[R] apply and cousins

[Bert Gunter]

John:

1. Please read and follow the posting guide. In particular, provide a
small reproducible example so that we know what your data and looping
code look like.

2. apply-type commands are *not* vectorized; they are disguised loops
that may or may not offer any speedup over explicit loops.

3. A guess at a possible strategy is to convert character date-time
data to POSIXct dates using as.POSITct and then just choose those rows
with the maximum value . e.g.

x[x==max(x)]

These operations *are* vectorized.

However, this guess might be completely useless with your unspecified
data, so beware.

Cheers,
Bert

Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Wed, Jun 8, 2016 at 9:41 AM, John Logsdon
<j.logsdon at quantex-research.com> wrote:
> Folks
>
> Is there any way to get the row index into apply as a variable?
>
> I want a function to do some sums on a small subset of some very long
> vectors, rolling through the whole vectors.
>
> apply(X,1,function {do something}, other arguments)
>
> seems to be the way to do it.
>
> The subset I want is the most recent set of measurements only - perhaps a
> couple of hundred out of millions - but I can't see how to index each
> value.  The ultimate output should be a matrix of results the length of
> the input vector.  But to do the sum I need to access the current row
> number.
>
> It is easy in a loop but that will take ages. Is there any vectorised
> apply-like solution to this?
>
> Or does apply etc only operate on each row at a time, independently of
> other rows?
>
>
> Best wishes
>
> John
>
> John Logsdon
> Quantex Research Ltd
> +44 161 445 4951/+44 7717758675
>
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