r - How to extract matrix object from list without being a list anymore?

You could use x[['b']] or x$b

x = list(a = matrix(sample(1:5,4) , nrow=2, ncol=2),
         b = matrix(sample(5:10,4) , nrow=2, ncol=2),
         c = matrix(sample(10:15,4) , nrow=2, ncol=2))
x[['b']]
     [,1] [,2]
[1,]    6   10
[2,]    9    7
x$b
     [,1] [,2]
[1,]    6   10
[2,]    9    7

Here is the microbenchmark between the 2 solutions :

microbenchmark(x[['b']],x$b)
Unit: nanoseconds
     expr min  lq   mean median   uq   max neval cld
 x[["b"]] 351 701 756.80    701  701  3851   100   a
      x$b 700 701 942.33    701 1050 15400   100   a