The default for matrix is to have 1 column. To explicitly have 0 columns, you need to write
matrix(, nrow = 15, ncol = 0)
A better way would be to preallocate the entire matrix and then fill it in
mat <- matrix(, nrow = 15, ncol = n.columns)
for(column in 1:n.columns){
mat[, column] <- vector
}