Difference between revisions of "cpp/algorithm/remove"

Revision as of 23:53, 10 August 2015

Removes all elements satisfying specific criteria from the range [first, last) and returns a past-the-end iterator for the new end of the range.

The first version removes all elements that are equal to value, the second version removes all elements for which predicate p returns true.

Removing is done by shifting (by means of move assignment) the elements in the range in such a way that the elements that are not to be removed appear in the beginning of the range. Relative order of the elements that remain is preserved and the physical size of the container is unchanged. Iterators pointing to an element between the new logical end and the physical end of the range are still dereferenceable, but the elements themselves have unspecified values (as per Template:concept post-condition). A call to remove is typically followed by a call to a container's erase method, which erases the unspecified values and reduces the physical size of the container to match its new logical size.

Parameters

Return value

Past-the-end iterator for the new range of values (if this is not end, then it points to an unspecified value, and so do iterators to any values between this iterator and end)

Complexity

Exactly std::distance(first, last) applications of the predicate.

Notes

The similarly-named container member functions list::remove, list::remove_if, forward_list::remove, and forward_list::remove_if erase the removed elements.

These algorithms cannot be used with associative containers such as std::set and std::map because ForwardIt does not dereference to a MoveAssignable type (the keys in these containers are not modifiable)

Possible implementation

template< class ForwardIt, class T >
ForwardIt remove(ForwardIt first, ForwardIt last, const T& value)
{
    first = std::find(first, last, value);
    if (first != last)
        for(ForwardIt i = first; ++i != last; )
            if (!(*i == value))
                *first++ = std::move(*i);
    return first;
}

template<class ForwardIt, class UnaryPredicate>
ForwardIt remove_if(ForwardIt first, ForwardIt last, UnaryPredicate p)
{
    first = std::find_if(first, last, p);
    if (first != last)
        for(ForwardIt i = first; ++i != last; )
            if (!p(*i))
                *first++ = std::move(*i);
    return first;
}

Examples

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

Textwithsomespaces
Textwithsomewhitespaces

See also