Appending/Growing List Elements Separately in r - r

Question

I have a loop with an embedded function inside, which creates a list with the same elements but different length each time. I want the created list of elements to grow or merge in each loop. Here is a very simplified visualization of it:
# Code Body-------------------------------------------------------------
desiredList <- list()
for (i in 1:3){
# "existingList" has three dataframes of A, B embedded
# A is a dataframe with x1 and x2 columns
# B is a vector
# A, B values are calculated using "somefunction" &
# in each loop their lenght differ
existingList <- somefunction(variable[i])
# "desiredList" should also have three dataframes of A, B
# In each loop, gererated Ai, Bi append to A, B elements of "desiredList"
desiredList <- append(desiredList, (existingList))
}
# existingList in each loop--------------------------------------------
# i=1................................
A:'data.frame': 3 obs. of 2 variables:
..$ x1: num [1:3] 1 2 3
..$ x2: num [1:3] 13 26 39
B:'data.frame': 1 obs. of 1 variables:
..$ b: num [1:1] 2.6
# i=2................................
A:'data.frame': 2 obs. of 2 variables:
..$ x1: num [1:2] 4 5
..$ x2: num [1:2] 52 65
B:'data.frame': 3 obs. of 1 variables:
..$ b: num [1:3] 5.2 7.8 10.4
# i=3................................
A:'data.frame': 5 obs. of 2 variables:
..$ x1: num [1:5] 6 7 8 9 10
..$ x2: num [1:5] 78 91 104 117 130
B:'data.frame': 2 obs. of 1 variables:
..$ b: num [1:2] 13 15.6
# desiredList at the end of the loop
A:'data.frame': 10 obs. of 2 variables:
..$ x1: num [1:10] 1 2 3 4 5 6 7 8 9 10
..$ x2: num [1:10] 13 26 39 52 65 78 91 104 117 130
B:'data.frame': 6 obs. of 1 variables:
..$ b: num [1:6] 2.6 5.2 7.8 10.4 13 15.6
I have tried "append", "lapply", "Map", and bunches of other functions. However, none gives the correct answer.

Answer 1

Your desired output requires "row-binding" the dataframes, as in:
library(tibble)
a1 <- tribble(
~A1, ~B1, ~C1,
1, 13 , 2.6,
2, 26 , 5.2,
3, 39 , 7.8,
7, 91 , 18.2,
8, 104, 20.8
)
a2 <- tribble(
~A2, ~B2,~C2,
4, 52, 10.4,
5, 65, 13,
6, 78, 15.6
)
a3 <- tribble(
~A3, ~B3, ~C3,
9, 117, 23.4,
10, 130, 26
)
out <- lapply(
X = list(a1, a2, a3),
FUN = function(x) `names<-`(x, substr(names(x), 1,1))
)
do.call("rbind", out)
#> # A tibble: 10 x 3
#> A B C
#> <dbl> <dbl> <dbl>
#> 1 1 13 2.6
#> 2 2 26 5.2
#> 3 3 39 7.8
#> 4 7 91 18.2
#> 5 8 104 20.8
#> 6 4 52 10.4
#> 7 5 65 13
#> 8 6 78 15.6
#> 9 9 117 23.4
#> 10 10 130 26
dplyr::bind_rows(out)
#> # A tibble: 10 x 3
#> A B C
#> <dbl> <dbl> <dbl>
#> 1 1 13 2.6
#> 2 2 26 5.2
#> 3 3 39 7.8
#> 4 7 91 18.2
#> 5 8 104 20.8
#> 6 4 52 10.4
#> 7 5 65 13
#> 8 6 78 15.6
#> 9 9 117 23.4
#> 10 10 130 26
Created on 2020-07-21 by the reprex package (v0.3.0)
If you have control of the data frame names and can avoid appending numbers to them, you can row bind them more easily. dplyr::bind_rows() is also an easy way to bind them.

Answer 2

One way to solve this is by using the "Map" function. The only problem with Map is that it can not merge an empty list with a list containing elements. This can be solved by applying an if-else statement. When the loop is running for the first time, the empty list is set to the existing list and for the remaining loops, the "Map" function is applied to update the desired list.
# Code Body-------------------------------------------------------------
desiredList <- list()
for (i in 1:3){
# "existingList" generated using an external function
existingList <- somefunction(variable[i])
# "desiredList" generation
if (i == 1){ # define the list in first loop
desiredList <- existingList
} else { # append the list
desiredList <- Map(rbind, desiredList, existingList)
}
}