subgraph}. The elements of the list
correspond to the nodes in the \code{subgraph}. The elements are lists
of the nodes in \code{graph} which share an edge with the
respective node in \code{subgraph}.
}
\author{ Elizabeth Whalen and R. Gentleman }
\seealso{ \code{\link{subGraph}}, \code{\link{graph-class}}
}
\examples{
set.seed(123)
g1 <- randomGraph(letters[1:10], 1:4, p=.3)
##both should be "a"
boundary(c("g", "i"), g1)
}
\keyword{ manip }
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