Convert from factor to numeric a column in data.frames within a list

Question

Here an example of my two lists and relative code:

df1 = data.frame(a = c(1,1,1,2,3,3,4,4,5,6,6,7,8,9,9,10))
df2 = data.frame(a = c(1,2,2,2,3,4,5,5,6,6,7,8,9,9,10,10,11))

lst = list(df1, df2)

lst_table = lapply(lst, function(x) data.frame(table(x$a))) 

> class(lst_table[[1]]$Var1)
[1] "factor"
> class(lst_table[[2]]$Var1)
[1] "factor"

Because of my code purpose I need the column Var1 within each data.frame in the list to be a numeric vector.

Starting from How to convert a factor to an integer\numeric without a loss of information? I applied the following code to the single data.frames and it works:

> df1$a = as.numeric(levels(df1$a))[df1$a]
> df2$a = as.numeric(levels(df2$a))[df2$a]

> class(df1$a)
[1] "numeric"
> class(df2$a)
[1] "numeric"

But how can I apply the above to a list?

I tried:

lst_table = lapply(lst_table, function(y) {y$Var1 = as.numeric(levels(y$Var1))[y$Var1]})

but it does not work.

Any suggestion? Thanks

will you always have the same amount of numbers in each array or could they both be 5 then later both be 10? — mast3rd3mon
– mast3rd3mon, Commented Mar 16, 2017 at 13:34
my real data.frames within my real list differ in length and type of observations. thanks — aaaaa
– aaaaa, Commented Mar 16, 2017 at 13:36
ok, will they be the same length as each other or can the length of each be different at the same time? — mast3rd3mon
– mast3rd3mon, Commented Mar 16, 2017 at 13:37
no, their length can be different at the same time, e.g. df1 = 22380 rows and df2 = 11654 rows, and the list contains df1 and df2. — aaaaa
– aaaaa, Commented Mar 16, 2017 at 13:39

MeetMrMet · Accepted Answer · 2017-03-16 13:44:36Z

2

I think the problem is that your function in your second lapply is only returning the vector of the numeric factor levels, not your entire data.frame. I believe the following should work:

foo <- function(y) {
  y$Var1 <- as.numeric(levels(y$Var1))[y$Var1]
  return(y)
}

lst_table <- lapply(lst_table, foo)

answered Mar 16, 2017 at 13:44

MeetMrMet

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