Export matrix in r

You don't get a new column, the row names are saved as a first column in the text file. So either you specify the column where the row names are given in read.table, or use the row.names=FALSE option in write.table.

Demonstration :

mat <- matrix(1:10,ncol=2)
rownames(mat) <- letters[1:5]
colnames(mat) <- LETTERS[1:2]

mat
write.table(mat,file="test.txt") # keeps the rownames
read.table("test.txt",header=TRUE,row.names=1) # says first column are rownames
unlink("test.txt")
write.table(mat,file="test2.txt",row.names=FALSE) # drops the rownames
read.table("test.txt",header=TRUE) 
unlink("test2.txt")

I assume that by "new column" you mean the row names that get written out by default. To suppress them, set row.names = FALSE when calling write.table or write.csv.

write.table               package:utils                R Documentation

Data Output

Description:

     ‘write.table’ prints its required argument ‘x’ (after converting
     it to a data frame if it is not one nor a matrix) to a file or
     connection.

Usage:

     write.table(x, file = "", append = FALSE, quote = TRUE, sep = " ",
                 eol = "\n", na = "NA", dec = ".", row.names = TRUE,
                 col.names = TRUE, qmethod = c("escape", "double"))

     write.csv(...)
     write.csv2(...)

...

row.names: either a logical value indicating whether the row names of
          ‘x’ are to be written along with ‘x’, or a character vector
          of row names to be written.

col.names: either a logical value indicating whether the column names
          of ‘x’ are to be written along with ‘x’, or a character
          vector of column names to be written.  See the section on
          ‘CSV files’ for the meaning of ‘col.names = NA’.