Digital data from the Internet has in many ways become part of our daily lives. Broadband facilities, the separation of data and design, as well as a broader adoption of certain web technology standards increasingly facilitate the integration of data across different software applications and hardware devices over the web. The Internet is increasingly becoming a programmable web,1 where data is published not only in HTML-based websites for human readers, but also in standardized machine-readable formats to be “shared and reused across application, enterprise, and community boundaries” . The technical ecology of this programmable web consists essentially of web servers providing data in formats such as Extensible Markup Language (XML) via Application Programming Interfaces (APIs)2 to other server- or client-side applications (see, e.g.,  for a detailed introduction to web technologies for R-programmers). In the conceptual framework of this paper, APIs serve a dual function: they are the central nodes between different web applications and, at the same time, the central access points for empirical researchers when they want to systematically collect and analyze data from the programmable web. More and more of these access points are becoming available every day. Moreover, this trend is likely to continue with the increasing number of devices to access the Internet and the increasing amount of data recorded by embedded systems (i.e., sensors and applications in devices such as portable music players or cars that automatically feed data to web services).3
While the advantages of accessing the programmable web and integrating it into research projects are very promising, the practical utilization of this technology comes at a cost and demands that researchers possess a specific skill set and a certain knowledge of web technologies. RWebData substantially reduces these costs by providing several high-level functions that facilitate the exploration and systematic collection of data from APIs based on a Representational State Transfer (REST) architecture.4 Moreover, the package contains a unified framework to summarize, visualize, and convert nested/tree-structured web data that works independently of the data’s initial format (XML/RSS, JSON, YAML). The package is aimed at empirical researchers using R as their daily data analysis- and statistics tool, but who do not have a background in computer science or data science. In addition, several lower level functions of RWebData might also be useful for more advanced R-programmers who wish to develop client applications to interact with REST APIs. The package thus bridges the gap between the fundamental and very valuable R-packages that integrate several web technologies into the R-environment (see, e.g., [26, 24, 1, 14]) and the statistical analysis of the programmable web universe.
Interacting with a REST API from within the R-environment, therefore, means, sending HTTP requests to a web server and handling the server’s response with R. In addition to the basic functionality necessary for this, which is delivered in the base-package , more detailed functions for HTTP requests are provided in packages such as RCurl , curl , and httr . RWebData builds on the libcurl-based RCurl package, which is designed to interact with web servers hosting a REST API. The implementation is focused on a robust download of resources that checks the received HTTP response for potential problems (e.g., unexpected binary content of the HTTP responses’ body-entity) to make sure that the user does not have to specify anything other than the URL.
Most functions related to the fitting and testing of statistical models as well as to exploratory data analysis and data visualization in the R computing environment work on data represented in data-frames (a table-like flat representation in which each row represents a data-record/observation and each column a variable describing these records).5 Data-frames (and their newer variants, data-tables  and tibbles ) are likely the most common R-objects used by researchers when conducting an empirical analysis with R. Many input/output-functionalities in R serve to import (export) data into (from) R as data-frames.6 However, when it comes to reading data from web APIs into R, this is not necessarily the case. The reason lies in the nature of web data formats that allow for more flexibility than solely table-like data representation and come (in the context of web APIs) typically in a nested structure. While a conversion from one table in, e.g., an XML-document to a data-frame is straightforward (see, i.e., xmlToDataFrame() in Temple Lang ; see also ), the conversion of a more complex XML-document with a nested data structure to a flat table-like data representation in R or any other computing environment is ex ante less clear and depends on the nature of the data and the purpose the data is being converted for.7 This is particularly the case for data provided by a non-scientific API that is explicitly made for web developers in order to integrate the data in dynamic websites or mobile applications and not for researchers to integrate the data in their analysis. Not surprisingly, there are different solutions offered in the R-environment to map web data formats such as JSON and XML to R-objects.8 Most packages represent web data as nested lists containing objects of different classes such as atomic vectors or data-frames. After converting the original web data to R-objects, the user, therefore, often has to extract and rearrange the data records of interest in order to obtain a data representation for statistical analysis. This process becomes even more complex, as queries to the same API method might provide slightly different results depending on the amount of detail embedded in the data records. In these cases, data extraction can become particularly costly if the data set that needs to be compiled depends on many API requests involving various API methods.
