[Boost-users] Re: Key iterators

I believe you could make your loop cleaner by using std::advance and std::count. Something like: multimap 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::iterator dir = filesystem.begin(); i != filesystem.end()) { pair::iterator, multimap::iterator> bounds; bounds = filesystem.equal_range(dir->first); cout << dir->first << "----" << endl; for (multimap::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 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::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 T& operator()(std::pair& p) const { return p.first }

template T const& operator()(std::pair const& p) const { return p.first } };

HTH,