Hi,
I agree with Frank on the processing block: it has nothing to do with summing.
Mathematically, defining adding (sum) leads to a neutral element (zero). But
you only need it if the collection is empty.
The trick of the #zero selector is elegant, but not needed with the #anyOne
trick.
Currently, #sum does not work with empty collections in Pharo, which surprised
me.
I would add a
Collection>>#sumFrom: value
^ self inject: value into: [ :sum :each | sum + each ]
And change
Collection>>#sum
| sample sum |
self isEmpty ifTrue: [ ^ self sumFrom: 0 ].
sample := self anyOne.
sum := self inject: sample into: [ :sum :each | sum + each ].
^ sum - sample
A variant with an additional collect style processing block could be added
along these lines, #sum: and #sum:from:
Sven
On 22 Apr 2013, at 12:29, Frank Shearar <frank.shea...@gmail.com> wrote:
> "Sum" is usually well-defined, so I don't understand what the block's
> for. With the block it's more like "take this collection, map the
> values with some function, and sum the result".
>
> If so, it sounds like you're looking for a sugared form of
> (myCollection collect: aBlock) inject: self first class zero into: #+.
> ?
>
> (There's already Float class >> #zero and (in Squeak at least) Integer
> class >> #zero, and so on. Just add MyUnit class >> #zero and you're
> done.)
>
> frank
>
> On 22 April 2013 10:37, Tudor Girba <tu...@tudorgirba.com> wrote:
>> Hi,
>>
>> I am CC-ing the pharo mailing list because it can be of interest.
>>
>> I agree with Guillaume that there is a legitimate need for a generic
>> solution. However, at the same time we lose the convenience of simply adding
>> numbers, which is likely to be the most encountered use case.
>>
>> I see a couple of possibilities:
>> - add Collection>>sum: aBlock from: aZeroValue that takes aZeroValue as the
>> default.
>> - add Collection>>sumNumbers: aBlock that takes 0 as default rather than
>> anyOne.
>>
>> What do you think?
>>
>> Cheers,
>> Doru
>>
>>
>>
>> On Mon, Apr 22, 2013 at 10:14 AM, Guillaume Larcheveque
>> <guillaume.larcheve...@gmail.com> wrote:
>>>
>>> For example, in Artefact we use Units and I have tried to use sum but it
>>> doesn't works because Units doesn't allow to do 0 + 4 cm which is
>>> inconsistent.
>>>
>>>
>>> 2013/4/22 Alexandre Bergel <alexandre.ber...@me.com>
>>>>
>>>> Thanks for letting us know. I cannot see a case where having 0 as the
>>>> initial value does not work as expected.
>>>>
>>>> Cheers,
>>>> Alexandre
>>>>
>>>>
>>>> Le 21 avr. 2013 à 17:35, Tudor Girba <tu...@tudorgirba.com> a écrit :
>>>>
>>>>> Hi,
>>>>>
>>>>> Pharo 2.0 comes with Collection>>sum:
>>>>>
>>>>> Collection>>sum: aBlock
>>>>> "This is implemented using a variant of the normal inject:into:
>>>>> pattern.
>>>>> The reason for this is that it is not known whether we're in the
>>>>> normal
>>>>> number line, i.e. whether 0 is a good initial value for the sum.
>>>>> Consider a collection of measurement objects, 0 would be the
>>>>> unitless
>>>>> value and would not be appropriate to add with the unit-ed objects."
>>>>> | sum sample |
>>>>> sample := aBlock value: self anyOne.
>>>>> sum := self inject: sample into: [ :previousValue :each |
>>>>> previousValue + (aBlock value: each) ].
>>>>> ^ sum - sample
>>>>>
>>>>> To some extent, this is more generic than the one we had in Moose that
>>>>> considered only numbers:
>>>>> Collection>>sum: aSymbolOrBlock
>>>>> ^ self
>>>>> inject: 0
>>>>> into: [:sum :each | sum + (aSymbolOrBlock value: each)]
>>>>>
>>>>>
>>>>> However, with the Pharo 2.0 implementation the collection must not be
>>>>> empty, while the other implementation we get 0. If the collection is
>>>>> empty,
>>>>> you get an exception due to anyOne.
>>>>>
>>>>> This induced several errors in metric computations (like number of
>>>>> methods of a package when the package had no classes). These are now
>>>>> fixed,
>>>>> but I thought I would let you know just in case you want to rely on this
>>>>> method.
>>>>>
>>>>> I actually still believe we would benefit from a robust but more
>>>>> limited sum:. Perhaps we could have sumNumbers:.
>>>>>
>>>>> Cheers,
>>>>> Doru
>>>>>
>>>>>
>>>>> --
>>>>> www.tudorgirba.com
>>>>>
>>>>> "If you can't say why something is relevant,
>>>>> it probably isn't."
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Moose-dev mailing list
>>>>> moose-...@iam.unibe.ch
>>>>> https://www.iam.unibe.ch/mailman/listinfo/moose-dev
>>>>
>>>> _______________________________________________
>>>> Moose-dev mailing list
>>>> moose-...@iam.unibe.ch
>>>> https://www.iam.unibe.ch/mailman/listinfo/moose-dev
>>>
>>>
>>> --
>>> Guillaume Larcheveque
>>>
>>> _______________________________________________
>>> Moose-dev mailing list
>>> moose-...@iam.unibe.ch
>>> https://www.iam.unibe.ch/mailman/listinfo/moose-dev
>>
>> --
>> www.tudorgirba.com
>>
>> "Every thing has its own flow"
--
Sven Van Caekenberghe
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