C++ Algorithm stable_sort()

C++ Algorithm stable_sort() function is used to sort the elements in the range [first, last) into ascending order like sort but keeps the order of equivalent elements.

The elements are compared using operator < for the first version, and comp for the second version.

Syntax

template <class RandomAccessIterator>
  void stable_sort ( RandomAccessIterator first, RandomAccessIterator last );

template <class RandomAccessIterator, class Compare>
  void stable_sort ( RandomAccessIterator first, RandomAccessIterator last,
                     Compare comp );

Parameter

first: A bidirectional iterator pointing to the first element in the range to be sorted.

last: A bidirectional iterator pointing to the past last element in the range to be sorted.

comp: A user defined binary predicate function that accepts two arguments and returns true if the two arguments are in order and false otherwise. It follows the strict weak ordering to order the elements.

Return value

None

Complexity

The run time complexity depends on the amount of available memory.

If enough extra memory is available, then the complexity is linear in the distance between first and last. Performs up to N*log₂ (N) element comparisons where N = last - first.

If no extra memory is available then the complexity is polylinear in the distance between first and last. Performs up to N*log₂² (N) element comparisons where N = last - first.

Data races

The object in the range [first, last) are modified.

Exceptions

This function throws an exception if any of element comparisons, the element swaps or an operation on iterator throws an exception.

Please note that invalid parameters cause an undefined behavior.

Example 1

Let's see the simple example to demonstrate the use of stable_sort():

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

using namespace std;

int main()
{
  vector<int> v = {3, 1, 4, 2, 5};
  
    cout<<"Before sorting: ";
    for_each(v.begin(), v.end(), [](int x) {
    cout << x << " ";
  });

  stable_sort(v.begin(), v.end());
  
  cout<<"\nAfter sorting:  ";
  for_each(v.begin(), v.end(), [](int x) {
    cout << x << " ";
  });
  
  return 0;
}

Output:

Before sorting: 3 1 4 2 5 
After sorting:  1 2 3 4 5

Example 2

Let's see another simple example:

#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
 
using namespace std;
 
struct Employee {
    Employee(int age, string name) : age(age), name(name) { } 
    int age;
    string name;  // Does not particpate in comparisons
};
 
bool operator<(const Employee &lhs, const Employee &rhs) {
    return lhs.age < rhs.age;
}
 
int main()
{
    vector<Employee> v = { 
        Employee(58, "Robin"),
        Employee(23, "Bob"),
        Employee(60, "Devid"),
    };  
 
    stable_sort(v.begin(), v.end());
    
    cout<<"Age : Name "<<endl<<"-----------\n";
    for (const Employee &e : v) {
        cout << e.age << " : " << e.name << '\n';
    }
    
    return 0;
}

Output:

Age : Name 
-----------
23 : Bob
58 : Robin
60 : Devid

Example 3

Let's see another simple example:

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

using namespace std;
 
struct Student {
    string name;
    int sec;
    char group;
};
 
bool compBySec(Student a, Student b)
{
    return a.sec < b.sec;
}
 
bool compByGroup(Student a, Student b)
{
    return a.group < b.group;
}
 
bool compByName(Student a, Student b)
{
    return a.name < b.name;
}
 
void print(const vector <Student>& v)
{
    cout << "Name  \tSec\tGroup" << "\n-------------------------"<<endl;
    for (unsigned int i = 0; i < v.size(); i++)
    {   
        cout << v[i].name << "\t" << v[i].sec<< "\t"
                  << v[i].group << endl;
    }
}
 
int main()
{
    vector <Student> Students;
    string name[] = {"Anjali", "Bob", "Chinu  ", "Faizal ",
                          "Nikita ", "Deep ", "Aman", "Rohit "};
    int sec[] = {3, 4, 3, 3, 1, 4, 3, 2};
    int group[] = {'A', 'C', 'A', 'A', 'A', 'B', 'B', 'A'};
 
    for (int i = 0; i < 8; i++)
    {
        Student p;   
        p.name =  name[i];
        p.sec = sec[i];
        p.group = group[i];
        Students.push_back(p);
    }
    stable_sort(Students.begin(), Students.end(), compByName);
    cout << "Stable Sort by name" << endl;
    print(Students);
    cout << endl;
    stable_sort(Students.begin(), Students.end(), compBySec);
    cout << "Stable Sort by section" << endl;
    print(Students);
    
    return 0;
}

Output:

Stable Sort by name
Name  	Sec	Group
-------------------------
Aman	3	B
Anjali	3	A
Bob	4	C
Chinu  	3	A
Deep 	4	B
Faizal 	3	A
Nikita 	1	A
Rohit 	2	A

Stable Sort by section
Name  	Sec	Group
-------------------------
Nikita 	1	A
Rohit 	2	A
Aman	3	B
Anjali	3	A
Chinu  	3	A
Faizal 	3	A
Bob	4	C
Deep 	4	B

Example 4

Let's see another simple example:

#include <vector>  
#include <algorithm>  
#include <functional>      // For greater<int>( )  
#include <iostream>  
  
// Return whether first element is greater than the second  
bool UDgreater (int elem1, int elem2 )  
{  
   return elem1 > elem2;  
}  
  
int main( )  
{  
   using namespace std;  
   vector <int> v1;  
   vector <int>::iterator Iter1;  
  
   int i;  
   for ( i = 0 ; i <= 5 ; i++ )  
   {  
      v1.push_back( 2 * i );  
   }  
  
   for ( i = 0 ; i <= 5 ; i++ )  
   {  
      v1.push_back( 2 * i  );  
   }  
  
   cout << "Original vector v1 = ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")" << endl;  
  
   stable_sort(v1.begin( ), v1.end( ) );  
   cout << "Sorted vector v1 = ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")" << endl;  
  
   // To sort in descending order, specify binary predicate  
   stable_sort(v1.begin( ), v1.end( ), greater<int>( ) );  
   cout << "Resorted (greater) vector v1 = ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")" << endl;  
  
   // A user-defined (UD) binary predicate can also be used  
   stable_sort(v1.begin( ), v1.end( ), UDgreater );  
   cout << "Resorted (UDgreater) vector v1 = ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")" << endl;  
   
   return 0;
}

Output:

Original vector v1 = ( 0 2 4 6 8 10 0 2 4 6 8 10 )
Sorted vector v1 = ( 0 0 2 2 4 4 6 6 8 8 10 10 )
Resorted (greater) vector v1 = ( 10 10 8 8 6 6 4 4 2 2 0 0 )
Resorted (UDgreater) vector v1 = ( 10 10 8 8 6 6 4 4 2 2 0 0 )

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