The codes for producing sample dataset and converting from character to numeric is as below:
ff = data.frame(a = c('1','2','3'),b = 1:3, c = 5:7)
#data.frame is a type of list.
fff = list(ff,ff,ff,ff)
k = fff %>% map(~map(.x,function(x){x['a'] %<>% as.numeric
return(x)}))
However, the result is something like this...: There are 3 lists appear in each of the nested list ==> 33 = 9, which is very strange. I think the result should have 3 lists in a nested list.==> 31 = 3
what I want is to convert every a in each dataframe to be numeric.
> k
[[1]]
[[1]]$a
a
"1" "2" "3" NA
[[1]]$b
a
1 2 3 NA
[[1]]$c
a
5 6 7 NA
[[2]]
[[2]]$a
a
"1" "2" "3" NA
[[2]]$b
a
1 2 3 NA
[[2]]$c
a
5 6 7 NA
[[3]]
[[3]]$a
a
"1" "2" "3" NA
[[3]]$b
a
1 2 3 NA
[[3]]$c
a
5 6 7 NA
[[4]]
[[4]]$a
a
"1" "2" "3" NA
[[4]]$b
a
1 2 3 NA
[[4]]$c
a
5 6 7 NA
I cannot understand why I cannot convert a into numeric...
Like this, with mutate:
fff %>%
map(~ mutate(.x, a = as.numeric(a)))
Or, more base R style:
fff %>%
map(\(x) {x$a <- as.numeric(x$a); x})
You should use map only once, because you don't have a nested list. With the first map, you access to each dataframe, and then you can convert to numeric. With a second map, you are accessing the columns of each data frame (which you don't want).
With two maps, it's also preferable to use \ or function rather than ~ because it becomes confusing to use .x and x for different objects. In your question, .x is the dataframe, while x are columns of it.