std::string::remove_copy(), std::string::remove_copy_if() in C++

Difficulty Level : Easy
Last Updated : 29 May, 2018

remove_copy()

It is an STL function in c++ which is defined in algorithm library. It copies the elements in the range [first, last) to the range beginning at result, except those elements that compare equal to given elements.

The resulting range is shorter than [first,last) by as many elements as matches in the sequence, which are “removed”.
The relative order of the elements not removed is preserved.
The function uses operator == to compare the individual elements to given value.

Function Template

ResultIterator remove_copy(ForwardIterator first, ForwardIterator last, 
ResultIterator result ,const T& ele);

first, last :  Forward iterators to the initial and final positions
in a sequence. The range used is [first, last), which contains all the elements
between first and last, including the element pointed by first but not 
the element pointed by last.

result : Output iterator to the initial position of the range 
where the resulting sequence is stored. The pointed type shall support being 
assigned the value of an element in the range [first, last).

ele : element to be removed.

Examples:

Input : b d a f g h a k   given element is a
Output :b d f g h k _ _ 
Input : b k c s n m c l   given element is c
Output : b k s n m l _ _

'_' represent remove places

// CPP code to demonstrate remove_copy() 
#include <vector> 
#include <algorithm> 
#include <iostream> 
using namespace std; 
  
// Function to remove_copy from v1 result vector is v2 
void removecopyDemo(vector <int> &v1) 
{ 
    remove_copy(v1.begin(), v1.end(), v1.begin(), 3); 
} 
  
  
// Function to print content of vector 
void print(vector<int>&v) 
{ 
    int len = v.size(); 
    for (int i = 0; i < len; i++) 
        cout << v[i] << " "; 
    cout << endl; 
} 
  
// Driver code 
int main() 
{ 
    // vector  
    vector <int> v1, v2(10); 
      
    // push data in vector  
    for(int i = 10; i <= 25; i++) 
        v1.push_back(i % 6); 
      
    cout << "elements of v1 before remove_copy: "<<endl; 
    print(v1); 
  
   removecopyDemo(v1); 
      
   cout << "After removing element  3" <<endl; 
    print(v1); 
  
return 0; 
  
} 

Output:

elements of v1 before remove_copy: 
4 5 0 1 2 3 4 5 0 1 2 3 4 5 0 1 
After removing element 3
4 5 0 1 2 4 5 0 1 2 4 5 0 1 0 1

Complexity: Linear O(n)

remove_copy_if

It is a STL function in c++ which is defined in algorithm library. It copies the elements in the range [first, last) to the range beginning at result, except those elements that meets to a given condition (like odd number, even number, prime number, non-prime number etc ).

Copies the elements in the range [first, last) to the range beginning at result, except those elements for which condition function returns true.
The resulting range is shorter than [first, last) by as many elements as matches, which are “removed”.

Function Template

ResultIterator remove_copy_if(ForwardIterator first, ForwardIterator last,
                                 ResultIterator result, UnaryPredicate pred);
pred :  Unary function that accepts an element in the range as
argument, and returns a value convertible to bool. The value returned indicates
 whether the element is to be removed (if true, it is removed).

Examples:

Input : 1 2 3 4 5 6 7 8 9 check if a number is prime and remove
Output :1 4 6 8 9 0 0 0 0

Input :1 2 3 4 5 6 7 8 9   check if a number is even and remove
Output :1 3 5 7 9 0 0 0 0

// CPP code to demonstrate remove_copy_if() 
#include <vector> 
#include <algorithm> 
#include <iostream> 
using namespace std; 
  
bool IsOdd(int i) { return ((i % 2) != 0); } 
  
// Function to remove_copy from v1 result vector is v2 
void remove_copy_ifDemo(vector <int> &v1, vector<int> &v2) 
{ 
    remove_copy_if(v1.begin(), v1.end(), v2.begin(), IsOdd); 
} 
  
// Function to print content of vector 
void print(vector<int>&v) 
{ 
    int len = v.size(); 
    for (int i = 0; i < len; i++) 
        cout << v[i] << " "; 
    cout << endl; 
} 
  
// Driver code 
int main() 
{ 
    // declare vector v1, v2 
    vector <int> v1, v2(10); 
      
    // push data in vector  
    for(int i = 10; i <= 20; i++) 
        v1.push_back(i); 
      
    cout << "elements of v1 before remove_copy: "; 
    print(v1); 
  
remove_copy_ifDemo(v1,v2); 
      
    cout << "elements of v1 after remove_copy: "; 
    print(v1); 
      
cout << "After removing Odd Numbers from v1"
            " copy result in vector v2" <<endl; 
    print(v2); 
  
return 0; 
  
} 

Output:

elements of v1 before remove_copy: 10 11 12 13 14 15 16 17 18 19 20 
elements of v1 after remove_copy: 10 11 12 13 14 15 16 17 18 19 20 
After removing Odd Numbers from v1 copy result in vector v2
10 12 14 16 18 20 0 0 0 0

Complexity: Linear O(n)

List of <algorithms> library in C++ STL
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