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.shearar@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 <tudor@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.larcheveque@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.bergel@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 <tudor@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
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