对于循环将输出存储为列表，然后合并为矩阵

Question

I have a list of matrices I would like to first convert into individual vectors then combine them into one large matrix. I have been trying to implement a for loop to do this and have been unable to do so with any success.

Example data: 5 10x10 matrices in a list

m1 <- matrix(1:100, nrow = 10, ncol = 10)
m2 <- matrix(1:100, nrow = 10, ncol = 10)
m3 <- matrix(1:100, nrow = 10, ncol = 10)
m4 <- matrix(1:100, nrow = 10, ncol = 10)
m5 <- matrix(1:100, nrow = 10, ncol = 10)
mylist <- list(m1, m2, m3, m4, m5)

I can turn an individual matrix into a vector using the following:

unlist(mylist[1])

My for loop is as follows, and currently outputs them all into a single vector:

z = list()
for (i in mylist[length(mylist)]) {
    n <- unlist(mylist)
    z <- c(n)
} 

length(z)
[1] 500

The output I would like would be the df:

m1 <- unlist(mylist[1])
m2 <- unlist(mylist[2])
m3 <- unlist(mylist[3])
m4 <- unlist(mylist[4])
m5 <- unlist(mylist[5])
df <- cbind(m1, m2, m3, m4, m5)

Any help would be great! Thanks!

Answer 1

To use a for loop, I would do it like this:

z = list()
for (i in seq_along(mylist)) {
    z[[i]] = c(mylist[[i]])
} 

But when you're just applying a simple function to each list element, lapply is very nice:

z = lapply(mylist, c)

If you then want to cbind all of these vectors, we can either use Reduce or do.call :

do.call(what = cbind, args = z)
Reduce(f = cbind, x = z)

The do.call is equivalent to cbind(z[[1]], z[[2]], z[[3]], ...) , and the Reduce is equivalent to cbind(z[[1]], cbind(z[[2]], cbind(z[[3]], ...))) . Since cbind accepts any number of arguments, do.call is probably nicer. You would need Reduce if you wanted to add or multiply the vectors, for example.