How to access very first object in differently deep nested lists?

Question

I need to access the first element of a list. The problem is that the lists vary in the way how deep they are nested. Here is an example:

list1 <- list(ts(1:100),
              list(1:19,
                   factor(letters)))

list2 <- list(list(list(ts(1:100), data.frame(a= rnorm(100))),
                   matrix(rnorm(10))),
              NA)

My expected output is to get the time seriests(1:100) for both lists, i.e. list1[[1]] and list2[[1]][[1]][[1]]. I've tried different stuff, among others lapply(list2, `[[`, 1) which here does not work here.

Answer 1

Another base R solution - you could do this with a recursive function:

list1 <- list(ts(1:100),
              list(1:19,
                   factor(letters)))

list2 <- list(list(list(ts(1:100), data.frame(a= rnorm(100))),
                   matrix(rnorm(10))),
              NA)


recursive_fun <- function(my_list) {
  
  if (inherits(my_list, 'list')) {
    Recall(my_list[[1]])
  } else {
    my_list
  }
  
}

Output:

> recursive_fun(list1)
Time Series:
Start = 1 
End = 100 
Frequency = 1 
  [1]   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30
 [31]  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60
 [61]  61  62  63  64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79  80  81  82  83  84  85  86  87  88  89  90
 [91]  91  92  93  94  95  96  97  98  99 100

> recursive_fun(list2)
Time Series:
Start = 1 
End = 100 
Frequency = 1 
  [1]   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30
 [31]  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60
 [61]  61  62  63  64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79  80  81  82  83  84  85  86  87  88  89  90
 [91]  91  92  93  94  95  96  97  98  99 100