STL map: reverse iterator for lower_bound?

[qlin88@gmail.com]

Hi,

In STL multi-map, the lower_bound, upper_bound,equ al_range all
return an iterator. Now ifone wants to iterate from an upper bound
towards a lower bound, what would be the best way to do it?

I tried to interate backwards with an iterator, but the begin()
element was not properly included.

Thanks for your help.


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[=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?=]

Re: STL map: reverse iterator for lower_bound?

On 2008-04-03 23:07, qlin88@gmail.co m wrote:

Hi,
>
In STL multi-map, the lower_bound, upper_bound,equ al_range all
return an iterator. Now ifone wants to iterate from an upper bound
towards a lower bound, what would be the best way to do it?
>
I tried to interate backwards with an iterator, but the begin()
element was not properly included.

You can use reverse_iterato rs and rend():

#include <iostream>
#include <map>

int main()
{
std::multimap<i nt, intm;
for (int i = 0; i < 10;++i)
{
m.insert(std::m ake_pair(i, i));
m.insert(std::m ake_pair(i, 2*i));
}

//for (
std::multimap<i nt, int>::iterator it = m.begin();
it != m.end();
++it)
// std::cout << it->first << "\t" << it->second << "\n";


std::multimap<i nt, int>::iterator upper = m.upper_bound(4 );
std::multimap<i nt, int>::reverse_i terator r(upper);

for (;r != m.rend();++r)
std::cout << r->first << "\t" << r->second << "\n";
}


--
Erik Wikström

[Carl Barron]

Re: STL map: reverse iterator for lower_bound?

In article
<beab93ac-d4ff-441a-8ac3-06702c2ef8ab@d2 1g2000prf.googl egroups.com>,
<qlin88@gmail.c omwrote:

Hi,
>
In STL multi-map, the lower_bound, upper_bound,equ al_range all
return an iterator. Now ifone wants to iterate from an upper bound
towards a lower bound, what would be the best way to do it?
>

well n2521,pdf [mulitmap] states equal_range returns
pair<iterator,i terator>, [or possibly
pair<const_iter ator,const_iter ator>

where the range [pair::first,pai r::second) is the range of equal keys.

if your implimentation of multimap returns only an iterator it is not
conforming....

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[Jiri Palecek]

Re: STL map: reverse iterator for lower_bound?

qlin88@gmail.co m wrote:

Hi,
>
In STL multi-map, the lower_bound, upper_bound,equ al_range all
return an iterator. Now ifone wants to iterate from an upper bound
towards a lower bound, what would be the best way to do it?

You can convert any (bidirectional) iterator to its reverse by something
like that:

typedef std::reverse_it erator<Iterrev_ it;
std::for_each(r ev_it(end), rev_it(begin), ...);

Regards
Jiri Palecek



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[Andrew Koenig]

Re: STL map: reverse iterator for lower_bound?

<qlin88@gmail.c omwrote in message
news:beab93ac-d4ff-441a-8ac3-06702c2ef8ab@d2 1g2000prf.googl egroups.com...

In STL multi-map, the lower_bound, upper_bound,equ al_range all
return an iterator. Now ifone wants to iterate from an upper bound
towards a lower bound, what would be the best way to do it?

I tried to interate backwards with an iterator, but the begin()
element was not properly included.

Rule of thumb: decrement before you use the iterator; increment after you
use it. So:

it = x.begin();
while (it != x.end()) {
// do something with *it
++it;
};

And, going the other way:

it = x.end();
while (it != x.begin()) {
--it;
// do something with *it
}


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