for loops, eval(parse(text = paste0 ... into vectorised lists but keep getting error :attempt to select more than one element in vector Index

This was my original code, very messy and untidy:
Link to my previous question

library(dplyr); library(plyr)
library(magrittr); library(stringr) 
library(ExclusionTable)
library(lubridate)
library(tidyverse); library(tidyr)
library(janitor)
library(survival)
library(ggsurvfit); library(gtsummary)
library(zoo)
library(tidycmprsk)
# AA cohort (2 of 3)
## as
i=1
num_fu = c(1,2,3,4,5,6,7,8,9)
as <- data.frame()
df <- data.frame()
dfs <- data.frame()
data_dir <- 'C:/Users/thepr/Documents/data/as'
assign(paste0("flnames", i), list.files(path = paste0(data_dir, i), pattern = "\\.csv", full.names = TRUE))
assign(paste0("as", i, "_list"), lapply(get(paste0("flnames", i)),
function(x){base::as.data.frame(read.csv(x))}))
nm <- gsub(".csv", "", basename(eval(parse(text = paste0("flnames", i))))) %>% str_sub(., 1,6)
assign(paste0("as", i, "_list"), setNames(get(paste0("as", i, "_list")), nm))
df <- Reduce(full_join, get(paste0("as", i, "_list")))
assign(paste0("as",i), df[!duplicated(base::as.list(df))])
dfs <- df
for (i in 2:length(num_fu)){
RID_common <- as1$RID %in% get(paste0("as", i))$RID
assign(paste0("flnames", i), list.files(path = paste0(data_dir, i), pattern = "\\.csv", full.names = TRUE))
assign(paste0("as", i, "_list"), lapply(get(paste0("flnames", i)),
function(x){base::as.data.frame(read.csv(x))}))
nm <- gsub(".csv", "", basename(eval(parse(text = paste0("flnames", i))))) %>% str_sub(., 1,6)
assign(paste0("as", i, "_list"), setNames(get(paste0("as", i, "_list")), nm))
df <- Reduce(full_join, get(paste0("as", i, "_list")))
assign(paste0("as",i), df[!duplicated(base::as.list(df))])
dfs <- merge(dfs, df, by = "RID", all.x = TRUE)
dfs <- dfs[!duplicated(base::as.list(dfs))]
if(paste0("AS", i, "_AREA") %in% colnames(get(paste0("as", i)))){
assign(paste0("fu_",i-1), get(paste0("as", i))[RID_common, c("RID", paste0("AS", i, "_AREA"))])
assign(paste0("fu_loss_",i-1), get(paste0("as", i))[!RID_common, c("RID", paste0("AS", i, "_AREA"))])
# FU rate
assign(paste0("fu_rate_", i-1), nrow(get(paste0("as", i)))/nrow(as1))
}
else if(paste0("AS", i, "_DATA_CLASS") %in% colnames(get(paste0("as", i)))){
assign(paste0("fu_",i-1), get(paste0("as", i))[RID_common, c("RID", paste0("AS", i, "_DATA_CLASS"))])
assign(paste0("fu_loss_",i-1), get(paste0("as", i))[!RID_common, c("RID", paste0("AS", i, "_DATA_CLASS"))])
# FU rate
assign(paste0("fu_rate_", i-1), nrow(get(paste0("as", i)))/nrow(as1))
}
else{}
}

After helpful comments by @Gregor Thomas @joran, I read previous posts and finally started using lists and vectors. Here is what I have tried so far:

library(tidyverse) #Includes: dplyr, stringr, tidyr
library(magrittr)
library(lubridate)
library(ExclusionTable)
library(janitor)
library(survival)
library(ggsurvfit); library(gtsummary)
library(zoo)
library(tidycmprsk)
# AA cohort (2 of 3)
## as
i=1
data_dir = c("C:/Users/thepr/Documents/data/as")
num_fu = c(1,2,3,4,5,6,7,8,9)
dirs <- paste0(data_dir, num_fu) # character
as <- data.frame()
df <- data.frame()
dfs <- data.frame()
flnames <- list.files(path = dirs, pattern = "\\.csv", full.names = TRUE)
as_list[[num_fu]] <- lapply(flnames[[num_fu]],
function(x){base::as.data.frame(read.csv(x))})
names(as_list) <- gsub(".csv", "", basename(flnames[[num_fu]])) %>% str_sub(., 1,6)
df <- Reduce(full_join, as_list)
df <- df[!duplicated(base::as.list(df))]

somehow I keep getting error messages:
Error in flnames[[num_fu]] :
attempt to select more than one element in vectorIndex

Based on How do I make a list of data frames?, I think I am headed in the right direction. Please give some insights and thougths. Will be appreciated, thanks.

[[ always selects exactly one element. This is what the error message is trying to tell you.

What you want to do (based on your num_fu value) is to slice the list to extract a subset. You need to use [ to do that.

Furthermore, remove the indexing in the assignment target. That is, write:

as_list = lapply(flnames[num_fu], read.csv)

However, it's unclear that you actually want the indexing here at all, since it does not correspond to anything that I understand. Don't you simply want the following instead?

as_list = lapply(flnames[num_fu], read.csv)

Alternatively, if you want the entire thing in a nested list, where each subdirectory has its own sub-list, there are several solutions. One would be the following (note the nested invocation of lapply):

flnames = lapply(dirs, list.files, pattern = "\\.csv", full.names = TRUE)
as_list = lapply(
flnames,
function (subdir) {
setNames(
lapply(subdir, read.csv),
sub("\\.csv$", "", basename(subdir)) %>% str_sub(., 1,6)
)
}
)

etc.

However, I would instead recommend keeping the structure flat. And, especially if you want to subsequently merge the entire thing into one single data.frame, you can drastically simplify the entire code as follows:

data_dir = "C:/Users/thepr/Documents/data/as"
num_fu = 1 : 9
dirs = paste0(data_dir, num_fu)
files = list.files(dirs, pattern = "\\.csv$", full.names = TRUE)
df = purrr::map_dfr(files, read.csv, .id = "Filename") %>%
mutate(Filename = str_sub(sub("\\.csv$", "", basename(Filename)), 1, 6))

(Incidentally, avoid mixing = and <- for assignment, it makes the code messy; you can use either one, just don’t mix them.)