R: creating a matrix with unknown number of rows - r

Question

I have written the code below to generate a matrix containing what is, to me, a fairly complex pattern. In this case I determined that there are 136 rows in the finished matrix by trial and error.
I could write a function to calculate the number of matrix rows in advance, but the function would be a little complex. In this example the number of rows in the matrix = ((4 * 3 + 1) + (3 * 3 + 1) + (2 * 3 + 1) + (1 * 3 + 1)) * 4.
Is there an easy and efficient way to create matrices in R without hard-wiring the number of rows in the matrix statement? In other words, is there an easy way to let R simply add a row to a matrix as needed when using for-loops?
I have presented one solution that employs rbind at each pass through the loops, but that seems a little convoluted and I was wondering if there might be a much easier solution.
Sorry if this question is redundant with an earlier question. I could not locate a similar question using the search feature on this site or using an internet search engine today, although I think I have found a similar question somewhere in the past.
Below are 2 sets of example code, one using rbind and the other where I used trial and error to set nrow=136 in advance.
Thanks for any suggestions.
v1 <- 5
v2 <- 2
v3 <- 2
v4 <- (v1-1)
my.matrix <- matrix(0, nrow=136, ncol=(v1+4) )
i = 1
for(a in 1:v2) {
for(b in 1:v3) {
for(c in 1:v4) {
for(d in (c+1):v1) {
if(d == (c+1)) l.s = 4
else l.s = 3
for(e in 1:l.s) {
my.matrix[i,c] = 1
if(d == (c+1)) my.matrix[i,d] = (e-1)
else my.matrix[i,d] = e
my.matrix[i,(v1+1)] = a
my.matrix[i,(v1+2)] = b
my.matrix[i,(v1+3)] = c
my.matrix[i,(v1+4)] = d
i <- i + 1
}
}
}
}
}
my.matrix2 <- matrix(0, nrow=1, ncol=(v1+4) )
my.matrix3 <- matrix(0, nrow=1, ncol=(v1+4) )
i = 1
for(a in 1:v2) {
for(b in 1:v3) {
for(c in 1:v4) {
for(d in (c+1):v1) {
if(d == (c+1)) l.s = 4
else l.s = 3
for(e in 1:l.s) {
my.matrix2[1,c] = 1
if(d == (c+1)) my.matrix2[1,d] = (e-1)
else my.matrix2[1,d] = e
my.matrix2[1,(v1+1)] = a
my.matrix2[1,(v1+2)] = b
my.matrix2[1,(v1+3)] = c
my.matrix2[1,(v1+4)] = d
i <- i+1
if(i == 2) my.matrix3 <- my.matrix2
else my.matrix3 <- rbind(my.matrix3, my.matrix2)
my.matrix2 <- matrix(0, nrow=1, ncol=(v1+4) )
}
}
}
}
}
all.equal(my.matrix, my.matrix3)

Answer 1

If you have some upper bound on the size of the matrix,
you can create a matrix
large enough to hold all the data
my.matrix <- matrix(0, nrow=v1*v2*v3*v4*4, ncol=(v1+4) )
and truncate it at the end.
my.matrix <- my.matrix[1:(i-1),]

Answer 2

This is the generic form to do it. You can adapt it to your problem
matrix <- NULL
for(...){
...
matrix <- rbind(matriz,vector)
}
where vector contains the row elements

Answer 3

I stumbled upon this solution today: convert the matrix to a data.frame. As new rows are needed by the for-loop those rows are automatically added to the data.frame. Then you can convert the data.frame back to a matrix at the end if you want. I am not sure whether this constitutes something similar to iterative use of rbind. Perhaps it becomes very slow with large data.frames. I do not know.
my.data <- matrix(0, ncol = 3, nrow = 2)
my.data <- as.data.frame(my.data)
j <- 1
for(i1 in 0:2) {
for(i2 in 0:2) {
for(i3 in 0:2) {
my.data[j,1] <- i1
my.data[j,2] <- i2
my.data[j,3] <- i3
j <- j + 1
}
}
}
my.data
my.data <- as.matrix(my.data)
dim(my.data)
class(my.data)
EDIT: July 27, 2015
You can also delete the first matrix statement, create an empty data.frame then convert the data.frame to a matrix at the end:
my.data <- data.frame(NULL,NULL,NULL)
j <- 1
for(i1 in 0:2) {
for(i2 in 0:2) {
for(i3 in 0:2) {
my.data[j,1] <- i1
my.data[j,2] <- i2
my.data[j,3] <- i3
j <- j + 1
}
}
}
my.data
my.data <- as.matrix(my.data)
dim(my.data)
class(my.data)