[R] Reduce woes

[David Winsemius]

> 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]]
> 
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David Winsemius
Alameda, CA, USA