[R] creating matrices from vectors

Erik Iverson iverson at biostat.wisc.edu
Sat Nov 15 17:56:09 CET 2008

James Rudge wrote:
> If I have 4 vectors (a, b, c, and d) each of length 1000, how do I then create 1000 two by two matrices from these vectors, such that:
> myMatrix[i] = matrix(c(a[i],b[i],c[i],d[i]),2)
> Then I'd like to create a single vector containing the largest eigenvalues of each matrix?
> (Sorry I am quite new to R)
> Many thanks,
> James

Something like this might get you started:

## use mapply to construct the matrices, returns list of matrices
m1 <- mapply(function(...) matrix(as.numeric(list(...)), nrow = 2,
       byrow = TRUE), a, b, c, d, SIMPLIFY = FALSE)

## apply the eigen function to the list
m2 <- lapply(m1, eigen, only.values = TRUE)

## the largest eigenvalue is in the first slot
sapply(sapply(m2, "[", "values"), "[", 1)

Where a, b, c, d are your vectors.  I tried this on a small example, 
with vectors of length 10, seemed to work.  In general, watch for naming 
objects 'c', as that is the name of an important built-in R function.

The apply functions can help a lot in cases like these, see their 
respective help pages.

Hope this helps,

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