C++ Sorting Multiple Dimensional Array by 2 Columns

Question

I have a 3D array where I keep 3 columns of numeric data. I want to sort this Nx3 matrix in ascending order, by first column, then for the cases where the values are the same on this column, I want them to be sorted according to second column.

For example: If there are multiple of min elements in column 1, I want so sort them furthermore by second column, again in ascending order. So it would look like this:

[1, 2, 3]
[1, 3, 1]
[2, 4, 2]
[3, 5, 6]

etc.

Since there were multiple of min element "1" in column 1, furthermore sorting is carried out for these min values of column 1, according to column 2 values.

I know this was possible in Pandas of Python but is there any convenient way to do this in C++?

Write a custom comparator that will do what you need. See, for example, this question. — 1201ProgramAlarm
– 1201ProgramAlarm, Commented Dec 7, 2019 at 18:30
Is this about a 2 dimensional array (where the first dimension happens to be of size 3) or am I missing something? — Peter - Reinstate Monica
– Peter - Reinstate Monica, Commented Dec 7, 2019 at 18:47

prog-fh · Accepted Answer · 2019-12-07 18:41:50Z

I would try the standard algorithm std::sort() with a comparator which is specific to your need.
( https://en.cppreference.com/w/cpp/algorithm/sort )

/*
  g++ -std=c++17 -o prog prog.cpp \
      -pedantic -Wall -Wextra -Wconversion \
      -g -O0 -fsanitize=address,undefined
*/

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

struct Vec3
{
  int x, y, z;
};

int
main()
{
  auto v=std::vector<Vec3>{{3, 5, 6},
                           {2, 4, 2},
                           {1, 3, 1},
                           {1, 2, 3}};
  const auto show=
    [&]()
    {
      for(const auto &e: v)
      {
        std::cout << e.x << ' ' << e.y << ' ' << e.z << '\n';
      }
    };
  show();
  std::sort(begin(v), end(v),
    [&](const auto &lhs, const auto &rhs)
    {
      if(lhs.x<rhs.x) return true;
      if(lhs.x>rhs.x) return false;
      if(lhs.y<rhs.y) return true;
      if(lhs.y>rhs.y) return false;
      return lhs.z<rhs.z;
    });
  std::cout << "~~~~~~~~~~~~~~~~\n";
  show();
  return 0;
}

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