code: public Map<String, Integer> buildIndex() {
          Map<String, Integer> index = new HashMap<String, Integer>();
          for (int i = 0; i < columns.length; ++i) {
              index.put(columns[i], i);
          }
          return index;
      }
docs: Builds a name-to-column index for quick access to data by columm names.
  
  @return a {@code Map} that maps column names to column positions starting from 0