[R] Reduce woes

[Ulrik Stervbo]

Hi Stefan,

in that case,lapply(data, length) should do the trick.

Best wishes,
Ulrik

On Thu, 28 Jul 2016 at 12:57 Stefan Kruger <stefan.kruger at gmail.com> wrote:

> David - many thanks for your response.
>
> What I tried to do was to turn
>
> data <- list(one = c(1, 1), three = c(3), two = c(2, 2))
>
> into
>
> result <- list(one = 2, three = 1, two = 2)
>
> that is creating a new list which has the same names as the first, but
> where the values are the vector lengths.
>
> I know there are many other (and better) trivial ways of achieving this -
> my aim is less the task itself, and more figuring out if this can be done
> using Reduce() in the fashion I showed in the other examples I gave. It's a
> building block of doing map-filter-reduce type pipelines that I'd like to
> understand how to do in R.
>
> Fumbling in the dark, I tried:
>
> Reduce(function(acc, item) { setNames(c(acc, length(data[item])), item },
> names(data), accumulate=TRUE)
>
> but setNames sets all the names, not adding one - and acc is still a
> vector, not a list.
>
> It looks like 'lambda.tools.fold()' and possibly 'purrr.reduce()' aim at
> doing what I'd like to do - but I've not been able to figure out quite how.
>
> Thanks
>
> Stefan
>
>
>
> On 27 July 2016 at 20:35, David Winsemius <dwinsemius at comcast.net> wrote:
>
> >
> > > On Jul 27, 2016, at 8:20 AM, Stefan Kruger <stefan.kruger at gmail.com>
> > wrote:
> > >
> > > Hi -
> > >
> > > I'm new to R.
> > >
> > > In other functional languages I'm familiar with you can often seed a
> call
> > > to reduce() with a custom accumulator. Here's an example in Elixir:
> > >
> > > map = %{"one" => [1, 1], "three" => [3], "two" => [2, 2]}
> > > map |> Enum.reduce(%{}, fn ({k,v}, acc) -> Map.update(acc, k,
> > > Enum.count(v), nil) end)
> > > # %{"one" => 2, "three" => 1, "two" => 2}
> > >
> > > In R-terms that's reducing a list of vectors to become a new list
> mapping
> > > the names to the vector lengths.
> > >
> > > Even in JavaScript, you can do similar things:
> > >
> > > list = { one: [1, 1], three: [3], two: [2, 2] };
> > > var result = Object.keys(list).reduceRight(function (acc, item) {
> > >  acc[item] = list[item].length;
> > >  return acc;
> > > }, {});
> > > // result == { two: 2, three: 1, one: 2 }
> > >
> > > In R, from what I can gather, Reduce() is restricted such that any init
> > > value you feed it is required to be of the same type as the elements of
> > the
> > > vector you're reducing -- so I can't build up. So whilst I can do, say
> > >
> > >> Reduce(function(acc, item) { acc + item }, c(1,2,3,4,5), 96)
> > > [1] 111
> > >
> > > I can't use Reduce to build up a list, vector or data frame?
> > >
> > > What am I missing?
> > >
> > > Many thanks for any pointers,
> >
> > This builds a list:
> >
> > > Reduce(function(acc, item) { c(acc , item) }, c(1,2,3,4,5), 96,
> > accumulate=TRUE)
> > [[1]]
> > [1] 96
> >
> > [[2]]
> > [1] 96  1
> >
> > [[3]]
> > [1] 96  1  2
> >
> > [[4]]
> > [1] 96  1  2  3
> >
> > [[5]]
> > [1] 96  1  2  3  4
> >
> > [[6]]
> > [1] 96  1  2  3  4  5
> >
> > But you are not saying what you want. The other examples were doing
> > something with names but you provided no names for the R example.
> >
> > This would return a list of named vectors:
> >
> > > Reduce(function(acc, item) { setNames( c(acc,item), 1:(item+1))  },
> > c(1,2,3,4,5), 96, accumulate=TRUE)
> > [[1]]
> > [1] 96
> >
> > [[2]]
> >  1  2
> > 96  1
> >
> > [[3]]
> >  1  2  3
> > 96  1  2
> >
> > [[4]]
> >  1  2  3  4
> > 96  1  2  3
> >
> > [[5]]
> >  1  2  3  4  5
> > 96  1  2  3  4
> >
> > [[6]]
> >  1  2  3  4  5  6
> > 96  1  2  3  4  5
> >
> >
> >
> >
> > > Stefan
> > >
> > >
> > >
> > > --
> > > Stefan Kruger <stefan.kruger at gmail.com>
> > >
> > >       [[alternative HTML version deleted]]
> > >
> > > ______________________________________________
> > > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> > > https://stat.ethz.ch/mailman/listinfo/r-help
> > > PLEASE do read the posting guide
> > http://www.R-project.org/posting-guide.html
> > > and provide commented, minimal, self-contained, reproducible code.
> >
> > David Winsemius
> > Alameda, CA, USA
> >
> >
>
>
> --
> Stefan Kruger <stefan.kruger at gmail.com>
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

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