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List:       r-help
Subject:    Re: [R] using S4 objects in "with"?
From:       Spencer Graves <spencer.graves () effectivedefense ! org>
Date:       2018-09-26 21:23:17
Message-ID: 243b9ca4-10f9-b331-841b-6d4c69677559 () effectivedefense ! org
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On 2018-09-26 15:34, Duncan Murdoch wrote:
> On 26/09/2018 4:16 PM, Spencer Graves wrote:
>>         Is there anything comparable to "with" for S4 objects?
>>
>>
>> EXAMPLE:
>>
>>
>>         A "Wave" object in the tuneR package has slots "left" and
>> "right", plus others.  I'd like to be able to do something like the
>> following:
>>
>>
>> library(tuneR)
>> x <- seq(0, 2*pi, length = 6)
>> all.equal(x, rev(x))
>> channel <- round(32000 * sin(440 * x))
>> Wobj <- Wave(left = channel, right=rev(channel))
>>
>> with(Wobj, quantile(left-right))
>>
>>
>>         ** This last statement throws "Error ... object 'left' not 
>> found".
>>
>>
>>         Is there something comparable to "with" that can do this?
>
> I don't know of anything that is "officially sanctioned".  A couple of 
> ideas:
>
> 1.  Slots in S4 are stored in attributes.  So
>
>   with(attributes(Wobj), quantile(left - right))
>
> works.  BUT:  as far as I recall, this is an undocumented 
> implementation detail, and you aren't supposed to count on it.
>
> 2.  You could write an as.list() method for the Wave class, then
>
>   with(as.list(Wobj),
>
> would work.  This may be the "right" way to do this.


       Thanks.  I'd prefer to have as.list.default convert every S4 
object to a list.  And have with(S4_object, ...) interpret it equivalent 
to with(as.list(S4_object), ...).


       I think I'll do it other ways for the time being.


       Best Wishes,
       Spencer

>
> Duncan Murdoch

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