Web data provided via REST APIs are typically in a format such as XML, JSON, or YAML and are not structured in tables containing data values in rows and columns, but are rather organized in a nested (tree-like) structure. However, independent of the data format and data structure, documents provided by a particular REST API have basic aspects in common. Based on these commonalities, this section presents a conceptual and terminological framework as well as a description of how this framework is used to develop the data mapping strategy applied in RWebData and serves as the foundation of the data mapping algorithm that will be outlined later on.
From the researcher’s point of view, the data sets provided by web APIs obey, independent of the raw data format and nesting structure, some very basic data semantics. For these data semantics, I use mainly the same terminology that is nicely outlined by Wickham [31, p. 3]:
The following fictional XML example further illustrates the basic data semantics outlined above.
<?xml version=”1.0” encoding=”UTF-8”?> <firm> <employees> <employee> <firstName>John</firstName> <secondName>Smith</secondName> </employee> <employee> <firstName>Peter</firstName> <secondName>Pan</secondName> </employee> </employees> <shareholders> <shareholder> <ID>S1</ID> <Name>Karl Marx</Name> </shareholder> <shareholder> <ID>S2</ID> <Name>Bill Gates</Name> </shareholder> </shareholders> <firmName>MicroCapital Ltd</firmName> <firmID>123</firmID> </firm>
The XML document contains a data set describing a firm. The variables “firstName” and “secondName” describe observations of type “employee”, while observations of type “shareholder” are described by the variables “ID” and “Name”. Finally, the variables “firmName” and “firmID” describe the firm itself, which forms another type of observation. The following subsection illustrates how the data would be mapped according to the procedure implemented in RWebData.
The core idea behind the data mapping procedure in RWebData is built around the basic semantics outlined above. According to this system, documents returned from APIs can contain data describing observational units of one type or several different types. RWebData returns one data-frame for each observation type. Observations and variables belonging to different types are, thus, collected in different data-frames, each describing one type of observational unit. Some observations might describe the document (or main type of observational unit) itself (i.e., metadata). These are collected in a separate data-frame. Table 1 (a–c) present the XML-document from the example above after mapping to data-frames according to the outlined mapping procedure. The next subsection explains how this process is implemented in RWebData.
|(a) Firm type|
|(b) [Employee type]|
|(c) [Shareholder type]|
In order to map nested web data to data-frames, RWebData applies, in a first step, existing parsers for different formats (mime-types) of the data in the body of the HTTP response from the API to read the data into R.9 Independent of the initial format, the data is parsed and coerced into a (nested) list representing the tree-structure of the raw data. In a next step, the data is mapped to one or several data-frames depending on the nesting structure and the recurrence of observation types and variables. The data-frames are returned to the user either directly (in a list) or as part of an apiresponse-object.
The core idea behind the generic approach to web data conversion in the RWebData package is thus to disentangle the parsing of web data from the mapping of the data to data-frames. This allows the parsers for different web data formats to be relatively simple, while the same mapping algorithm is focused on the data semantics and can be applied independent of the initial raw data format. In addition, it allows different parsers’ advantages to be combined in order to make the parsing process more robust. The suggested procedure goes hand in hand with the object-oriented approach applied in RWebData and provides summary and plot methods that work independent of the initial data format. The modular design of mapping web data to data-frames facilitates the extension of RWebData’s compatibility with additional web data formats or alternative parsers that are, for example, optimized for vast web documents. Parsers simply need to read the web data as (nested) lists or in a format that conserves the tree-structure of the raw web data and can be easily coerced into a (nested) list.
