[R] extract from a list of lists

[Bill Dunlap]

I find your original sapply(List, function(element)element$name) easy to
understand.  However replacing sapply with vapply makes for more robust
code.  vapply requires you to supply the expected mode (type) and length of
element$name and if any of the elements don't comply with that, vapply
gives an error (sapply returns a list in this case).  Also, for lists of
length zero, it returns a 0-long result of the mode you specified (instead
of sapply's list()).  In the usual happy case it returns what sapply does.
 It happens to be a bit faster than sapply and uses less memory, but I
think the checks that it does make it worth using instead of sapply.

E.g.,
> L <- list(list(A=1, B=c(TRUE,FALSE)), list(A=2, B=c(FALSE,FALSE)),
list(A=3))
> vapply(L, function(el)el$A, FUN.VALUE=numeric(1))
[1] 1 2 3
> vapply(L, function(el)el$B, FUN.VALUE=logical(2))
Error in vapply(L, function(el) el$B, FUN.VALUE = logical(2)) :
  values must be length 2,
 but FUN(X[[3]]) result is length 0
> vapply(L, function(el){ z <- el$B; if (is.null(z)) rep(NA,2) else z},
FUN.VALUE=logical(2))
      [,1]  [,2] [,3]
[1,]  TRUE FALSE   NA
[2,] FALSE FALSE   NA

-Bill

On Tue, Dec 27, 2022 at 1:10 PM Therneau, Terry M., Ph.D. via R-help <
r-help using r-project.org> wrote:

> 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"
> >>>
> >>>          [[alternative HTML version deleted]]
> >>>
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> >>> and provide commented, minimal, self-contained, reproducible code.
> >>
> >>
> >> --
> >> Gregory (Greg) L. Snow Ph.D.
> >> 538280 using gmail.com
> >>
> >> ______________________________________________
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> >>
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> >> PLEASE do read the posting guidehttps://
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> >> and provide commented, minimal, self-contained, reproducible code.
>
>         [[alternative HTML version deleted]]
>
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