Question

I have a set of points (x,y,z). I want to sort these data using first column and 2nd and 3rd columns should be rearranged according to the sorting 1st column. is it possible to do this in c++, if so could you pls help me.

Herewith I am attaching codes of my implementation but I got a error message “invalid conversion from ‘const’ vod*’ to ‘const int [*][3]’ “ in line 31 and 32. I tried this using several methods but my effort was not success yet. I used here ‘qsort’ for this, are there any other methods or can I use ‘sort’ in to do this. Since I have a very big data set I wish to use a fast method.
So what I need at the end the data set which is sorted using only 1st column like following example:
before sort

34  12  12
12  34 15
24  20  34
13  11  10
40  23  32

after sort

12  34 15
13  11  10
24  20  34
34  12  12
40  23  32

if any good methods help me to write codes …thanks

#include <iostream>
#include <cstdlib>
#include <vector>
#include <stdio.h>
#include <stdlib.h>

using namespace std;

class Point
{
   private:          
    double x;
    double y;
    double z;

   public:
     Point(){};
     ~Point(){};

   Point(double X, double Y, double Z){
     x=X;y=Y;z=Z; }

   double X(){return x;}
   double Y(){return y;}
   double Z(){return z;}
};

int cmp ( const void *pa, const void *pb ) {
const int (*a)[3] = pa;
const int (*b)[3] = pb;
if ( (*a)[1] < (*b)[1] ) return -1;
if ( (*a)[1] > (*b)[1] ) return +1;
return 0;
}

int main ( ) {
vector<Point> points;
int input_x,input_y,input_z;       
   int i=0;
   while(i<6){//data set,it is a example, actual data come from a file 

         cout<<"x: ";cin>>input_x;
         cout<<"y: ";cin>>input_y;
         cout<<"z: ";cin>>input_z;
         Point point(input_x,input_y,input_z);                        
         points.push_back(point);
        i++;                         
         }        
for (int i=0;i<points.size();i++){//before sort
cout<<points[i].X()<<" "<<points[i].Y()<<" "<<points[i].Z()<<endl;
}

qsort( points, 6, sizeof points[0], cmp );

for (int i=0;i<points.size();i++){//after sort
cout<<points[i].X()<<" "<<points[i].Y()<<" "<<points[i].Z()<<endl;
}            
system("PAUSE");
return 0;
}

Answer 1

It’s almost certainly easiest to use std::sort instead of qsort:

class Point { 
    int x, y, z;
public:
    Point(int x, int y, int z) : x(x), y(y), z(z) {}

    bool operator<(Point const &other) { 
        return x < other.x;
    }
    // skipping the reading and other stuff for now...
};

int main() { 
    std::vector<Point> points;
    // add some Points to `points` here.

    // sort using order defined in Point::operator<:
    std::sort(points.begin(), points.end());
    return 0;
}

Edit: to keep the comparison separate from the items being compared, you use a separate function or functor to do the comparison, and pass that to std::sort. There are a few things about your class that you really want to change in any case though — at the very least, since your Point::X(), Point::Y() and Point::Z() don’t modify the Point object, you want to make them const member functions. Once you’ve done that, the sorting is fairly trivial:

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

class Point { 
    double x, y, z;
public:
    double X() const { return x; }
    double Y() const { return y; }
    double Z() const { return z; }

    Point(double x=0.0, double y=0.0, double z=0.0) : x(x), y(y), z(z) {}
};

namespace std { 
ostream &operator<<(ostream &os, Point const &p) { 
    return os << "(" << p.X() << ", " << p.Y() << ", " << p.Z() << ")";
}
}
struct byX { 
    bool operator()(Point const &a, Point const &b) { 
        return a.X() < b.X();
    }
};

int main(){ 
    std::vector<Point> points;

    for (int i=0; i<10; i++)
        points.push_back(Point(rand(), i, i));

    std::cout << "Unsorted:\n";
    std::copy(points.begin(), points.end(), 
        std::ostream_iterator<Point>(std::cout, "\n"));

    std::sort(points.begin(), points.end(), byX());

    std::cout << "\nSorted:\n";
    std::copy(points.begin(), points.end(), 
        std::ostream_iterator<Point>(std::cout, "\n"));
    return 0;
}

Technically, I suppose I should add one more minor detail: if the x value in any of your points is a NaN, this won’t work correctly. A NaN isn’t equal to anything (not even itself) which violates the strict weak ordering required for std::sort.