Re: [Boost-users] Re: Key iterators

From: Caleb Epstein (caleb.epstein_at_[hidden])
Date: 2004-11-18 17:51:55

On Thu, 18 Nov 2004 15:21:06 -0700, Chris Goller
<cgoller_at_[hidden]> 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.

You can use multimap::upper_bound instead of iter++ to help navigate
the unique key space:

using namespace std;
using namespace boost::assign;

typedef multimap<int, int> mymap;
mymap m = map_list_of (1,1)(1,2)(1,3)(2,1)(2,2)(2,3)(3,1)(3,2)(4,9)(5,10);

for (mymap::const_iterator i = m.begin (); i != m.end (); i =
m.upper_bound (i->first))
   cout << i->first << endl;

outputs
1
2
3
4
5

-- 
Caleb Epstein
caleb dot epstein at gmail dot com