code: private String[] readLine() throws IOException {
          // Read the next number of non zeros.
          String line = reader.readLine();
          if (line == null)
              throw new IllegalArgumentException(
                      "The matrix file is improperly formatted");
          return line.split("\\s+");
      }
docs: Reads a line and splits it into [col value] pairs.
  
  @throws IllegalArgumentException If there are no more lines to read.