Get the length of an element on nested list - r

Question

I have a list (of lists) that came from JSON (jsonlite) like this one (dput below)
{
"1":["123", "131", "342"],
"2":["123", "131"],
"3":["123", "131", "352"],
"4":["31", "352"],
"5":["153", "131"],
"6":["153", "131", "382"]
}
structure(list(`1` = c("123", "131", "342"), `2` = c("123", "131" ), `3` = c("123", "131", "352"), `4` = c("31", "352"), `5` = c("153", "131"), `6` = c("153", "131", "382")), .Names = c("1", "2", "3", "4", "5", "6"))
Then, I'm trying to convert it to a data frame with the key and the length of the nested list, like
V1 V2 V3 V4 V5 V6
1 1 2 3 4 5 6
2 3 2 3 2 2 3
with that code:
a = (read_file("ghist.json") %>% fromJSON)$hist # Reads my list from a JSON file
dates = data.frame() #Creates an empty data frame
#Iterate my list element by element
for(i in 1:length(a)){
dates[1, i] = strtoi(names(a)[i]) #Appends to my data frame on the first row, line 'i' the key from my list (index 'i'), as Integer
dates[2, i] = length(a[i]) #Here is my problem, it returns '1', not the real length of my list (index 'i')
}
print(dates) #Just debug
With the code above I'm getting
V1 V2 V3 V4 V5 V6
1 1 2 3 4 5 6
2 1 1 1 1 1 1
Note: I know the numbers on key are just crescent numbers, but it will become dates in ms

Answer 1

You can just use the built in lengths function to construct your data frame. This gives you the length of list elements, which is exactly what you want.
a <- structure(list(`1` = c("123", "131", "342"), `2` = c("123", "131"), `3` = c("123", "131", "352"), `4` = c("31", "352"), `5` = c("153", "131"), `6` = c("153", "131", "382")), .Names = c("1", "2", "3", "4", "5", "6"))
dates <- data.frame(
matrix(
data = c(names(a), lengths(a)),
ncol = length(a),
byrow = TRUE
)
)
dates
#> X1 X2 X3 X4 X5 X6
#> 1 1 2 3 4 5 6
#> 2 3 2 3 2 2 3
The bug in your code is very minor, thought I wouldn't recommend this approach. It's that you need length(a[[i]]). I suggest you look at some resources on subsetting in R, but to illustrate compare the two at the bottom. a[1] returns a list containing the desired element, a[[1]] returns the actual element. The length of a[1] is 1.
dates = data.frame()
for(i in 1:length(a)){
dates[1, i] = strtoi(names(a)[i])
dates[2, i] = length(a[[i]]) # changed here
}
dates
#> V1 V2 V3 V4 V5 V6
#> 1 1 2 3 4 5 6
#> 2 3 2 3 2 2 3
a[1]
#> $`1`
#> [1] "123" "131" "342"
a[[1]]
#> [1] "123" "131" "342"
Created on 2018-06-26 by the reprex package (v0.2.0).

Answer 2

Try nrow(a[i]) instead of length(a[i]) in your loop.