Arrays - Inserting and moving data

[Timon Gehr]

On 02/14/2012 12:34 AM, James Miller wrote:
> On 14 February 2012 06:25, Timon Gehr<timon.gehr at gmx.ch>  wrote:
>> On 02/13/2012 03:19 PM, James Miller wrote:
>>>
>>> On 11 February 2012 10:45, Jonathan M Davis<jmdavisProg at gmx.com>    wrote:
>>>>
>>>> On Friday, February 10, 2012 13:32:56 Marco Leise wrote:
>>>>
>>>>> I know that feeling. I had no exposure to functional programming and
>>>>> options like chain never come to my head. Although "map" is a concept
>>>>> that
>>>>> I made friends with early.
>>>>
>>>>
>>>> It would benefit your programming in general to learn a functional
>>>> programming
>>>> language and become reasonably proficient in it, even if you don't intend
>>>> to
>>>> program in it normally. It'll increase the number of tools in your
>>>> programming
>>>> toolbox and improve your programming in other programming languages. It's
>>>> something that not enough programmers get sufficient exposure to IMHO.
>>>>
>>>> - Jonathan M Davis
>>>
>>>
>>> I found that learning Haskell made me significantly better at what I
>>> do. New paradigms are good for reminding you to think outside the box,
>>> I also learnt Prolog for a university course (AI) and that was an
>>> interesting challenge. Logical programming, where you define the
>>> boundaries of the program and then it works out the possible answers
>>> for you, amazingly useful for BNF grammars and similar constructs.
>>>
>>> If fact it's got to the point where I feel hamstrung if I can't do at
>>> least function passing (fortunately C, C++ and D can do this), and I
>>> prefer to work with languages that support closures and anonymous
>>> functions, since you can do wonders with simple constructs like map,
>>> fold (reduce) and filter. In fact a naive implementation of quicksort
>>> can be done succinctly in any language that supports filter.
>>>
>>>      T[] sort(T)(T[] array) {
>>>          pivot = array[array.length/2];
>>>          return sort(filter!("a<    "~pivot)(array)~pivot~sort(filter!("a
>>>>
>>>> "~pivot)(array));
>>>
>>>      }
>>>
>>> (Disclaimer, this is probably a very slow implementation, possibly
>>> very broken, may cause compiler demons to possess your computer, DO
>>> NOT USE!)
>>>
>>> I have left out some details for brevity, and it probably won't work
>>> in alot of situations, but it demonstrates the power of functional
>>> programming, quicksort in 4 lines (sort of, its not like Haskell's
>>> "quicksort in 2 lines" is any better mind you, its slow as balls
>>> because of all the memory allocation it has to do).
>>>
>>> Anyway, yay for functional programming and thread derailment.
>>>
>>> James
>>
>>
>> If it is slow and uses an awful lot of auxiliary memory it is not quicksort
>> as much as it may conceptually resemble quicksort. Try to implement in-place
>> quicksort in Haskell. It will look like C code. Also see:
>> http://stackoverflow.com/questions/5268156/how-do-you-do-an-in-place-quicksort-in-haskell
>>
>
> It is still conceptually quicksort, the divide-and-conquer method
> based on partitioning the list.

Hoare's original quicksort algorithm is more detailed than what is 
sketched here. The main point is the in-place partition operation with 
the two pointers approaching each other.

> I wasn't writing it to show a valid
> implementation (I didn't even test it, it probably doesn't compile), I
> even warned of compiler demons! Its a demonstration of the
> succinctness of functional techniques for certain problems, not a show
> that functional languages "are teh awesum and all other langauges
> suck".

The approach given does not solve the problem (it does not implement 
Quicksort). Quicksort in Haskell looks like Quicksort in D, because the 
algorithm depends on destructive updates. Functional techniques can be 
succinct for certain problems, but implementing Quicksort is not one of 
them.

> Haskell is almost a pure functional language, therefore, under
> normal circumstances, every change to the array will allocate a new
> array,

Haskell can do destructive array updates that look like pure operations 
just fine. 
http://hackage.haskell.org/packages/archive/array/0.2.0.0/doc/html/Data-Array-MArray.html

> this is because of the whole immutability thing that it has going on.

This is confusing the abstraction with its implementation. It is 
impossible to recreate Haskell's execution semantics in D using only 
immutable types.

> Of course I would never use such an implementation in real
> life, and Haskellers tend to avoid algorithms that do these kinds of
> things, using sorts like mergesort instead.
>

Mostly lazy mergesort if I'm not mistaken. And they don't usually use it 
to sort arrays, they sort lists. Haskell arrays ought to be sorted with 
introsort if the comparison operation is cheap.

> Saying "it is not quicksort as much as it may conceptually resemble
> quicksort" is kinda odd, its like saying "it is not a car, as much as
> it may conceptually resemble a car" because it doesn't run on petrol
> or gas, but instead runs on environment destroying orphan tears.
>

It is more like saying "a handcart is not a car, as much as it may 
conceptually resemble a car" (the engine is missing!).