R: Recalling variables dynamically within loop - r

Question

I need to recall the variable within the loop and the use that as my column name:
This is an example dataset:
mtcars.df <- mtcars
Expected output:
mtcars.df <- mtcars.df %>% add_column(a1 = sample (1:4, 32, replace = TRUE), b1 = sample (1:4, 32, replace = TRUE), c1 = sample (1:4, 32, replace = TRUE))
So a1, b1 and c1 need to be created within the loop and renamed to a1, b1 and c1. The names are longer (and different combinations) in the original dataset but this is defined in variable mpg.filename.
This is what I have tried so far: The 1st obviously doesn't give the desired result but it's just to show what I want to achieve.
mpg.filename <- c("a1.file", "b1.file", "c1.file")
for (i in mpg.filename) {
sample.name <- unlist(strsplit(as.character(i), '.', fixed = TRUE))[1]
mtcars.df$i <- sample (1:4, 32, replace = TRUE)
}
for (i in mpg.filename) {
sample.name <- unlist(strsplit(as.character(i), '.', fixed = TRUE))[1]
mtcars.df$temp.var <- sample (1:4, 32, replace = TRUE)
temp.name <- paste0 (sample.name) %>% rlang::parse_expr()
mtcars.df <- mtcars.df %>% rename (eval (sample.name) = temp.var)
}
for (i in mpg.filename) {
sample.name <- unlist(strsplit(as.character(i), '.', fixed = TRUE))[1]
mtcars.df$temp.var <- sample (1:4, 32, replace = TRUE)
temp.name <- paste0 (sample.name) %>% rlang::parse_expr()
mtcars.df <- mtcars.df %>% rename (syms(sample.name) = temp.var)
}
I have tried get, as.symbol, parse (text = "sample.name") as well but didn't work either.
Thanks for the help. I have tried looking at other answers on forums but they do not seem to apply or work.
using eval() on string to access object attributes in R
call columns from inside a for loop in R
Getting strings recognized as variable names in R
How to evaluate an expression with variables in R?

Answer 1

Your first attempt was close! When you want to access / create columns in a data.frame using a character, you need to use [[ instead of $. No need for symbols / parsing / other complicated nonsense if this is all you need to do.
for(i in mpg.filename) {
sample.name <- unlist(strsplit(as.character(i), '.', fixed = TRUE))[1]
mtcars.df[[sample.name]] <- sample(1:4, 32, replace=TRUE)
}

Answer 2

Let me know if this did the trick-
mpg.filename <- c("a1.file", "b1.file", "c1.file")
for (i in 1:length(mpg.filename)) {
sample.name <- unlist(strsplit(as.character(mpg.filename[i]), '.', fixed = TRUE))[1]
mtcars.df$i <- sample (1:4, 32, replace = TRUE)
colnames(mtcars.df)[length(mtcars.df)]<-paste(sample.name)
}