if-expressions + list comprehensions

[Bent Rasmussen]

In light of these suggestions, let me emphatically repeat: I retract my 
proposal! ;-)

R, Bent

"Kirk McDonald" <kirklin.mcdonald at gmail.com> wrote in message 
news:f3n97d$1s4o$1 at digitalmars.com...
> Leandro Lucarella wrote:
>> Daniel Keep, el  1 de junio a las 02:44 me escribiste:
>>
>>>Leandro Lucarella wrote:
>>>
>>>>Daniel Keep, el 29 de mayo a las 12:19 me escribiste:
>>>>
>>>>>double[] as = [1.,2.,3.,4.,5.];
>>>>>double[] bs = foreach(a;as) a*a;
>>>>>
>>>>>List comprehensions would be cooler still, but I'm not holding my 
>>>>>breath
>>>>>on that one :P
>>>>
>>>>More pythonic:
>>>>
>>>>double[] bs = a * a foreach(a; as) if (a % 2);
>>>>
>>>>would give you filtering too (leaving bs = [1.0, 9.0, 25.0]) =)
>>>
>>>Problem with that one is that it's ambiguous.  If we pretend that the
>>>"double[]" isn't there (for the sake of argument), then it could
>>>legitimately mean:
>>>
>>>(bs = a * a) foreach(a; as) if (a % 2);
>>>
>>>bs = (a * a) foreach(a; as) if (a % 2);
>>>
>>>bs = a * (a) foreach(a; as) if (a % 2);
>>>
>>>(bs = a * (a) foreach(a; as)) if (a % 2);
>>>
>>>Which is one reason why I'm not a big fan of postfix keywords :P
>>
>>
>> it's a question of precedence, just like bs = a * a, which could mean (bs
>> = a) * a or bs = (a * a).
>>
>> Python use that syntax (bs = a * a for a in as if a % 2) and it's not
>> ambiguous.
>>
>> The last varian is not possible because (in python) there's no
>> "expression" if without an else (and there is no possible else in
>> "expression" for).
>>
>
> Note that, in Python, assignment is not an expression, it's a statement. 
> This is why the above is not ambiguous.
>
> Also note that Python has two forms of this concept: List comprehensions 
> and generator expressions.
>
> A list comprehension looks like this:
>     [a * a for a in as if a % 2]
> Note the surrounding brackets. A list comprehension is an expression that 
> evaluates to a list (the Python equivalent of a vector). It computes all 
> of the elements when the expression is evaluated.
>
> A generator expression looks just like that, but without the brackets (or 
> with regular parentheses, if you need that additional clarity).
>     (a * a for a in as if a % 2)
> A generator expression evaluates to a particular kind of object called a 
> generator, which can be iterated over. The elements of the generator are 
> computed lazily, as it is iterated over. If 'as' is a very large sequence, 
> this can drastically reduce memory usage. This also implies that you 
> cannot index a generator as you can a list.
>
> -- 
> Kirk McDonald
> http://kirkmcdonald.blogspot.com
> Pyd: Connecting D and Python
> http://pyd.dsource.org