How to transform a data frame into a list in r?

问题

Could you please help me?

I want to make a list based on information contained in an R data frame. I found a solution, but it is not general.

For instance, let's start with this data frame:

df <- data.frame(vertices = paste(letters[1:20]), modules = rep(1:4, 5))

I want to use df$modules to turn the data frame into a list. The items of the list should contain data from df$vertices. So I found this solution:

list1 <- split(df, df$modules)

list2 <- vector(mode = "list", length = length(unique(df$modules)))
list2[[1]] <- as.vector(list1[[1]]$vertices)
list2[[2]] <- as.vector(list1[[2]]$vertices)
list2[[3]] <- as.vector(list1[[3]]$vertices)
list2[[4]] <- as.vector(list1[[4]]$vertices)

As you can see, it works, but it's not general, because you need to manually add so many lines to the code's end as length(unique(df$modules)).

I've also tried a while loop, but it didn't work:

i <- 1

while (i == length(list2)) {
    list2[[i]] <- as.vector(list1[[i]]$vertices)
    list2
}

Is there a general solution to this problem? Thank you!

回答1:

You're almost there:

split(df$vertices, df$modules)

回答2:

> lapply(split(df, df$modules), "[", , 1)
$`1`
[1] "a" "e" "i" "m" "q"

$`2`
[1] "b" "f" "j" "n" "r"

$`3`
[1] "c" "g" "k" "o" "s"

$`4`
[1] "d" "h" "l" "p" "t"

来源：https://stackoverflow.com/questions/64523750/how-to-transform-a-data-frame-into-a-list-in-r