BytedanceRPC commented on pull request #2469:
URL: https://github.com/apache/thrift/pull/2469#issuecomment-958691924


   > Re "sparse": It was not about the feature, but about terminology. A sparse 
array is an array where most values are unused (e.g. NULL/nil), and the most 
significant problem about it is to store it with a minimum amount of memory, 
e.g. imagine an 2-sided matrix with 5 mio by 5 mio possible combinations, where 
only a few thousand are really used. The challenge here is to get it small AND 
still fast. With that in mind, a map or dictionary is by definition an 
efficient, sparse data structure with something in the range of O(1) for reads. 
Hence, "sparse map" makes not much sense, plus what you really mean is sth 
completely different: The entry is used, only the value happens to be null. An 
existing null entry and a non-existing entry are two different things.
   
   Thanks for explanation.


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