[R] Reduce woes
jeremiah rounds
roundsjeremiah at gmail.com
Thu Jul 28 21:14:50 CEST 2016
Re:
"What I'm trying to
work out is how to have the accumulator in Reduce not be the same type as
the elements of the vector/list being reduced - ideally it could be an S3
instance, list, vector, or data frame."
Pretty sure that is not true. See code that follows. I would never solve
this task in this way though so no comment on the use of Reduce for what
you described. (Note the accumulation of "functions" in a list is just a
demo of possibilities). You could accumulate in an environment too and
potentially gain a lot of copy efficiency.
lookup = list()
lookup[[as.character(1)]] = function() print("1")
lookup[[as.character(2)]] = function() print("2")
lookup[[as.character(3)]] = function() print("3")
data = list(c(1,2), c(1,4), c(3,3), c(2,30))
r = Reduce(function(acc, item) {
append(acc, list(lookup[[as.character(min(item))]]))
}, data,list())
r
for(f in r) f()
On Thu, Jul 28, 2016 at 5:09 AM, Stefan Kruger <stefan.kruger at gmail.com>
wrote:
> Ulrik - many thanks for your reply.
>
> I'm aware of many simple solutions as the one you suggest, both iterative
> and functional style - but I'm trying to learn how to bend Reduce() for the
> purpose of using it in more complex processing tasks. What I'm trying to
> work out is how to have the accumulator in Reduce not be the same type as
> the elements of the vector/list being reduced - ideally it could be an S3
> instance, list, vector, or data frame.
>
> Here's a more realistic example (in Elixir, sorry)
>
> Given two lists:
>
> 1. data: maps an id string to a vector of revision strings
> 2. dict: maps known id/revision pairs as a string to true (or 1)
>
> find the items in data not already in dict, returned as a named list.
>
> ```elixir
> data = %{
> "id1" => ["rev1.1", "rev1.2"],
> "id2" => ["rev2.1"],
> "id3" => ["rev3.1", "rev3.2", "rev3.3"]
> }
>
> dict = %{
> "id1/rev1.1" => 1,
> "id1/rev1.2" => 1,
> "id3/rev3.1" => 1
> }
>
> # Find the items in data not already in dict. Return as a grouped map
>
> Map.keys(data)
> |> Enum.flat_map(fn id -> Enum.map(data[id], fn rev -> {id, rev} end)
> end)
> |> Enum.filter(fn {id, rev} -> !Dict.has_key?(dict, "#{id}/#{rev}")
> end)
> |> Enum.reduce(%{}, fn ({k, v}, d) -> Map.update(d, k, [v], &[v|&1])
> end)
> ```
>
>
>
>
> On 28 July 2016 at 12:03, Ulrik Stervbo <ulrik.stervbo at gmail.com> wrote:
>
> > 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.
> >>
> >
>
>
> --
> 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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