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

February 2, 2022

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.

>Solution :

You can use rrapply::rrapply:

library(rrapply)
firstList1 <- rrapply(list1, how = "flatten")[[1]]
firstList2 <- rrapply(list2, how = "flatten")[[1]]

all.equal(firstList1, firstList2)
# [1] TRUE

output

> rrapply(list1, how = "flatten")[[1]]

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]  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52
 [53]  53  54  55  56  57  58  59  60  61  62  63  64  65  66  67  68  69  70  71  72  73  74  75  76  77  78
 [79]  79  80  81  82  83  84  85  86  87  88  89  90  91  92  93  94  95  96  97  98  99 100