Algorithms

STL provides implementation of common algorithms that we can use. They are template based.The below example shows how we can use the built in linear search and binary search.

File: find1.cpp

#include <iostream>
#include <algorithm>

using namespace std ;



int main()
{

      vector<int> numbers = {10, 20, 30, 40, 50};
      //Linear search example

      // Value to search for
      int searchValue1 = 30;
      int searchValue2 = 60;

      // Use find to search for searchValue1
      vector<int>::iterator it1 = find(numbers.begin(), numbers.end(), searchValue1);

      // Check if the element was found
      if ( it1 != numbers.end())
      {
          cout << "Element " << searchValue1 << " found at position: "
                    << distance(numbers.begin(), it1) << endl;
      }
      else
      {
          cout << "Element " << searchValue1 << " not found." << endl;
      }

      it1 = find(numbers.begin(), numbers.end(), searchValue2 );

      // Check if the element was found
      if ( it1 != numbers.end())
      {
          cout << "Element " << searchValue2 << " found at position: "
                    << distance(numbers.begin(), it1) << endl;
      }
      else
      {
          cout << "Element " << searchValue2 << " not found." << endl;
      }

     if (binary_search(numbers.begin(), numbers.end(), searchValue1))
     {
        cout << "Binary search: " << searchValue1 << " found in the vector." << endl;
     } else {
        cout << "Binary search: " << searchValue1 << " not found in the vector." << endl;
      }

    return 0 ;
}

Exercise
The below program does not compile. Fix the code.

File: find_ex1.cpp

Functor

A functor is a class that has the overloaded operator"()" . It can be used as a function. Functors are used in STL algorithms that expect either a function or a Functor. A functor can retain state and because it is a class it can be passed as a template parameter.

File: functor1.cpp

// C++ program to illustrate the begin and end iterator
#include <iostream>
#include <map>
#include <string>
using namespace std;

class functor1
{
   public:
     void operator()( int& x1 )
       {
           cout << "Inside operator() : " << x1 << endl ;
           x1++ ;
       }

};

void function1( int& x1 )
  {
           cout << "Inside function1() : " << x1 << endl ;
           x1++ ;
  }


int main()
{
    functor1 functor1Object ;
    int y1 = 10 ;
    functor1Object( y1 ) ;
    cout << "y1: " << y1 << endl ;

    function1( y1 ) ;
    cout << "y1: " << y1 << endl ;

    return 0;
}

Sort

File: sort1.cpp

#include <iostream>     // cout
#include <algorithm>    // sort
#include <vector>       // vector

using namespace std;

bool myfunction (int i,int j)
{
    return (i<j);
}

class myclass
{
   public:
     bool operator() (int i,int j) { return (i<j);}
} ;

int main ()
{
  int myints[] = {32,71,12,45,26,80,53,33};
  vector<int> myvector (myints, myints+8);
  myclass  myobject ;

  // using default comparison (operator <):
  sort (myvector.begin(), myvector.begin()+4);
  //sort the first 4 elements only
  //(12 32 45 71)26 80 53 33

  cout << "myvector contains:";
  for (vector<int>::iterator it=myvector.begin(); it!=myvector.end(); ++it)
    cout << ' ' << *it;
  cout << '\n';


  // using function as comp
  sort (myvector.begin()+4, myvector.end(), myfunction);
   // 12 32 45 71(26 33 53 80)
  cout << "myvector contains:";
  for (vector<int>::iterator it=myvector.begin(); it!=myvector.end(); ++it)
    cout << ' ' << *it;
  cout << '\n';

  // using object as comp
  sort (myvector.begin(), myvector.end(), myobject);     //(12 26 32 33 45 53 71 80)


  // print out content:
  cout << "myvector contains:";
  for (vector<int>::iterator it=myvector.begin(); it!=myvector.end(); ++it)
    cout << ' ' << *it;
  cout << '\n';

  cout << "myobject:" << myobject( 12, 5 ) << endl ;
  return 0;
}

$ g++ sort1.cpp ; ./a.exe
myvector contains: 12 32 45 71 26 80 53 33
myvector contains: 12 32 45 71 26 33 53 80
myvector contains: 12 26 32 33 45 53 71 80
myobject:0

Complete the TODO parts in the following exercise. A student is less than another student if a student's id is less than the other student's id.

File: sort_ex1.cpp

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

using namespace std ;

class student
{
    public:
     int id  ;
     student( )
      {
          id=0 ;
      }
     student( int idP )
      {
          id = idP ;
      }
     student( const student& obj1 )
      {
          id = obj1.id ;
      }

     ~student(  )
      {
      }

};

bool studentCompareFunction (student obj1, student obj2  )
{
    //TO DO Complete the code


}

class studentCompareClass
{
   public:
     bool ascend = true ;
     bool operator() (student obj1, student obj2  )
     {
         //TO DO Complete the code
         //If ascend is true the sorted order is ascending
         //else it is descending.

     } ;



int main()
{
    student  studentObject1(1)  ;
    student  studentObject2(2)  ;
    student  studentObject3(3)  ;
    vector< student >   vec ;


    vec.push_back( studentObject1 ) ;
    vec.push_back( studentObject3 ) ;
    vec.push_back( studentObject2 ) ;
    studentCompareClass studentCompareClassObj1 ;
    studentCompareClassObj1.ascend = false ;
    sort( vec.begin() , vec.end() ,  studentCompareClassObj1  ) ;
    for( student x1 : vec )
      cout << x1.id << " " ;
    cout << endl ;

    vec.clear() ;
    vec.push_back( studentObject1 ) ;
    vec.push_back( studentObject3 ) ;
    vec.push_back( studentObject2 ) ;
    sort( vec.begin() , vec.end() ,  studentCompareFunction  ) ;
    for( student x1 : vec )
      cout << x1.id << " " ;
    cout << endl ;



    return 0 ;
}
$ g++ sort_ex1s.cpp ; ./a.exe
3 2 1
1 2 3

The "for_each" function allows us to call a function for a range of elements in a container.

For Each

File: for1.cpp

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

using namespace std ;

// A regular function to be used with for_each
void print_element(int n)
{
    cout << n << " ";
}

int main()
{
    vector<int> numbers = {1, 2, 3, 4, 5};

    // Using for_each with a function pointer
    for_each( numbers.begin(), numbers.end(), print_element );
    cout << endl;

    return 0;
}

Exercise

File: for_ex1.cpp

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

using namespace std ;

// A regular function to be used with for_each
//Change the below function to use templates
void print_element(int n)
{
    cout << n << " ";
}



int main()
{
     vector<int> numbers = {1, 2, 3, 4, 5};

      // Using for_each with a function pointer
      for_each(numbers.begin(), numbers.end(), print_element<int>  );
      cout << endl;

      vector<string> strings = { "One", "Two", "Three"  };


      for_each(strings.begin(), strings.end(), print_element<string>  );
      cout << endl;

    return 0;
}

Transform applies a function to a range of elements in a container and

Transform

File: transform1.cpp

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

using namespace std ;
// Free function to double an integer
int double_value(int i)
{
    return i * 2;
}

int main() {
    vector<int> input = {1, 2, 3, 4, 5};
    vector<int> output(input.size());

    // Apply double_value to each element of input and store in output
    transform(input.begin(), input.end(), output.begin(), double_value);

    for (int val : output) {
        cout << val << " "; // Output: 2 4 6 8 10
    }
    cout << endl;

    return 0;
}

Exercise

1)

File: algo_ex1.cpp

Solutions

File: vector_ex1_s.cpp


File: sort_ex1s.cpp


File: for_ex1s.cpp


File: algo_ex1_s.cpp