having extra values in a loop output in R - r

Question

I have a graph like below:
adj <- read.table(text = "
A B C D
A 0 1 0 0
B 1 0 1 1
C 0 1 0 0
D 0 1 0 0
", header = T)
g <- graph_from_adjacency_matrix(as.matrix(adj))
I want to compute each node's distance like below:
distMat <- 1/2^distances(g)
for (i in 1:nrow(distMat)) {
res[i] <- sum(distMat[i, ]) - distMat[i, i]
}
names(res) <- V(g)$name[V(g)]
res
But the number of values in the result should be equal to the number of nodes in the graph but it's not and I have 5 values instead of 4. Any idea how to fix it?

Answer 1

Your code does not return 5 values. One reason that you may run into trouble is that your code is much more complicated than it needs to be. Look at what it does:
res <- 1:nrow(distMat)
for (i in 1:nrow(distMat)) {
res[i]<-sum( distMat[i,]) - distMat[i,i]
}
is a loop of your distance matrix where each row is summarised before subtracting distMat[i,i] which must always be 1 since distMat[i,i] refer's to each node's distance to itself. A nicer rewrite for the same would be:
res <- rowSums(1/2^distances(g))-1
It then becomes easier to see that kind of calculations are really centrality measures. The lowest sum of distances to all other nodes will be associated with high centrality.
iGraph has functions to compute a whole range of documented and established centrality measures. See degree(), closeness(), or betweenness(). What is the advantage of yours?
Look at your centrality measure and play around using this code:
# Make random graph with more nodes and calculate your centrality measure as res
graph <- erdos.renyi.game(80, 100, "gnm", directed=FALSE)
res <- rowSums(1/2^distances(graph))-1
# Colour each node according to their distance to every other node
colfunc <- colorRampPalette(c("yellow", "red"))
gradient <- colfunc(max(res, na.rm=T))
V(graph)$color <- gradient[round(res)+1]
# Plot it
plot(graph, vertex.label="")
Play around by using res <- betweenness(graph) for example.