Standard C++ Library
Copyright 1998, Rogue Wave Software, Inc.
NAME
remove_if
- Moves desired elements to the front of a container, and
returns an iterator that describes where the sequence of
desired elements ends.
SYNOPSIS
#include <algorithm>
template <class ForwardIterator, class Predicate>
ForwardIterator remove_if (ForwardIterator first,
ForwardIterator last,
Predicate pred);
DESCRIPTION
The remove_if algorithm eliminates all the elements referred
to by iterator i in the range [first, last) for which the
following corresponding condition holds: pred(*i) == true.
remove_if returns an iterator that points to the end of the
resulting range. remove_if is stable, which means that the
relative order of the elements that are not removed is the
same as their relative order in the original range.
remove_if does not actually reduce the size of the sequence.
It actually: 1) copies the values that are to be retained to
the front of the sequence, and 2) returns an iterator that
describes where the sequence of retained values ends. Ele-
ments that follow this iterator are simply the original
sequence values, left unchanged. Here's a simple example:
Say we want to remove all even numbers from the following
sequence:
123456789
Applying the remove_if algorithm results in the following
sequence:
13579|XXXX
The vertical bar represents the position of the iterator
returned by remove_if. Note that the elements to the left of
the vertical bar are the original sequence with the even
numbers removed. The elements to the right of the bar are
simply the untouched original members of the original
sequence.
If you want to actually delete items from the container, use
the following technique:
container.erase(remove(first,last,value),container.end());
COMPLEXITY
Exactly last1 - first1 applications of the corresponding
predicate are done.
EXAMPLE
//
// remove.cpp
//
#include <algorithm>
#include <vector>
#include <iterator>
#include <iostream>
using namespace std;
template<class Arg>
struct all_true : public unary_function<Arg, bool>
{
bool operator()(const Arg& x){ return 1; }
};
int main ()
{
int arr[10] = {1,2,3,4,5,6,7,8,9,10};
vector<int> v(arr, arr+10);
copy(v.begin(),v.end(),
ostream_iterator<int,char>(cout," "));
cout << endl << endl;
// remove the 7
vector<int>::iterator result =
remove(v.begin(), v.end(), 7);
// delete dangling elements from the vector
v.erase(result, v.end());
copy(v.begin(),v.end(),
ostream_iterator<int,char>(cout," "));
cout << endl << endl;
// remove everything beyond the fourth element
result = remove_if(v.begin()+4,
v.begin()+8, all_true<int>());
// delete dangling elements
v.erase(result, v.end());
copy(v.begin(),v.end(),
ostream_iterator<int,char>(cout," "));
cout << endl << endl;
return 0;
}
Program Output
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 8 9 10
1 2 3 4
1 2 4
WARNINGS
If your compiler does not support default template parame-
ters, then you always need to supply the Allocator template
argument. For instance, you need to write:
vector<int, allocator<int> >
instead of:
vector<int>
If your compiler does not support namespaces, then you do
not need the using declaration for std.
SEE ALSO
remove, remove_copy, remove_copy_if