access to iterator in lambda function of an algorithm cause me segmentation fault

I need to sort a table by column. My tables are represents by a single vector.

example :

col_name A B C

vector : 1 2 3 6 5 4 7 8 9

that give me the table :

A B C

1 6 7
2 5 8
3 4 9

After a sort on column B , I need to obtain :

A B C

3 4 9
2 5 8
1 6 7

my code :

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

int main()
{

  std::vector<std::string> vec = {"1","8","1","2","3","2","3",
                                  "5","5","2","5","6","5","6",
                                  "9","3","3","4","8","3","9"};

  int size = 7;
  int col_idx = 1;


  for(int i = 0; i<3;++i)
    {
      if(i==col_idx)
        continue;

      std::sort(vec.begin() + i*size, vec.begin() + (i+1)*size,
                [col_idx, size, i](std::string& s1,std::string& s2)
                       {
                         std::cout << s1 << " "
                                   << s2 << " "
                                   << *(&s1 +(col_idx - i)*size) << " "
                                   << *(&s2 +(col_idx - i)*size) << " "
                                   << (*(&s1 +(col_idx - i)*size) < *(&s2 +(col_idx - i)*size)) << std::endl;
                         return *(&s1 +(col_idx - i)*size) < *(&s2 +(col_idx - i)*size);
                       });
    }
  std::sort(vec.begin() + col_idx*size, vec.begin() + (col_idx+1)*size);
}

I have a segmentation fault error : only the first line appear from the cout.