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Rank Ordering A map<string, int>

Posted on 2007-03-25
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Last Modified: 2010-04-01
I have map<string, int> which I want to order descending by the int value. Can Anyone Help? Perhaps with a pre-written rank ordering function.

Many Thanks
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Question by:inghfs
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6 Comments
 
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Expert Comment

by:Infinity08
ID: 18789625
Then why not use a map<int, string> instead ???
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Author Comment

by:inghfs
ID: 18789638
This is what I am currently doing with the map. I'm not sure it makes scence to use map<int, string>.

while (wordsfile >> s) {  
             
              if (occurrences[s] == 0)  
                        words.push_back(s);
             
              occurrences[s]++;                                
        }  
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LVL 53

Expert Comment

by:Infinity08
ID: 18789675
Well, there are two obvious solutions :

1) copy the data in a vector and sort the vector

http://www.cplusplus.com/reference/stl/vector/
http://www.cplusplus.com/reference/algorithm/sort.html

2) create a new map<int, string> (or probably a multimap since the ints can be duplicate), and fill it with the same data

http://www.cplusplus.com/reference/stl/multimap/


Note that you don't need to sort in descending order. The default sort is in ascending order, and you can just read the data from last to first element.
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LVL 86

Accepted Solution

by:
jkr earned 500 total points
ID: 18790435
>>I'm not sure it makes scence to use map<int, string>.

It makes sense when it comes to have the different ordering for a certain situation. The idea is to keep the original map<string, int> to map the occurrances, but to use a map<int,string> temporarily to have the opposite ordering for the ranking, i.e.

map<string, int> occurances;
map<int,string> ranking;

// ... read values

while (wordsfile >> s) {  
             
              if (occurrences[s] == 0)  
                        words.push_back(s);
             
              occurrences[s]++;                                
        }  

//...

// now, get the ranking


map<string, int>::iterator i;

for (i = occurances.begin(); i != occurances.end(); ++i) {

    ranking.insert(map<int,string>::value_type(i->second,i->first)); // insert using the reverse
}

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LVL 39

Expert Comment

by:itsmeandnobodyelse
ID: 18792696
>>>> or probably a multimap since the ints can be duplicate
That might give the overkill. If the int values are not unique (even for rankings some may share a rank) a multimap helps cause each int key can point to more than one value, but I never experienced a more uncomfortable interface than with a multimap. I strongly recommend to put key and value into a struct and use two std::set containers providing two different compare functions, where the first compares the (unique) string members while the second needs to compare the int member *and* the string member to guarantee uniqueness.  If you don't want to have redundancies you might store pointers rather than values.  

Regards, Alex
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Author Comment

by:inghfs
ID: 18961966
ok I will have to look at this again but will be away for a few weeks - so please excuse continued silence.
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