Re: [Texmacs-dev] Efficiency of C++ switch-statements

[Bob McElrath]

Joris van der Hoeven address@hidden wrote:
> 
> Hi *,
> 
> Can anyone provide me with some information on how well
> the C++ switch-statement is being optimized?
> 
> I am interested in the following situation: I have 150 built-in
> TeXmacs tags and a few places in the code where some action has
> to be undertaken which is different for about 10 tags and
> the same for the remaining 140 tags. So I use a switch with
> a "default:". My question is: does C++ use table lookup for
> immediately jumping to the right location (the tables would
> clearly be quite sparse)?

Yes, it does use a table.

This page is useful:
    http://www.eventhelix.com/RealtimeMantra/Basics/CToAssemblyTranslation3.htm

...it says compliers sometimes use a binary search if the table would be
sparse.

> Another question: if I do not put the cases in the same order
> as the enumeration, does this alter the efficiency?

If I remember correctly, switch used to essentially be a <= so you had
to make sure they were in order or the wrong case would be executed.  (I
just tested this, and it is not true -- either I remember wrong or this
is a C/C++ difference)  I don't know if this varies from compiler to
compiler.

Also consider using the STL hash_map object, which gives you O(1)
lookups and does not waste space when your table is sparse.

Cheers,
Bob McElrath [Univ. of California at Davis, Department of Physics]

signature.asc
Description: Digital signature