Question

I am having trouble to figure out, how to sort a vector of vector of strings, here is the testing code.

#include <iostream>
#include <vector>
#include <boost/algorithm/string.hpp>

int main(int argc, char** argv) {
  std::vector <std::vector <std::string> > data_var;
  std::vector <std::string> temp;

  std::string str1 = "1,hello3,temp2";
  std::string str2 = "2,hello2,temp1";
  std::string str3 = "3,hello1,temp3";

  boost::split(temp, str1, boost::is_any_of(","));
  data_var.push_back(temp);
  boost::split(temp, str2, boost::is_any_of(","));
  data_var.push_back(temp);
  boost::split(temp, str3, boost::is_any_of(","));
  data_var.push_back(temp);

  // sorting code here...
}

Thanks in advance…

Answer 1

If you only want to sort based on the second column, then you just need to provide a custom comparison operator. Once way to do that is:

struct StringListCompare
{
  bool operator()(const vector<string>& lhs, const vector<string>& rhs)
  {
    // what do we do if lhs or rhs don't have two elements?
    if (lhs.size() < 2 || rhs.size() < 2)
    {
      // ?
    }
    else
    {
      return lhs[1] < rhs[1];
    }
  }
} StringListComparer;

int main()
{
  // ...
  sort(data_var.begin(), data_var.end(), StringListComparer);
}

Edit: If you won’t know until runtime which column you’ll be sorting on, you can encode that in the sorting object:

class StringListCompare
{
public:
  explicit StringListCompare(int column) : m_column(column) {}
  bool operator()(const vector<string>& lhs, const vector<string>& rhs)
  {
    // what do we do if lhs or rhs don't have (m_column + 1) elements?
    return lhs[m_column] < rhs[m_column];
  }
private:
  int m_column;
};

Notice how we’ve added a constructor that takes which column it’ll act on. You can use it like this:

  // We set it up so the columns are 0-based:
  StringListCompare compare_column_0(0), compare_column_1(1), compare_column_2(2);

  cout << "Original:\n" << data_var << endl;
  sort(data_var.begin(), data_var.end(), compare_column_2);
  cout << "Sorted on column 2:\n" << data_var << endl;
  sort(data_var.begin(), data_var.end(), compare_column_1);
  cout << "Sorted on column 1:\n" << data_var << endl;
  sort(data_var.begin(), data_var.end(), compare_column_0);
  cout << "Sorted on column 0:\n" << data_var << endl;

You don’t even need to make the local variable if you don’t want to:

  sort(data_var.begin(), data_var.end(), StringListCompare(2));
  cout << "Sorted on 2, no local sort variable:\n" << data_var << endl;

[Code at ideone]