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C++ algorithm unique() function

C++ algorithm unique() function with c++ tutorial for beginners and professionals with examples on adjacent_find(),any_of(), copy(), copy_if(), count(), count_if(), equal(), find(), find_end(), find_first_of(), find_if(), find_if_not(), for_each() etc.

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C++ Algorithm unique()

C++ Algorithm unique() function is used to transform a sequence in such a way that each duplicate consecutive element becomes a unique element.

The first version uses operator== to compare the elements and the second version uses the given binary predicate pred.

Syntax

equality (1)	template <class ForwardIterator>
                          ForwardIterator unique (ForwardIterator first, ForwardIterator last);

predicate (2)	template <class ForwardIterator, class BinaryPredicate>
                          ForwardIterator unique (ForwardIterator first, ForwardIterator last,
                          BinaryPredicate pred);

Parameter

first: A forward iterator pointing the position of the first element in the range to be scanned for duplicate removal.

last: A forward iterator pointing the position one past the final element in the range to be scanned for duplicate removal.

pred: A user-defined predicate function object that defines the condition to be satisfied if two elements in a range are to be taken as equivalent. A binary predicate returns two arguments and returns true when satisfied and false when not satisfied. 0

Return value

A forward iterator pointing to the new end of the range[first, last) that contains no consecutive duplicates.

Complexity

Complexity is linear in the range [first, last): compares each pair of consecutive elements, and performs assignment operation on some of them.

Data races

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

Exception safety

This function throws an exception if any of pred, the element comparisons, the element assignments or the operations on iterator throws an exception.

Note: The invalid parameters cause an undefined behavior.

Example 1

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

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

using namespace std;
 
int main() 
{
    // remove duplicate elements
    vector<int> v{1,2,3,1,2,3,3,4,5,4,5,6,7};
    sort(v.begin(), v.end()); 
    auto last = unique(v.begin(), v.end());
    // v now holds {1 2 3 4 5 6 7 x x x x x x}, where 'x' is indeterminate
    v.erase(last, v.end()); 
    for (int i : v)
      cout << i << " ";
    cout << "\n";
    
    return 0;
}

Output:

1 2 3 4 5 6 7

Example 2

Let's see another simple example:

#include <iostream>     // std::cout
#include <algorithm>    // std::unique, std::distance
#include <vector>       // std::vector

using namespace std;

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

int main () {
  int myints[] = {10,20,20,20,30,30,20,20,10};           // 10 20 20 20 30 30 20 20 10
  vector<int> myvector (myints,myints+9);

  // using default comparison:
  vector<int>::iterator it;
  it = unique (myvector.begin(), myvector.end());   // 10 20 30 20 10 ?  ?  ?  ?
                                                         //                ^

  myvector.resize( distance(myvector.begin(),it) ); // 10 20 30 20 10

  // using predicate comparison:
  unique (myvector.begin(), myvector.end(), myfunction);   // (no changes)

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

  return 0;
}

Output:

myvector contains: 10 20 30 20 10

Example 3

Let's see another simple example:

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

using namespace std;

void print(const char* tag, const vector<int>& v) {
  cout << tag << " : ";
  bool first = true;
  for (int x : v) {
    if (first) {
      first = false;
    }
    else {
      cout << ',';
    }
    cout << x;
  }
  cout << endl;
}

int main() {

  {
    vector<int> v = { 2,5,3,3,1,2,4,2,1,1,4,4,3,3,3 };

    decltype(v)::iterator result = unique(v.begin(), v.end());

    v.erase(result, v.end());

    print("unsorted unique", v);
  }

  {
      vector<int> v = { 2,5,3,3,1,2,4,2,1,1,4,4,3,3,3 };

    sort(v.begin(), v.end());
    decltype(v)::iterator result = unique(v.begin(), v.end());

    v.erase(result, v.end());

    print("sorted unique", v);
  }
}

Output:

unsorted unique : 2,5,3,1,2,4,2,1,4,3
sorted unique : 1,2,3,4,5

Example 4

Let's see another simple example:

