C++ 2D Array Sorting

Question

I am trying to sort the elements in 2d array like this:

338 640 723.771 62.1603

364 804 882.56 65.642

199 664 693.179 73.3166

I need to sort them regards to 3rd and 4th columns.

For instance for the 3rd column:

199 664 693.179 73.3166

338 640 723.771 62.1603

364 804 882.56 65.642

For the 4th column:

338 640 723.771 62.1603

364 804 882.56 65.642

199 664 693.179 73.3166

I hope I could explain what I want to do. Thanks for help already..

Answer:

I've found what need it. I am putting the codes here maybe it could be helpful for someone else.

this is compare function for columns:

bool Compare(vector<double> A, vector<double> B) {
    return (A[2] < B[2]); // 2 means which column that you want to compare

}

and this is the sorting code:

std::sort(dots.begin(), dots.end(), &Compare); // "dots" needs to be a vector. in this case its a 2d double vector

the source is : https://stackoverflow.com/a/37516971/5331586

It seems a multivariable objective function. How do you handle the following case: r1 and r2 are the two row of a matrix; r1[2] < r2[2] but r2[3] < r2[3]? (Indices starts from 0) — BiagioF
– BiagioF, Commented Apr 21, 2017 at 16:16

marc_s · Accepted Answer · 2017-05-05 21:30:33Z

-1

Use std::sort with your own comparator. I would solve it like this:

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

using std::cout;
using std::cin;
using std::endl;

class Comparate2DArrayByColumn {
public:
    Comparate2DArrayByColumn(int column)
        : column(column)
    {
    }
    template <typename T>
    bool operator()(const std::vector<T>& array1, const std::vector<T>& array2)
    {
        // do not use [] here, it will be UB
        return array1.at(column) > array2.at(column);
    }

private:
    int column;
};

void printArray(const std::vector<std::vector<double> >& array)
{
    for (auto& line : array) {
        for (auto val : line) {
            cout << val << " ";
        }
        cout << endl;
    }
}

int main()
{
    std::vector<std::vector<double> > array = {
        { 33, 640, 723.771, 62.1603 },
        { 364, 804, 882.56, 65.642 },
        { 199, 664, 693.179, 73.3166 },
    };

    printArray(array);
    cout << endl
         << endl;

    std::sort(array.begin(), array.end(), Comparate2DArrayByColumn(2));

    printArray(array);

    return 0;
}

edited May 5, 2017 at 21:30

marc_s

760k186 gold badges1.4k silver badges1.5k bronze badges

answered Apr 21, 2017 at 16:02

devalone

5856 silver badges23 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

NathanOliver Over a year ago

Well, it wasn't really useful and should have been a comment until you added an example.

NathanOliver Over a year ago

Also Comparate2DArrayByCollumn should not be templated and instead operator() should be templated. That way you don't need to do Comparate2DArrayByCollumn<double>(2) but can instead use Comparate2DArrayByCollumn(2)

BiagioF Over a year ago

That takes into account just the third column, what about the fourth?

devalone Over a year ago

@biagio-festa use std::sort(array.begin(), array.end(), Comparate2DArrayByCollumn<double>(3);

BiagioF Over a year ago

@devalone then the sorting respect to (2) will be lost :D

|

Collectives™ on Stack Overflow

C++ 2D Array Sorting

1 Answer 1

6 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related