code: @Override
    public int[] columns(String[] names) {
      int[] idxs = new int[_strs.length];
      for (int i = 0; i < _strs.length; i++) {
        int idx = idxs[i] = water.util.ArrayUtils.find(names, _strs[i]);
        if (idx == -1) throw new IllegalArgumentException("Column " + _strs[i] + " not found");
      }
      return idxs;
    }
docs: Select columns by number or String.