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Servant of the Lord

Iterating in order over an unordered map

4 posts in this topic

I have an std::unordered_map<MyStruct>. It's unordered for extra-fast lookups which is what will be needed most of the time. However, once, at startup, I'd like to iterate over it in order of MyStruct::myInt.

 

The container currently contains upwards of 2000 elements, and might reach as high as 5-10 thousand.

 

Is there some way I can iterate over it in an order other than its internal hashing order?

I'm not worried about speed.

Edited by Servant of the Lord
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No.

 

If you know the keys used you can sort them and then look up the items in order.

 

You could put them in a map and iterate over them (and use the map to fill the unordered_map) then throw the map away.

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Alright, thanks.

 

I'm now using a temporary map of references:

std::map<Key, std::reference_wrapper<Value>> sortedMap(std::begin(unsortedMap), std::end(unsortedMap));

for(const auto &pair : sortedMap)
{ ... }
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I'm pretty sure boost::multi_index (http://www.boost.org/doc/libs/1_54_0/libs/multi_index/doc/index.html) can do what you're asking for.  You can set up a sequential index and an unordered index.


Thanks for the tip about multi_index. Just used it to clean up a monstrosity with multiple containers that I created a while back.

Boost contains so much, it's tricky to really get an overview of the stuff you acually need sometimes...
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