Dependent Origination

how to use customized key in unordered_map

Posted on: November 15, 2012

let’s say you want such a map

unordered_map<pair<int, int>, pair<int, int>> m;

you have to provide a hash function, otherwise you are going to run into compiler complaints about std:hash<your customized key type> is not specialized or something like that. in fact, if your type is really customized, you have to provide an equality function too since the map needs to know which bucket to put a key in, and how to tell if a key is equal to another key.

since we are using pair, which has already defined == operators, we just need to provide a hash function to the definition of the map.

the function is defined like this. note we are using the first component of the pair to be the hash key. you can do any fancy algorithms here.

typedef struct {

long operator() (const pair<int, int> &k) const {

std::hash<int> hasher;

return hasher(k.first);

}

} PairKeyHash;

then

unordered_map<pair<int, int>, pair<int, int>, PairKeyHash> variable_;

and all done!

References:

1. pair type reference

2. unordered_map reference

3. this article helps the most

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

November 2012
M T W T F S S
« Oct   Dec »
 1234
567891011
12131415161718
19202122232425
2627282930  

Twitter

  • is looking at other people's intentions, not their capabilities, and feeling much happier every day :) 11 months ago
  • is planning on how to spend the next two weeks until the new year, at home. 11 months ago
  • is going to have human company for Thanksgiving; a rare event for the past like twenty years. 1 year ago
  • living by myself again; a strange feeling in a bustling city with thirteen million people 1 year ago
  • just came back home after sixteen days on the road; when i set out, the plan was a mere three-day trip 1 year ago

Flickr Photos

IMG_3150

IMG_3175

IMG_3169

IMG_3139

IMG_3120

IMG_3111

IMG_3086

IMG_3079

IMG_3054

IMG_3048

More Photos
%d bloggers like this: