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C++ algorithm remove_if() function

C++ algorithm remove_if() 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 remove_if()

C++ Algorithm remove_if() function is used to eliminate all the elements that satisfy a predicate from a given range [first, last) without disturbing the order of the remaining elements.

  • This function cannot alter the size of the container.
  • It returns an iterator to the new end of the range.
  • Remove is stable, means that the relative order of the elements that are not removed is remain unchanged.

Syntax

template <class ForwardIterator, class UnaryPredicate>
  ForwardIterator remove_if (ForwardIterator first, ForwardIterator last,
                             UnaryPredicate pred);

Parameter

first: A forward iterator pointing the position of the first element in the range from which elements are being removed.

last: A forward iterator pointing the position one past the final element in the range from which elements are being removed.

pred: The unary predicate function which accepts an element as an argument that must be satisfied is the value of an element is to be replaced.

Return value

A forward iterator pointing the new end position (last) of the modified range or first element if first and last is equal.

Complexity

Complexity is linear in the range [first, last): Applies pred to each element, and possibly performs assignments 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 assignments or the operation on an 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 remove_if():

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

using namespace std;

bool IsOdd(int i) 
{ 
    return ((i % 2) == 1); 
} 
  
int main ()  
{ 
    vector <int> vec2 { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; 
    
    //store the position of last element 
    vector <int>::iterator pend; 
        
    // Print original vector 
    cout << "\nOriginal vector : "; 
    for(int i=0; i < vec2.size(); i++) 
        cout << " " << vec2[i]; 
    cout << "\n"; 
  
    // remove_if function call 
    pend = remove_if (vec2.begin(), vec2.end() , IsOdd); 
      
    // Print the vector 
    cout << "After remove_if : "; 
    for ( vector<int> :: iterator q=vec2.begin(); q != pend; ++q) 
        cout << ' ' << *q; 
    cout << '\n'; 
  
return 0; 
}

Output:

Original vector :  1 2 3 4 5 6 7 8 9 10
After remove_if : 2 4 6 8 10

Example 2

Let's see another simple example:

#include <vector>  
#include <algorithm>  
#include <iostream>  
using namespace std;  
  
bool greater6 ( int value ) {  
   return value >6;  
}  
  
int main( ) {  

   vector <int> v1, v2;  
   vector <int>::iterator Iter1, Iter2, new_end;  
  
   int i;  
   for ( i = 0 ; i <= 9 ; i++ )  
      v1.push_back( i );  
  
   int ii;  
   for ( ii = 0 ; ii <= 3 ; ii++ )  
      v1.push_back( 7 );  
  
   random_shuffle ( v1.begin( ), v1.end( ) );  
   cout << "Vector v1 is ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")." << endl;  
  
   // Remove elements satisfying predicate greater6  
   new_end = remove_if (v1.begin( ), v1.end( ), greater6 );  
  
   cout << "Vector v1 with elements satisfying greater6 removed is\n ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")." << endl;  
  
   // To change the sequence size, use erase  
   v1.erase (new_end, v1.end( ) );  
  
   cout << "Vector v1 resized elements satisfying greater6 removed is\n ( " ;  
   for ( Iter1 = v1.begin( ) ; Iter1 != v1.end( ) ; Iter1++ )  
      cout << *Iter1 << " ";  
   cout << ")." << endl;  
   
   return 0;
}

Output:

 Vector v1 is ( 4 7 7 7 0 5 7 1 6 9 3 7 8 2 ).
Vector v1 with elements satisfying greater6 removed is
 ( 4 0 5 1 6 3 2 1 6 9 3 7 8 2 ).
Vector v1 resized elements satisfying greater6 removed is
 ( 4 0 5 1 6 3 2 ).

Example 3

Let's see another simple example:

#include <iostream>
#include <functional>
#include <vector>
#include <algorithm>
 
typedef std::vector<std::string>::iterator iterator;
 
struct startsWithA : public std::unary_function<std::string, bool> {
    bool operator() (std::string s)
    {
        if(s[0] == 'A')
        {
            return true;
        }
        else
            return false;
    }
};
 
void print(iterator b, iterator e)
{
    iterator i;
    for(i = b; i != e; i++)
    {
        std::cout << *i << "    ";
    }
    std::cout << std::endl;
}
 
int main()
{
    startsWithA s;
    std::vector<std::string> v;
    v.push_back("China");
    v.push_back("India");
    v.push_back("Korea");
    v.push_back("America");
    v.push_back("Australia");
    v.push_back("Pakistan");
 
    std::cout << "Vector : ";
    print(v.begin(), v.end());
    iterator i = remove_if(v.begin(), v.end(), s);
    std::cout << "Vector : ";
    print(v.begin(), i);
    
    return 0;
}

Output:

Vector : China    India    Korea    America    Australia    Pakistan    
Vector : China    India    Korea    Pakistan

Example 4

Let's see another simple example to remove all spaces from a string by shifting all non-space characters to the left and then erasing the extra space.

#include <algorithm>
#include <string>
#include <iostream>
#include <cctype>
 
using namespace std; 
 
int main()
{
    string str1 = "Text with some   spaces";
    str1.erase(remove(str1.begin(), str1.end(), ' '),
               str1.end());
    cout << str1 << '\n';
 
    string str2 = "Text\n with\tsome \t  whitespaces\n\n";
    str2.erase(remove_if(str2.begin(), 
                              str2.end(),
                              [](unsigned char x){return std::isspace(x);}),
               str2.end());
    cout << str2 << '\n';
    
    return 0;
}

Output:

Textwithsomespaces
Textwithsomewhitespaces

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