[R] Using lapply when there are dependencies
blue honour
bluehonour at yahoo.com
Thu May 28 08:42:50 CEST 2015
Thank you. May I ask, what would you recommend reading in order to learn how to vectorise loops with dependencies using data.table facilities? I have been searching online on this topic since last night but no luck yet.
--------------------------------------------
On Thu, 5/28/15, David Winsemius <dwinsemius at comcast.net> wrote:
Subject: Re: [R] Using lapply when there are dependencies
Cc: r-help at r-project.org
Date: Thursday, May 28, 2015, 7:02 AM
On May
27, 2015, at 4:34 PM, blue honour via R-help wrote:
> Hi all,
>
> Let's say I have
a vector:
>
>
vv<-c(1,2,3)
>
>
> And suppose I have a function f(a,b),
which is a function of 2 scalar inputs. I would like to
evaluate this function separately for each element of the vv
vector while the second input to f( ) will be the previous
output from f( ). So, the valuation of f() has a dependency
on the previous f( ) valuation (recursive). This type of
calculation is easy to set up with a for loop but that will
run slow. How can I achieve this with apply family of
functions please?
The speed
of loops is determined by the speed of their inner
functions. The Reduce() function does what you request but
for the application described it will need an init value.
Reduce(f, vv,
init=<something>)
> I
have the same question for the case when vv is a data.table
instead of a vector.
I could be wrong but that doesn't sound
like an effective use of the data.table facilities.
--
David Winsemius
Alameda, CA,
USA
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