Ggtcf

I want to create a data frame from the last elements of a list. I have describing two cases here, one easy and one a bit complecated.

Easy case

Let's assume a list or two vectors, v

v <- list("22" = c(2, 3, 5), "23" = c("aa", "bb")) 
> str(v)
List of 2
 $ 22: num [1:3] 2 3 5
 $ 23: chr [1:2] "aa" "bb"

I want to have a data frame where in the first column I will have the element names (here 22 and 23) and in the second column I will have the last element of that vector.

I can write the following codes to generate what I want,

last_elems1 <- lapply(v, tail, n = 1L)
last_elems1 <- data.frame(last_elems1) 
tidyr::gather(last_elems1)

> tidyr::gather(last_elems1)
 key value
1 X22 5
2 X23 bb

I have the following questions here,

1. How can I avoid the "X"s in the "key" column, e.i, I want only number 22, not X22.


2. 
   

   Should I worry about the error message?

   
   
   

   Warning message:
   attributes are not identical across measure variables;
   they will be dropped

   
   

Slightly complecated example

Here I have list of two (or more) data frames with two vectors inside. The list can be generated as follows,

w <- list("22" = data.frame(a = c(2, 3, 5), 
 b = c(5, 6, 8)), 
 "23" = data.frame(a = c(9, 10), 
 b = c(11, 12))
 )
> str(w)
List of 2
 $ 22:'data.frame': 3 obs. of 2 variables:
 ..$ a: num [1:3] 2 3 5
 ..$ b: num [1:3] 5 6 8
 $ 23:'data.frame': 2 obs. of 2 variables:
 ..$ a: num [1:2] 9 10
 ..$ b: num [1:2] 11 12

I want to have to last element of the variable b from each data frame inside the list. I am using the same codes described above, and that gives me the following,

last_elems2 <- lapply(w, tail, n = 1L)
last_elems2 <- data.frame(last_elems2)
tidyr::gather(last_elems2)

> tidyr::gather(last_elems2)
 key value
1 X22.a 5
2 X22.b 8
3 X23.a 10
4 X23.b 12

Here are my questions,

3. How to have the value in column key like 22 instead of X22.b.


4. I want only the row 2 and row 4. That means, the output should be,

tidyr::gather(last_elems2)
 key value
1 22 8
2 23 12

Any idea how to fine tune what I am doing?

A base R option

stack(lapply(w, function(x) 
 x[dim(x)[1], "b"] # return last element of column "b"
))
# values ind
#1 8 22
#2 12 23

When we use dim, we assume that w does not contain vectors - as a vector does not have a dimensions attribute. You would need to change that part to x[length(x)] then.

data

w <- list("22" = data.frame(a = c(2, 3, 5), 
 b = c(5, 6, 8)), 
 "23" = data.frame(a = c(9, 10), 
 b = c(11, 12))
)