C++ sort

https://www.geeksforgeeks.org/sorting-2d-vector-in-c-set-1-by-row-and-column/

// C++ code to demonstrate sorting of a 
// 2D vector on basis of a column 
#include<iostream> 
#include<vector> // for 2D vector 
#include<algorithm> // for sort() 
using namespace std;
// Driver function to sort the 2D vector 
// on basis of a particular column 
bool sortcol( const vector<int>& v1, 
   const vector<int>& v2 ) { 
return v1[1] < v2[1]; 
}
int main() 
{ 
 // Initializing 2D vector "vect" with 
 // values 
 vector< vector<int> > vect{{8, 5, 1}, 
        {4, 8, 6}, 
        {7, 2, 9}};
// Number of rows; 
 int m = vect.size();
// Number of columns (Assuming all rows 
 // are of same size). We can have different 
 // sizes though (like Java). 
 int n = vect[0].size(); 
 
 // Displaying the 2D vector before sorting 
 cout << "The Matrix before sorting is:\n"; 
 for (int i=0; i<m; i++) 
 { 
  for (int j=0; j<n ;j++) 
   cout << vect[i][j] << " "; 
  cout << endl; 
 }
// Use of "sort()" for sorting on basis 
 // of 2nd column 
 sort(vect.begin(), vect.end(),sortcol);
// Displaying the 2D vector after sorting 
 cout << "The Matrix after sorting is:\n"; 
 for (int i=0; i<m; i++) 
 { 
  for (int j=0; j<n ;j++) 
   cout << vect[i][j] << " "; 
  cout << endl; 
 } 
 return 0; 
}

Output

The Matrix before sorting is:
8 5 1 
4 8 6 
7 2 9 
The Matrix after sorting is:
7 2 9 
8 5 1 
4 8 6

In the default mode, if we don’t customize the compare function, the first column will be sorted.

The Matrix before sorting is:
8 5 1 
4 8 6 
7 2 9 
The Matrix after sorting is:
4 8 6 
7 2 9 
8 5 1

We can also write a lambda expression

// C++ code to demonstrate sorting of a 
// 2D vector on basis of a column 
#include<iostream> 
#include<vector> // for 2D vector 
#include<algorithm> // for sort() 
using namespace std;
// Driver function to sort the 2D vector 
// on basis of a particular column 
bool sortcol( const vector<int>& v1, 
   const vector<int>& v2 ) { 
return v1[1] < v2[1]; 
}
int main() 
{ 
 // Initializing 2D vector "vect" with 
 // values 
 vector< vector<int> > vect{{8, 5, 1}, 
        {4, 8, 6}, 
        {7, 2, 9}};
// Number of rows; 
 int m = vect.size();
// Number of columns (Assuming all rows 
 // are of same size). We can have different 
 // sizes though (like Java). 
 int n = vect[0].size(); 
 
 // Displaying the 2D vector before sorting 
 cout << "The Matrix before sorting is:\n"; 
 for (int i=0; i<m; i++) 
 { 
  for (int j=0; j<n ;j++) 
   cout << vect[i][j] << " "; 
  cout << endl; 
 }
// Use of "sort()" for sorting on basis 
 // of 2nd column 
 sort(vect.begin(), vect.end(),[](const vector<int>&v1, const vector<int>&v2){
     return v1[1]<v2[1];
 });
// Displaying the 2D vector after sorting 
 cout << "The Matrix after sorting is:\n"; 
 for (int i=0; i<m; i++) 
 { 
  for (int j=0; j<n ;j++) 
   cout << vect[i][j] << " "; 
  cout << endl; 
 } 
 return 0; 
}