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To: Joe NYC who wrote (64998)12/5/2001 5:07:48 PM
From: TGPTNDRRead Replies (1) | Respond to of 275872
 
Joe, Re: <but my recollection is that hash table, the order of magnitude of the time to evaluate n cases is 1, meaning a fixed time.>

That's about a 30K' view. Order of magnitude 1 would cover values from 1 to 9 or 900% range. In the program we're talking about the compiler can figure exactly the table size so it should be able to figure out how to index directly without a hash. But that's the kind of reason why you have to look at the machine code(as first choice) or the .asm(for a good clue). Stupid compilers may build a stupid lookup -- and that's why well hand assembled code will outrun straight C every time(sometimes by factors of 1000s).

tgptndr



To: Joe NYC who wrote (64998)12/5/2001 6:33:57 PM
From: Neil BoothRead Replies (1) | Respond to of 275872
 
I don't know how the case statement is compiled to assembler.

GCC orders the cases in a binary tree. It then creates a
set of if / then / else branches to get to the right conclusion as efficiently as possible.

Further, any subsets of the tree that are dense get done by
a jump table. Thus a single case statement can be translated into 0, 1 or many jump tables. GCC is pretty good at that kind of thing; other compilers often not as good.

Neil.


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