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

Timon Gehr timon.gehr at
Tue Jan 3 03:27:08 PST 2012

On 01/03/2012 12:22 PM, Jonathan M Davis wrote:
> 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

No, I love declaring variables in if statements and would like it to be 
extended to while statements as well. What I meant is the fact that 
something called 'in' returns a pointer. And the two code snippets I was 
referring to were the two in Matej's post.

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