gistfile1.txt

struct Row {
    float& operator[](int column) {
      return coeffs[column];
    }

    typedef std::tr1::unordered_map<int, float> RowData;
    RowData coeffs;
  };


  static float Dot(const VectorX &x, const Row &row) {
    float sum = 0.0f;
    for (Row::RowData::const_iterator it = row.coeffs.begin();
         it != row.coeffs.end(); ++it) {
      sum += x[it->first] * it->second;
    }
    return sum;
  }