Why there are no bijective datatypes

Asked May 29 '13 at 18:39

Active May 29 '13 at 18:39

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I recently ran into a situation where I needed to add reverse lookup functionalities in my mappings, and then I was surprised by the fact there there are simply no datatypes designed for this kind of bijection.

I was able to get around using two maps, one key-value, one value-key

but I am really really curious about why there are no bijective datatypes

asked May 29 '13 at 18:39

Matthew Yang

1

Please add the relevant language tag to your question. – Oliver Charlesworth May 29 '13 at 18:40
its general, I can't find any bijective types in static typing languages such as C++, Java, C#, as for Js, python and etc it's really meaningless – Matthew Yang May 29 '13 at 18:42
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Well, C++ has [`boost::bimap`](http://www.boost.org/doc/libs/1_42_0/libs/bimap/doc/html/index.html) for example. – Oliver Charlesworth May 29 '13 at 18:44
as much as I know it relies on 2 unidirection maps – Matthew Yang May 29 '13 at 18:47
Well, how else would you implement it? – Oliver Charlesworth May 29 '13 at 18:49
that is my question, why there are no native implementations of bijective datatypes, is it because there are no efficient algorithms or what.. – Matthew Yang May 29 '13 at 18:51
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One way to look at it is that you need to be able to do lookups on `x` or lookups on `y`. Therefore, you need two lookup mechanisms (i.e. two hashtables or whataver). Theoretically you could do it all with one merged hashtable, but I don't know whether that offers any advantage in practice. – Oliver Charlesworth May 29 '13 at 18:52
@OliverCharlesworth make it an answer. – Euri Pinhollow Mar 06 '17 at 10:20

Why there are no bijective datatypes

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