Applying a function to a specific element of a list of a list

Question

I have a function that returns a list of vectors and matrices. I then create a variable that is a list of several of the resulting lists from calls to the function. So I have a list of lists. My question is how do I apply a function over the elements of these lists (note this is not the same as applying a function over the lists themselves). Here is a simple example that retains all the essential features of what I am doing ``` numtrials = 5 x = rep(list(NULL),numtrials) testfunction = function(){return( list( c(1,2,3,4,5), matrix(runif(10), 2,5), matrix(0,2,2) ) )} for(index in 1:numtrials){ x[[index]] = testfunction() } ``` I want to now calculate the mean of say the (2,3) element of `x[[index]][[2]]` across all "index" lists. Or even better get a matrix of means, `xbar`, such that `xbar[i,j] = mean(x[[]][[2]][i,j])`. I tried to play around with (and of course read the help file for) `lapply`, and `apply`, but couldn't get it to work. One of the reasons is that `x[[]][[2]][i,j]` app…

Accepted Answer

Using `abind` you can create a list which contains arrays for the relevent components of the internal lists..

eg

```
library(abind)

xl <- do.call(mapply, c('abind', x, rev.along = 0))

# the second element from each inner list is now within a 3-d array 
# which is the 2nd element of  xl

# you can now construct your matrix of mean values by using `apply`

means <- apply(xl[[2]], 1:2, mean)
means
##            [,1]      [,2]      [,3]      [,4]      [,5]
## [1,] 0.4576039 0.5185270 0.7099742 0.3812656 0.4529965
## [2,] 0.6528345 0.2304651 0.5534443 0.4404609 0.7361132
```

Applying a function to a specific element of a list of a list

Solution

Problem