[R] extract from a list of lists

[Therneau, Terry M., Ph.D.]

Thanks everyone for prodding my gray matter, which seems to be getting stiffer as I 
approach 70 (< 90 days).

  --  I'll continue to use the $ or [[ forms.   That will suffice.

--  I thought there might be a base R variant, e.g. something like  extract( list, 
element-name); probably cross talk in my brain from the rstan library

-- Gregg's note shows such a function in purr.  But I rather like having as few 
dependencies as possible in a package, one usage is normally not enough, at least for 
something this simple.

Terry T.

On 12/27/22 14:38, Bert Gunter wrote:
> Well, I prefer Greg's approach, but if you want to avoid calls to $ or
> `[[` then you could do:
>
> unlist(fits)[ rep(names(fits[[1]]) == 'iter', length(fits))]
>
>
> Cheers,
> Bert
>
> On Tue, Dec 27, 2022 at 9:46 AM Greg Snow<538280 using gmail.com>  wrote:
>> Another option is the map family of functions in the purrr package
>> (yes, this depends on another package being loaded, which may affect
>> things if you are including this in your own package, creating a
>> dependency).
>>
>> In map and friends, if the "function" is a string or integer, then it
>> is taken as the piece to be extracted, so you should be able to do
>> something like:
>>
>> library(purrr)
>> map(fits, 'iter')
>> # or
>> map_int(fits, 'iter')
>> # or
>> map_dbl(fits, 'iter')
>>
>> which of the last 2 to use depends on how `iter` is stored.
>>
>> On Tue, Dec 27, 2022 at 10:16 AM Therneau, Terry M., Ph.D. via R-help
>> <r-help using r-project.org>  wrote:
>>> I not uncommonly have the following paradym
>>>      fits <- lapply(argument, function)
>>>
>>> resulting in a list of function results.   Often, the outer call is to mclapply, and the
>>> function encodes some long calculation, e.g. multiple chains in an MCMC.
>>> Assume for illustration that each function returns a list with elements   beta, loglik,  iter.
>>>
>>> Then  sapply(fits,  function(x) x$iter)
>>> will give me a vector, with the number of iterations used by each instance.
>>>
>>> I've often been suspicious that there is some simple shorthand for the "grab all the
>>> elements named iter" that skips the explicit x$iter function.   Am I indeed overlooking
>>> something?    I don't expect a speed increase, just cleaner code.
>>>
>>> Terry T.
>>>
>>> --
>>> Terry M Therneau, PhD
>>> Department of Quantitative Health Sciences
>>> Mayo Clinic
>>> therneau using mayo.edu
>>>
>>> "TERR-ree THUR-noh"
>>>
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>>>
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>>
>>
>> --
>> Gregory (Greg) L. Snow Ph.D.
>> 538280 using gmail.com
>>
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>> and provide commented, minimal, self-contained, reproducible code.

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