write.table is not outputting a header for row names [duplicate]

Question

I try to write out a matrix to csv while retaining rownames (c.f. Export matrix in r).

However when I do it using write.table() all the columns get shifted to the left (so the first data column header appears above the rownames column).

"PC1","PC2","PC3","PC4"
"Murder",0.0417043206282872,-0.04482165626967,0.0798906594208106,-0.994921731246978
"Assault",0.995221281426497,-0.0587600278572231,-0.0675697350838042,0.03893829763516
"UrbanPop",0.0463357461197111,0.976857479909889,-0.200546287353865,-0.0581691430589317
"Rape",0.0751555005855469,0.200718066450337,0.974080592182492,0.0723250196376096

I tried the following (to manually add an extra column):

merged.pca <- prcomp(USArrests)

write.table(merged.pca$rotation, file = "rotation.csv", sep = ",", col.names = c("rowname",colnames(merged.pca$rotation)))

Unfortunately this fails with:

Error in write.table(merged.pca$rotation, file = "rotation.csv", sep = ",",  : 
  invalid 'col.names' specification

TBH I have no clue what this error means. Is it something about the argument being a list and not a vector?

Answer 1

If all else fails, you could make the rownames as column, e. g.

res <- transform(merged.pca$rotation, rowname=rownames(merged.pca$rotation))
write.table(res, "rotation.csv", row.names=FALSE)

Answer 2

Per the help for write.table, you want to specify col.names=NA:

write.table(merged.pca$rotation, file="rotation.csv", col.names=NA, sep=",")

Yes, I think it's a bit silly too. Note that write.csv will do this for you automatically.