Re: The meaning of 'sugar' [Forked from Re: Why do we have ipairs?]

[Ross Bencina <rossb-lists@...>]

On 11/06/2014 10:47 PM, Roberto Ierusalimschy wrote:
> Probably I am missing something, because I fail to see what in a
> conventional switch statement (for a language like Lua) could be
> different than sugar for a chain of if-elseif-else (unless you
> are talking about "fall through", in which case you should use
> gotos).

Hello Roberto,

To answer "what could be different than sugar for if-elseif-else?":

A linear chain of if-elseif-else conditions has N comparisons. I would 
expect a switch to use one or both of the following strategies:

a) generate a balanced tree of tests to give log(N) comparisons (if the 
value range is sparse)

or

b) a single lookup (ie a jump table to program locations indexed by the 
switch value).

or maybe even:

c) a perfect hash combined with a jump table.


Perhaps what you are saying is that as there are no symbolic constants 
in Lua, it is not possible to generate optimized switch statements for 
all but the simplest scenario where the switch cases are literals?

Ross.