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

using namespace std;

int main() 
{
    // remove duplicate elements (normal use)
    vector<int> v{1,2,3,1,2,3,3,4,5,4,5,6,7};
    sort(v.begin(), v.end()); // 1 1 2 2 3 3 3 4 4 5 5 6 7 
    auto last = unique(v.begin(), v.end());
    // v now holds {1 2 3 4 5 6 7 x x x x x x}, where 'x' is indeterminate
    v.erase(last, v.end()); 
    for (int i : v)
      cout << i << " ";
    cout << "\n";
 
    // remove consecutive spaces
    string s = "wanna go    to      space?";
    auto end = unique(s.begin(), s.end(), [](char l, char r){
        return isspace(l) && isspace(r) && l == r;
    });
    // s now holds "wanna go to space?xxxxxxxx", where 'x' is indeterminate
    cout << string(s.begin(), end) << '\n';
}

Output:

1 2 3 4 5 6 7 
wanna go to space?

Example 5

Let's see another example:

#include <vector>  
#include <algorithm>  
#include <functional>  
#include <iostream>  
#include <ostream>  
  
using namespace std;  
  
// Return whether modulus of elem1 is equal to modulus of elem2  
bool mod_equal ( int elem1, int elem2 )  
{  
   if ( elem1 < 0 )   
      elem1 = - elem1;  
   if ( elem2 < 0 )   
      elem2 = - elem2;  
   return elem1 == elem2;  
};  
  
int main( )  
{  
   vector <int> v1;  
   vector <int>::iterator v1_Iter1, v1_Iter2, v1_Iter3,  
         v1_NewEnd1, v1_NewEnd2, v1_NewEnd3;  
  
   int i;  
   for ( i = 0 ; i <= 3 ; i++ )  
   {  
      v1.push_back( 5 );  
      v1.push_back( -5 );  
   }  
  
   int ii;  
   for ( ii = 0 ; ii <= 3 ; ii++ )  
   {  
      v1.push_back( 4 );  
   }  
   v1.push_back( 7 );  
  
   cout << "Vector v1 is ( " ;  
   for ( v1_Iter1 = v1.begin( ) ; v1_Iter1 != v1.end( ) ; v1_Iter1++ )  
      cout << *v1_Iter1 << " ";  
   cout << ")." << endl;  
  
   // Remove consecutive duplicates  
   v1_NewEnd1 = unique ( v1.begin ( ) , v1.end ( ) );  
  
   cout << "Removing adjacent duplicates from vector v1 gives\n ( " ;  
   for ( v1_Iter1 = v1.begin( ) ; v1_Iter1 != v1_NewEnd1 ; v1_Iter1++ )  
      cout << *v1_Iter1 << " ";  
   cout << ")." << endl;  
  
   // Remove consecutive duplicates under the binary prediate mod_equals  
   v1_NewEnd2 = unique ( v1.begin ( ) , v1_NewEnd1 , mod_equal );  
  
   cout << "Removing adjacent duplicates from vector v1 under the\n "  
        << " binary predicate mod_equal gives\n ( " ;  
   for ( v1_Iter2 = v1.begin( ) ; v1_Iter2 != v1_NewEnd2 ; v1_Iter2++ )  
      cout << *v1_Iter2 << " ";  
   cout << ")." << endl;  
  
   // Remove elements if preceded by an element that was greater  
   v1_NewEnd3 = unique ( v1.begin ( ) , v1_NewEnd2, greater<int>( ) );  
  
   cout << "Removing adjacent elements satisfying the binary\n "  
        << " predicate mod_equal from vector v1 gives ( " ;  
   for ( v1_Iter3 = v1.begin( ) ; v1_Iter3 != v1_NewEnd3 ; v1_Iter3++ )  
      cout << *v1_Iter3 << " ";  
   cout << ")." << endl;  
   
   return 0;
}

Output:

Vector v1 is ( 5 -5 5 -5 5 -5 5 -5 4 4 4 4 7 ).
Removing adjacent duplicates from vector v1 gives
 ( 5 -5 5 -5 5 -5 5 -5 4 7 ).
Removing adjacent duplicates from vector v1 under the
  binary predicate mod_equal gives
 ( 5 4 7 ).
Removing adjacent elements satisfying the binary
  predicate mod_equal from vector v1 gives ( 5 7 ).

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