Function to split a matrix into sub-matrices in R

by

If you have a 16×12 array like this

mb <- structure(c("a1", "a2", "a3", "a4", "e1", "e2", "e3", "e4", "i1", 
"i2", "i3", "i4", "m1", "m2", "m3", "m4", "a5", "a6", "a7", "a8", 
"e5", "e6", "e7", "e8", "i5", "i6", "i7", "i8", "m5", "m6", "m7", 
"m8", "a9", "a10", "a11", "a12", "e9", "e10", "e11", "e12", "i9", 
"i10", "i11", "i12", "m9", "m10", "m11", "m12", "b1", "b2", "b3", 
"b4", "f1", "f2", "f3", "f4", "j1", "j2", "j3", "j4", "n1", "n2", 
"n3", "n4", "b5", "b6", "b7", "b8", "f5", "f6", "f7", "f8", "j5", 
"j6", "j7", "j8", "n5", "n6", "n7", "n8", "b9", "b10", "b11", 
"b12", "f9", "f10", "f11", "f12", "j9", "j10", "j11", "j12", 
"n9", "n10", "n11", "n12", "c1", "c2", "c3", "c4", "g1", "g2", 
"g3", "g4", "k1", "k2", "k3", "k4", "o1", "o2", "o3", "o4", "c5", 
"c6", "c7", "c8", "g5", "g6", "g7", "g8", "k5", "k6", "k7", "k8", 
"o5", "o6", "o7", "o8", "c9", "c10", "c11", "c12", "g9", "g10", 
"g11", "g12", "k9", "k10", "k11", "k12", "o9", "o10", "o11", 
"o12", "d1", "d2", "d3", "d4", "h1", "h2", "h3", "h4", "l1", 
"l2", "l3", "l4", "p1", "p2", "p3", "p4", "d5", "d6", "d7", "d8", 
"h5", "h6", "h7", "h8", "l5", "l6", "l7", "l8", "p5", "p6", "p7", 
"p8", "d9", "d10", "d11", "d12", "h9", "h10", "h11", "h12", "l9", 
"l10", "l11", "l12", "p9", "p10", "p11", "p12"), .Dim = c(16L, 
12L))

You can define matsplitter as

matsplitter<-function(M, r, c) {
    rg <- (row(M)-1)%/%r+1
    cg <- (col(M)-1)%/%c+1
    rci <- (rg-1)*max(cg) + cg
    N <- prod(dim(M))/r/c
    cv <- unlist(lapply(1:N, function(x) M[rci==x]))
    dim(cv)<-c(r,c,N)
    cv
}

Then 

matsplitter(mb,4,3)

will return (output clipped)

, , 1

     [,1] [,2] [,3] 
[1,] "a1" "a5" "a9" 
[2,] "a2" "a6" "a10"
[3,] "a3" "a7" "a11"
[4,] "a4" "a8" "a12"

, , 2

     [,1] [,2] [,3] 
[1,] "b1" "b5" "b9" 
[2,] "b2" "b6" "b10"
[3,] "b3" "b7" "b11"
[4,] "b4" "b8" "b12"

, , 3

     [,1] [,2] [,3] 
[1,] "c1" "c5" "c9" 
[2,] "c2" "c6" "c10"
[3,] "c3" "c7" "c11"
[4,] "c4" "c8" "c12"

...

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