[Boost-users] Re: Key iterators

From: Jeffrey Holle (jeff.holle_at_[hidden])
Date: 2004-11-18 17:57:54

I believe you could make your loop cleaner by using std::advance and 
std::count.  Something like:
   multimap<string,string> MyMap;
   for (MyMap::iterator dir=filesystem.begin();
        dir!=filesystem.end();
        advance(dir,filesystem.count(*dir))) {
   }

Chris Goller wrote:
> David-
> 
> Thanks.
> 
> Upon reading my poorly phrased email I realized this:
> 
> I want unique keys so that if the data looks like this:
> 
> key         value
> 3            1
> 1            2
> 1            4
> 4            3
> 5            5
> 5            9
> 
> So, what I would like to do is to a print each value for a particular 
> key in such a way:
> 
> key:         value:
> 1----
>    |------2
>    |------4
> 3----
>    |------1
> 4----
>    |------3
> 5----
>    |------5
>    |------9
> 
> 
> Or, for example, if I had a list of directories as keys and and files 
> are their values then for each directory I could print out the file.
> Right now, I do this:
> 
> for (multimap<string, string>::iterator dir = filesystem.begin(); i != 
> filesystem.end())
> {
>    pair<multimap<string, string>::iterator, multimap<string, 
> string>::iterator> bounds;
>    bounds = filesystem.equal_range(dir->first);
>    cout << dir->first << "----" << endl;
>    for (multimap<string, string>::iterator file = bounds.first;  file != 
> bounds.second; file++)
>    {
>       cout << "    |------" << file->second  << endl;
>    }
>    // Moves the directory iterator to the next directory key.
>    dir = file;
> }
> 
> 
> So, I'm trying to get better at using iterators much smarter.  I would 
> like to make this code much more concise.
> 
> I figured that if I could jump from key to key I write the above code 
> much cleaner.
> 
> Thanks,
> 
> Chris
> 
> 
> David Abrahams wrote:
> 
>> Chris Goller <cgoller_at_[hidden]> writes:
>>
>>  
>>
>>> I'm just getting into boost and I really like it.
>>>
>>> I would like to iterate over keys of a multimap.
>>>
>>> so it would look like this:
>>>
>>>
>>> std::multimap<type, type>::key_iterator i = map.beginkey(); i !=
>>> map.endkey(); i++
>>> at each i++ it would move to the next key.
>>>
>>> -or-
>>>
>>> for_each(map.beginkey(), map.endkey(), whatever);
>>>
>>>
>>> Is this possible with the boost iterators?
>>>   
>>
>>
>> You can't use that syntax, because there's no way to inject a
>> key_iterator member into std::multimap.  You could use a
>> transform_iterator over the multimap's native iterator with select1st,
>> defined as:
>>
>> struct select1st
>> {
>>    template <class T, class U>
>>    T& operator()(std::pair<T,U>& p) const
>>    { return p.first }
>>
>>    template <class T, class U>
>>    T const& operator()(std::pair<T,U> const& p) const
>>    { return p.first }
>> };
>>
>> HTH,
>>  
>>