[R] Vectorized expression to extrapolate matrix columns with columns of another matrix

[Abiel X Reinhart]

Gabor,

This comes close to solving my problem, but I am still left with the problem of how I can extrapolate, not just interpolate. In our example, if I define m as,

m <- zoo(cbind(c(1, 2, NA, NA, 5, NA, NA), seq(7)^2), as.Date(1:7))

instead of 

m <- zoo(cbind(c(1, 2, NA, NA, 5, NA, 7), seq(7)^2), as.Date(1:7))

then I will only get five values back, when I really want m[,1] to be fully extrapolated so that there are seven values. Is there a workaround?

Abiel Reinhart

-----Original Message-----
From: Gabor Grothendieck [mailto:ggrothendieck at gmail.com] 
Sent: Wednesday, May 12, 2010 8:37 AM
To: Abiel X Reinhart
Cc: r-help at r-project.org
Subject: Re: [R] Vectorized expression to extrapolate matrix columns with columns of another matrix

Try this using the zoo package.  See ?na.approx for more and note that
this functionality requires zoo 1.6-3 or later.

.> m <- zoo(cbind(c(1, 2, NA, NA, 5, NA, 7), seq(7)^2), as.Date(1:7))
> na.approx(m[, 1], x = m[, 2])
1970-01-02 1970-01-03 1970-01-04 1970-01-05 1970-01-06 1970-01-07 1970-01-08
  1.000000   2.000000   2.714286   3.714286   5.000000   5.916667   7.000000
> na.approx(m[, 1])
1970-01-02 1970-01-03 1970-01-04 1970-01-05 1970-01-06 1970-01-07 1970-01-08
         1          2          3          4          5          6          7


On Tue, May 11, 2010 at 4:48 PM, Abiel X Reinhart
<abiel.x.reinhart at jpmchase.com> wrote:
> I have two identically sized matrices of data that represent time series (I am storing the data in zoo objects, but the idea should apply to any matrix of data). The time series in the second matrix extend further than in the first matrix, and I would like to use the data in matrix 2 to extrapolate the data in matrix 1. In other words, if mat1[i,j] == NA, then mat1[i,j] <- mat1[i-1, j]*mat2[i,j]/mat2[i-1,j]. Of course, before we can calculate mat1[i,j] we may need to calculate mat1[i-1,j], and that in turn may require the computation of mat1[i-2,j], etc. This could all clearly be done with loops, but I am wondering if anyone can think of a vectorized expression or other efficient method that would work.
>
> Thanks very much.
>
> Abiel Reinhart
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