I need to go from the beginning +3 and from the end -3 that is, if the total size is 10, then I have to iterate from 3 to 7 who it doesnt work?
std::multimap<int,int> _multimap;
for(auto it = _multimap.begin() + 3; it != _multimap.end() - 3; it++) {
std::cout << it->first;
}
The problem is that the class template std::multimap does not have random access iterators. It has bidirectional iterators.
So you may not write
_multimap.begin() + 3
or
_multimap.end() - 3
Instead you could write the for loop the following way using standard function std::next and std::prev
#include <iterator>
//...
for( auto first = std::next( _multimap.begin(), 3 ),
last = std::prev( _multimap.end(), 3 );
first != last;
++first ) {
/// TODO: something happen
}
Here is a demonstration program.
#include <iostream>
#include <map>
#include <iterator>
int main()
{
std::multimap<int, char> m =
{
{ 65, 'A' }, { 66, 'B' }, { 67, 'C' }, { 68, 'D' }, { 69, 'E' },
{ 70, 'F' }, { 71, 'G' }, { 72, 'H' }, { 73, 'I' }, { 74, 'J' },
};
for ( auto first = std::next( std::begin( m ), 3 ), last = std::prev( std::end( m ), 3 );
first != last;
++first )
{
std::cout << first->first << ": " << first->second << '\n';
}
}
The program output is
68: D
69: E
70: F
71: G
If you want to have the range of elements [3, 7 ] then instead of
last = std::prev( std::end( m ), 3 )
you need to write
last = std::prev( std::end( m ), 2 )
Do not forget to include the header <iterator>.
