Re: removing elements from vector<int> using <algorithm>
arnuld wrote:
On Wed, 10 Oct 2007 08:07:23 +0000, Erik Wikstr=F6m wrote:
[...]
By the way, the assignment wants you to use erase() (not sure if they
meant std::erase or std::vector<T>::erase(), I suspect the latter).
I tried it with vector's erase member function but that falls into the
infinite loop:
void rem_evens( std::vector<int>& ivec)
{
std::vector<int>::iterator begin = ivec.begin();
std::vector<int>::iterator end = ivec.end();
while( begin != end )
{
if( (*begin % 2) == 0 )
{
begin++ = ivec.erase( begin );
I'm not sure what this statement is supposed to do, but it
looks very, very wrong (and may not compile with some
implementations).
Note that the statement has a very different meaning
depending on whether vector<>::iterator is a typedef for a
pointer, or is a class type. Since the code shouldn't
compile if it is a typedef for a pointer, I will assume that
it is a class type, which means that you have the equivalent
of:
begin.operator++( 0 ).operator=( ivec.erase( begin ) ) ;
(I'm supposing member functions for the operator++ here, as
I think that's the most frequent implementation. If it's a
non-member, the call sequence will be different, but what
follows still holds.)
Note that begin.operator++( 0 ) returns a temporary object,
whose assignment operator is called to capture the results
of ivec.erase. Furthermore, ivec.erase invalidates begin
(since it removes the object begin points to); the order of
evaluation of the subexpressions above is not specified, so
you may be calling operator++ on an invalid iterator.
And of course, if begin happens to point to the last element
of the vector, after erase, it will point to end() (if it
were valid), and incrementing an iterator pointing to end()
is not a good idea at all.
Note that erase returns an iterator which points to the
element which was immediately behind the element you
removed. For something like this, the "classical" idiom is:
while ( begin != end ) {
if ( condition ) {
begin = vec.erase( begin ) ;
} else {
++ begin ;
}
}
You either increment, or you use the iterator returned by
erase.
Of course, the classical idiom would use remove here:
vec.erase(
remove_if( vec.begin(), vec.end(), condition ),
vec.end() ) ;
--
James Kanze (GABI Software) mailto:james.kanze@gmail.com
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