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[NEWBIE] is there an in-place map?
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Date: -- (:)
From: Brian Hurt <bhurt@j...>
Subject: Re: [Caml-list] [NEWBIE] is there an in-place map?
Jean-Christophe Filliâtre wrote:

>Kuba Ober a écrit :
>  
>
>>I need functionality of map, but done in such a way that the output array
>>is given as the argument, not as a return value. The closest I could get was
>>
>>let inplace_map f a b = Array.blit (map f a) 0 b 0 (Array.length a)
>>
>>Arrays a and b are of the same size.
>>This seems very inelegant.
>>    
>>
>
>I guess you mean Array.map. Indeed, this is not optimal because
>Array.map allocates a new array, whose contents is immediately copied
>into b.
>
>  
>
There's a reason for that...

>>One could use an adapter function and iteri, but that adds very noticeable 
>>overhead, and doesn't seem too elegant either.
>>
>>let inplace_map f a b = Array.iteri (fun i src -> b.(i) <- f src; ()) a
>>    
>>
>
>You may find this inelegant too, but it is clearly more efficient than
>your previous version using map. Note that "; ()" can be omitted, since
>the assignment <- already has type unit.
>
>Equivalently, you could use a for loop instead of Array.iteri.
>
>  
>

Both of these maps have the type ('a -> 'a) -> 'a array -> 'a array, as 
opposed to the normal map, which has the type ('a -> 'b) -> 'a array -> 
'b array.  Once you allocate an array, you can't change the type it 
holds.  So I'd probably give the function some other name, maybe 
"modify" or similiar.

Brian