Combining pages of JSON data with jsonlite and plyr

July 24, 2014


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The jsonlite package is a JSON parser/generator for R which is optimized for pipelines and web APIs. It is used by the OpenCPU system and many other packages to get data in and out of R using the JSON format.

A bidirectional mapping

One of the main strenghts of jsonlite is that it implements a bidirectional mapping between data frames and JSON. Thereby it can convert nested collections of JSON records, as they often appear on the web, immediately into the appropriate R structures, without complicated manual data munging by the user. For example, if a journalist wants to grab some data from ProPublica, she can simply use something like:

mydata <- fromJSON("")

Here, the mydata$geo object is a data frame which can be used directly for modeling or visualization, without the need for advanced data minipulation skills.

Paging with jsonlite and plyr

A question that comes up frequently is how to combine pages of data. Most web APIs limit the amount of data that can be retrieved per request. If the client needs more data than what can fits in a single request, it needs to break down the data into multiple requests that each retrieve a fragment (page) of data, not unlike pages in a book. In practice this is often implemented using a page parameter in the API. Below an example from the ProPublica Nonprofit Explorer API where we retrieve the first 3 pages of tax-exempt organizations in the USA, ordered by revenue:

baseurl <- ""
mydata0 <- fromJSON(paste0(baseurl, "&page=0"))
mydata1 <- fromJSON(paste0(baseurl, "&page=1"))
mydata2 <- fromJSON(paste0(baseurl, "&page=2"))

#The actual data is in the filings element

To analyze or visualize these data, we need to combine the pages into a single dataset. This is best done using rbind.fill from the plyr package. However because rbind.fill does not support nested data frames, we need to flatten the JSON data by passing the flatten = TRUE argument to fromJSON.

#Note flatten=TRUE requires jsonlite => 0.9.9
baseurl <- ""
mydata0 <- fromJSON(paste0(baseurl, "&page=0"), flatten = TRUE)
mydata1 <- fromJSON(paste0(baseurl, "&page=1"), flatten = TRUE)
mydata2 <- fromJSON(paste0(baseurl, "&page=2"), flatten = TRUE)

#Combine data pages
filings <- rbind.fill(mydata0$filings, mydata1$filings, mydata2$filings)

#Check output

Automatically combining many pages

We can write a simple loop that automatically downloads and combines many pages. For example to retrieve the first 20 pages with non-profits from the example above:

#requires jsonlite >= 0.9.9

#store all pages in a list first
baseurl <- ""
pages <- list()
for(i in 0:20){
  mydata <- fromJSON(paste0(baseurl, "&page=", i), flatten=TRUE)
  message("Retrieving page ", i)
  pages[[i+1]] <- mydata$filings

#combine all into one 
filings <- rbind.fill(pages)

#check output

From here, our journalist can go straight to analyzing the data without any further tedious, complicated and time consuming data manipulation.



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