Average of Two Matrices

Well, you're rather stuck writing them all out unless you have them in
some other data structure.

## three dimensional array (50 4 x 4 matrices)
x <- array(rnorm(16 * 50), dim = c(4, 4, 50))

apply(x, 1:2, mean)

[,1]        [,2]         [,3]        [,4]
[1,] -0.09460574  0.01572077 -0.166194625 -0.28176540
[2,] -0.26649044 -0.23546332 -0.003559647 -0.09857819
[3,]  0.12657883  0.10979631  0.368414189 -0.13653997
[4,] -0.02827496  0.07634844 -0.088057890  0.06208034
## list of length 50 each element is 4 x 4 matrix

y <- rep(list(matrix(rnorm(16), 4, 4)), 50)
length(y)

[1] 50

Reduce(`+`, y)/length(y)

[,1]        [,2]        [,3]       [,4]
[1,] -1.8334660  0.67812999  0.20159314  1.6163501
[2,] -0.6716821 -0.23942474 -0.50482638 -0.5765309
[3,]  0.7991496  0.38674047 -0.02128386 -1.1868209
[4,] -1.1654429  0.06773386  0.13538268  0.3847375

See ?array ?apply ?Reduce for documentation

Hope this helps,

Josh

What if you have over 50 matrices and you don't want to write them all out
one-by-one? ?I know there's something really quite simple, but I haven't
found it yet!...

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Joshua Wiley
Ph.D. Student, Health Psychology
Programmer Analyst II, ATS Statistical Consulting Group
University of California, Los Angeles
https://joshuawiley.com/