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