remove_if (3C++std) - Tru64 UNIX
Standard C++ LibraryCopyright 1996, Rogue Wave Software, Inc. NAME
remove_if - Move desired elements to the front of a container, and return
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 the end of the
resulting range. remove_if is stable, that is, 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
operates by: 1) copying the values that are to be retained to the front of
the sequence, and 2) returning an iterator that describes where the
sequence of retained values ends. Elements that are after 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.h>
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;
}
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
WARNING
If your compiler does not support default template parameters, then you
need to always supply the Allocator template argument. For instance, you
will need to write :
vector<int, allocator<int> >
instead of :
vector<int>
SEE ALSO
remove, remove_copy, remove_copy_if
STANDARDS CONFORMANCE
ANSI X3J16/ISO WG21 Joint C++ Committee
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