rbind fromJSON page: duplicate rowname error - json

Question

I was trying to rbind some json data scraped from api
library(jsonlite)
pop_dat <- data.frame()
for (i in 1:3) {
# Generate url for each page
url <- paste0('http://api.worldbank.org/v2/countries/all/indicators/SP.POP.TOTL?format=json&page=',i)
# Get json data from each page and transform it into dataframe
dat <- as.data.frame(fromJSON(url)[2],flatten = TRUE, row.names = NULL)
pop_dat <- rbind(pop_dat, dat)
}
However, it returns the following error:
Error in row.names<-.data.frame(*tmp*, value = value) :
duplicate 'row.names' are not allowed
In addition: Warning message:
non-unique values when setting 'row.names': ‘1’, ‘10’, ‘11’, ‘12’, ‘13’, ‘14’, ‘15’, ‘16’, ‘17’, ‘18’, ‘19’, ‘2’, ‘20’, ‘21’, ‘22’, ‘23’, ‘24’, ‘25’, ‘26’, ‘27’, ‘28’, ‘29’, ‘3’, ‘30’, ‘31’, ‘32’, ‘33’, ‘34’, ‘35’, ‘36’, ‘37’, ‘38’, ‘39’, ‘4’, ‘40’, ‘41’, ‘42’, ‘43’, ‘44’, ‘45’, ‘46’, ‘47’, ‘48’, ‘49’, ‘5’, ‘50’, ‘6’, ‘7’, ‘8’, ‘9’
Changing the row.names to null doesn't work. I heard from someone it is due to the fact that some data are stored as lists here, which I don't quite understand.
I understand that there is an alternative package WDI to access this data and it works well, but I want to know how to resolve the duplicates row name problem here in general so that I can deal with similar situation where no alternative package is available.

Answer 1

I heard from someone it is due to the fact that some data are stored as lists...
This is correct. The solution is fairly simple, but I find it really easy to get tripped up by this. Right now you're using:
dat <- as.data.frame(fromJSON(url)[2],flatten = TRUE, row.names = NULL)
The problem comes from fromJSON(url)[2]. This should be fromJSON(url)[[2]] instead. According to the documentation, the key difference between [ and [[ is a single bracket can select multiple elements whereas [[ selects only one.
You can see how this works with some fake data.
foo <- list(
a = rnorm(100),
b = rnorm(100),
c = rnorm(100)
)
With [, you can select multiple values inside this list.
foo[c("a", "b")]
length(foo["a"]) # Result is 1 not 100 like you might expect.
With [[ the results are different.
foo[[c("a", "b")]] # Raises a subscript error.
foo[["a"]] #This works.
length(foo[["a"]]) # Result is 100.
So, your answer will depend on which subset operator you're using. For your problem, you'll want to use [[ to select a single data.frame inside of the list. Then, you should be able to use rbind correctly.
final <- data.frame()
for (i in 1:10) {
url <- paste0(
'http://api.worldbank.org/v2/countries/all/indicators/SP.POP.TOTL?format=json&page=',
i
)
res <- jsonlite::fromJSON(url, flatten = TRUE)[[2]]
final <- rbind(final, res)
}
Alternative solution with lapply:
urls <- sprintf(
'http://api.worldbank.org/v2/countries/all/indicators/SP.POP.TOTL?format=json&page=%s',
1:10
)
resl <- lapply(urls, jsonlite::fromJSON, flatten = TRUE)
resl <- lapply(resl, "[[", 2) # Use lapply to select the 2 element from each list element.
resl <- do.call(rbind, resl) # This takes all the elements of the list and uses those elements as the arguments for rbind.