使用索引在R中的循环后命名矩阵

Question

Please excuse my naive question.

I have a loop which returns in every step, a matrix b . I would like to save each matrix from each loop, under a different name depending on the iteration number. For example, at the end of the first iteration, I want to get the matrix named b1 , at the end of the second iteration the b2 etc...

As an example, lets use the following code:

count=0
a=matrix(c(1,2,3,4,5,6,7,8,9,10,11,12), nrow=6)
for (count in 1:10)  {
  b<-cbind(a[,1],matrix(c( a[sample(nrow(a)),2]), nrow=nrow(a)) ) 
  print(b)
}
count+1

Here, the original matrix is matrix a which has 6 rows and 2 columns. I permute the order of the elements in the second column. The resulting matrix b , is the matrix that conatins as first column the first column of the original matrix a and as second column the permuted second column of a .

Can anyone help me?

Answer 1

You really don't want to store these as separate variables- it would be much better to keep them as a list of 10 matrices. That could be done very easily using replicate :

lst = replicate(10, cbind(a[,1],matrix(c( a[sample(nrow(a)),2]), nrow=nrow(a)) ),
          simplify=FALSE)

You can then access any of the 10 matrices like this:

lst[[1]]
#     [,1] [,2]
#[1,]    1    7
#[2,]    2   10
#[3,]    3   11
#[4,]    4    8
#[5,]    5    9
#[6,]    6   12

Similarly, you could loop over them like this:

for (m in lst) {
    print(m)
    # do something with your matrix m
}

Answer 2

As told before, list is a better option. But, if you still want to save each interation on different variables, you can use assign()

count=0
a=matrix(c(1,2,3,4,5,6,7,8,9,10,11,12), nrow=6)
for (count in 1:10)  {
  assign(paste('b',count,sep=''),cbind(a[,1],matrix(c( a[sample(nrow(a)),2]), nrow=nrow(a))))
}
b1
b2

Answer 3

As said in other option it is better to use a list. Here a version using sapply to get pretty named result:

 res <- sapply(paste('b',1:10,sep=''), 
           function(x) cbind(a[,1],matrix(c( a[sample(nrow(a)),2]), nrow=nrow(a)) ),
       simplify=F)

Then To get matrix b5 for example,

res$b5

   [,1] [,2]
[1,]    1    9
[2,]    2    7
[3,]    3    8
[4,]    4   11
[5,]    5   10
[6,]    6   12