Using "in" with associative arrays and then indexing them (efficiency)

Jonathan M Davis jmdavisProg at gmx.com
Tue Jan 3 03:22:48 PST 2012


On Tuesday, January 03, 2012 12:13:45 Timon Gehr wrote:
> On 01/03/2012 12:07 PM, Jonathan M Davis wrote:
> > On Tuesday, January 03, 2012 11:52:13 Matej Nanut wrote:
> >> Hello everyone,
> >> 
> >> I would like to know whether
> >> 
> >>          if (symbol in symbols)
> >>          
> >>                  return symbols[symbol];
> >> 
> >> is any less efficient than
> >> 
> >>          auto tmp = symbol in symbols;
> >>          if (tmp !is null)
> >>          
> >>                  return *tmp;
> >> 
> >> Without optimisation, it looks like the first example
> >> searches for `symbol' twice.
> > 
> > Of course it does. in does a search and returns a pointer to the element
> > in the AA (or null if it isn't there). The subscript operator also does
> > a search, returning the element if it's there and blowing up if it's
> > not
> > (OutOfRangeError IIRC without -release and who-knows-what with
> > -release). So, if you use in and then the subscript operator, of course
> > it's going to search twice. Part of the point of using in is to not
> > have to do a double lookup (like you would be doing if AAs had a
> > contains function and you called that prior to using the substript
> > operator).
> > 
> > The correct way to do it is the second way, though you should be able to
> > reduce it to
> > 
> > if(auto tmp = symbol in symbols)
> > 
> >      return *tmp;
> > 
> > - Jonathan M Davis
> 
> I think this is the single most ugly thing in the language. IIRC ldc
> will generate identical code for both code snippets.

What, declaring variables in if statements? It's fantastic IMHO. It allows you 
to restrict the scope of the variable to the if statement's scope and still 
use it in the if's condition. And yes, as far as the assembly goes, the 
generated code is identical. But the scoping for the variable is most 
definitely different - it won't exist past the if statement if it's declared in 
the if's condition - and it saves you a line of code. The reduced scope is the 
more important of the two though IMHO, as nice as saving a line of code is.

- Jonathan M Davis


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