build matrix in a for loop automatically in R

Question

Suppose I have a code like this

probv=c(0.5,0.1,0.2,0.3)
N=c(1,2,3,4)

g1=matrix(rbinom(n = 10, size = N[1], prob = probv[1]), nrow=5)
g2=matrix(rbinom(n = 10, size = N[2], prob = probv[2]), nrow=5)
g3=matrix(rbinom(n = 10, size = N[3], prob = probv[3]), nrow=5)
g4=matrix(rbinom(n = 10, size = N[4], prob = probv[4]), nrow=5)

I want to use a for loop for i in (1:J) {......} J=4 in this case use one line function to return the same output like this, I want to know how I create a matrix g_ in the loop which is also benefit for me when I increase the length of my vector into 5,6,7...... for example N=c(1,2,3,4,5) probv=c(0.5,0.1,0.2,0.3,0.5) I do not change my code to create another matrix called g5.The code can create it and I just need to change my input to achieve my goal

Thanks Akrun

what is my N is a three dimensional array, I want to map the last dimension of it? How to change in the map method?

probv=c(0.5,0.1,0.2,0.3)
N=array(1:24,c(3,2,4))

g1=matrix(rbinom(n = 10, size = N[,,1], prob = probv[1]), nrow=5)
g2=matrix(rbinom(n = 10, size = N[,,2], prob = probv[2]), nrow=5)
g3=matrix(rbinom(n = 10, size = N[,,3], prob = probv[3]), nrow=5)
g4=matrix(rbinom(n = 10, size = N[,,4], prob = probv[4]), nrow=5)

Here, the lengths of 'probv' is 5 and last dimension of N is 4 — akrun
– akrun, Commented Jul 2, 2020 at 20:47
thanks, the first method is cool, and the second method looks like just produce 1 2by20 matrix — hard worker
– hard worker, Commented Jul 2, 2020 at 20:54
lst2 is a list of matrices (5 x 2) and the length of the list is 4. When we rbind, we get the 20 x2 because we are rbinding the list elements. I was not sure whether you need a single matrix of a list of matrices — akrun
– akrun, Commented Jul 2, 2020 at 20:56
updated by assigning names to the list elements so that you can get the list elements by name. — akrun
– akrun, Commented Jul 2, 2020 at 20:59

akrun · Accepted Answer · 2020-07-02 20:59:09Z

1

We can use Map to loop over the 'N' and 'probv' vector, get the corresponding values into rbinom and create a matrix. It returns a list of matrices

lst1 <- Map(function(x, y) matrix(rbinom(n = 10,
        size = x, prob = y), nrow = 5), N, probv)

Or using for loop

lst2 <- vector('list', length(N))
for(i in seq_along(N)) {
     lst2[[i]] <- matrix(rbinom(n = 10, size = N[i], prob = probv[i]), nrow = 5)
   }
 names(lst2) <- paste0("g", seq_along(lst2))

For the updated question to extract from an array

mnLength <- min(length(probv), dim(N)[3])
lst2 <- vector('list', mnLength)
for(i in seq_len(mnLength)) {
     lst2[[i]] <- matrix(rbinom(n = 10, size = N[,,i], prob = probv[i]), nrow = 5)
   }

names(lst2) <- paste0("g", seq_along(lst2))
lst2$g1
lst2$g2

edited Jul 2, 2020 at 20:59

answered Jul 2, 2020 at 20:10

akrun

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4 Comments

hard worker Over a year ago

thanks what if my N is an array and I want to map the last dimension of it.

hard worker Over a year ago

```probv=c(0.5,0.1,0.2,0.3,0.5) N=array(1:24,c(3,2,4)) g1=matrix(rbinom(n = 10, size = N[,,1], prob = probv[1]), nrow=5) g2=matrix(rbinom(n = 10, size = N[,,2], prob = probv[2]), nrow=5) g3=matrix(rbinom(n = 10, size = N[,,3], prob = probv[3]), nrow=5) g4=matrix(rbinom(n = 10, size = N[,,4], prob = probv[4]), nrow=5)

hard worker Over a year ago

For the second method, could you change it to produce 4 matrices ,instead of 1 matrix?thanks

akrun Over a year ago

@hardworker For the second, just use lst2. It is a list of 4 matrices. you don't need to rbind it to create mat1. Or convert to an array if you want an array

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