weird output when partitioning a matrix into lists in R - r

Question

I've been trying to write a function that takes in matrix (x) and a vector (cut.vec) and outputs a list, where each element of the list is a combination of some of the columns in the input matrix. Each element in the input vector is an index where want to partition the matrix. Then I want to save every partition to an element in a list and return that list.
Here's what I've got so far:
This is the actual function that I'm running:
make.cut <- function(x, cut.vec){
ran.once <- 0 #This checks for first run
out <- list() #This creates the output list
temp.matrix <- matrix() #For holding data
for(i in 1:length(cut.vec)){
for(n in 1:cut.vec[i]){
if(cut.vec[i]<n){
#Do nothing
}else{
hold <- x[,n]
if(ran.once != 0){
temp.matrix <- cbind(temp.matrix, hold)
}else{
temp.matrix <- hold
ran.once <- 1
}
}
}
out[[i]] <- temp.matrix
temp.matrix <- matrix()
}
return(out)
}
When I run this I get a list out, but only the first element is correct. Each element except the first one only contains one column of the input matrix.
**Example Input**
x<-matrix(c(341, 435, 834, 412, 245, 532.2, 683.4, 204.2, 562.7, 721.5, 149, 356, 112, 253, 211, 53, 92, 61, 84, 69), nrow=4)
x= 341 435 834 412 245
532.2 683.4 204.2 562.7 721.5
149 356 112 253 211
53 92 61 84 69
cut.vec = c(2, 3, 5)
out <- make.cut(x, cut.vec):
a <- out[[1]]
b <- out[[2]]
c <- out[[3]]
**Intended Output**
a= 341 435
532.2 683.4
149 356
53 92
b= 834
204.2
112
61
c= 412 245
562.7 721.5
253 211
84 69
**Actual Output**
a= 341 435
532.2 683.4
149 356
53 92
b= 435
683.4
356
92
c= 834
204.2
112
61
I can do this from the console manually, one element at a time and it works, but every time I try and do it with the make.cut function it breaks.
This is how I did it by hand in terminal:
cut.vec<-c(3, 5)
a<-x[,1]
b<-x[,2]
c<-x[,3]
temp <- cbind(a,b,c)
out[[1]] <- temp
cut.vec[2] is equal to 5
a<-x[,4]
b<-x[,5]
temp <- cbind(a,b)
out[[2]] <- temp
However, when I try to apply the same methodology in a function it breaks.

Answer 1

You can use the following approach to "cut" a matrix along a vector:
Example data:
mat <- matrix(1:16, nrow = 2)
# [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
# [1,] 1 3 5 7 9 11 13 15
# [2,] 2 4 6 8 10 12 14 16
cutvec <- c(2,5)
First, cut the column numbers of mat along cutvec:
cuts <- cut(seq(ncol(mat)), c(0, cutvec - 1))
Then you can create a list with subsets with tapply:
tapply(seq(ncol(mat)), cuts, function(x) mat[, x, drop = FALSE])
# $`(0,1]`
# [,1]
# [1,] 1
# [2,] 2
#
# $`(1,4]`
# [,1] [,2] [,3]
# [1,] 3 5 7
# [2,] 4 6 8