extract a column from a matrix in a list in R

Question

I have a list of 100 elements. Each element is a list. And this list has two elements in them. One is just a key another is a matrix. This matrix has 5 columns in it. I am trying to extract one column from each of these matrices for all the elements. So basically I would have 100 matrices again as an element of the list. This is how the list looks like ``` a[[1]] [[1]] [1] "keyid" [[2]] [,1] [,2] [,3] [,4] [,5] [1,] "1329397986.572332" "x" "y" "z" "a" [2,] "1329242747.368294" "x" "y" "z" "a" [3,] "1329194224.351232 "x" "y" "z" "a" ``` So what you can see here is a is a list and it has 100 elements. Each element is a list of 2 elements. I am currently interested only in the second element. The second element has a matrix. I would like to extract the first column of this matrix. Just to see if I can extract the column into a vector I can do ``` a[[1]][[2]][,1] ``` and I get all the values as a vector, as follows. ``` a[[1]][[2]][,1] [1] "1329397986.572332" "1329242747.368294" "132919422…

Accepted Answer

This seems a perfect case for the little-used function `rapply`.

Until a reproducible example is posted, I will post this as a possible solution

```
set.seed(1)
# a possible reproducible example
a <- replicate(5, list('a', matrix(runif(10), ncol=5)), simplify = FALSE)

# the first column
a1 <- rapply(a, classes = 'matrix', how = 'list', f = function(x) x[, 1, drop = FALSE]) 
# a second `lapply` is required to drop `NULL` entries

a1Only <- lapply(a1, Filter, f = Negate(is.null))
```

extract a column from a matrix in a list in R

Solution

Problem