Once a document is downloaded from an API and coerced into a nested list, the algorithm consists essentially of two procedures:
Singly occurring leaf nodes that are not nested within one of the detected observation types are collected along the way and then returned in one data-frame. According to the logic of the data semantics and the algorithm outlined above, these remaining data can be seen as metadata describing the data set as a whole. The Appendix presents a detailed, more technical outline of the data mapping algorithm.
RWebData is specifically written to give practical researchers a high-level interface to compile data from the programmable web. The common user will thus normally not be confronted with the range of internal processes outlined above. The work-flow with RWebData for the average user thus mainly involves the specification of what data should be requested from what API (in the form of either a list, a data-frame or simply a string with a URL) to one of RWebData’s high-level functions in order to obtain the desired data mapped to data-frames. Figure 1 illustrates the basic architecture of RWebData, summarizing its internal functionality and the processing procedure when the package is used by the average user.
RWebData’s high level functions take either lists, data frames, or a URL (as a character string) as input values and return a list of data-frames or an apidata-object. First, RWebData generates an apirequest-object, based on which the HTTP GET and responses are handled. Upon successful interaction with the API, an apiresponse-object, containing, inter alia, the raw web data, is generated. Subject to consideration of the mime-type of the raw data, the data is then preprocessed, parsed, and coerced to a nested list. Finally, the data mapping algorithm is applied in order to extract the data in the form of data frames as outlined in the previous section. The latter builds the main part of the package. The procedure is straightforward and user-friendly in practice.
In order to describe the basic usage of RWebData, this section offers a step-by-step introduction to the high-level functionality of the package. Some of the functions described here offer more options, which are not all discussed in this section.10 The latest version of the package can be directly installed from the GitHub repository via devtools::install_github() as shown below.
R> # install.packages(“devtools”) R> library(devtools) R> install_github(“umatter/rwebdata”) R> # load RWebData R> library(RWebData)
RWebData’s implementation is motivated by the convention over configuration paradigm and thus provides a way for the user to interact with APIs using as few specifications as necessary to access the data. RWebData then converts data in the most common formats provided by APIs to one data-frame or a list of several data-frames. Hence, the user does not need to specify any HTTP options or understand what XML or JSON is and is able to obtain the data directly in the expected format for statistical analysis. There are primarily two high-level functions in RWebData that provide this functionality in different ways: getTabularData() and apiData(). The following subsection discusses how these functions work and how they can be applied in practice.
Querying data from a REST API is based on the same client-server principle as opening a website in a web browser. In order to visit a certain website, we normally type a website’s address in the web browser’s address bar. The browser (a type of web client) then sends a message to the web server behind that address, requesting a copy of the specific webpage and then parses and renders the returned document (usually an HTML document) in the browser window. Any website address consists of several components that specify the request to the server. For example, the address https://www.admin.ch/gov/en/start.html contains the components https (the scheme), www.admin.ch (the host name/domain name with the top-level domain ch), and the path to the specific webpage (here, an HTML document): gov/en/start.html.
In REST terminology, the website’s address is a URL and the website which the URL locates is a resource. The document that the server sends in response to the client’s request is a representation of that resource. Importantly, every URL points to only one resource and every resource should have only one URL (this is one of the REST principles and is referred to as addressability). The message that the browser sends to the web server is a HTTP GET request, a HTTP method that essentially requests a representation of a given resource.
REST APIs work basically the same way as the website outlined above. The crucial difference is that their design is intended for programmable clients (rather than humans using a web browser) and that they therefore consist of URLs pointing to resources that are not optimized for graphical rendering, but which instead contain the raw data (often in a format such as XML or JSON that facilitates the integration of the data in a HTML document). However, a URL pointing to an API basically follows the same logic and consists of the same components, including scheme, host name, path, and potentially additional parameters (key-value pairs separated by &). Thus, in order to query data from a REST API, we first have to construct the respective URL pointing to the data of interest and then apply the functions provided RWebData to query the data. The example below further illustrates this point.
The function getTabularData() provides a straightforward way to get data from web APIs as data-frames. In the simplest case, the function takes a string containing the URL to the respective API resource as an input.11 The function then handles the HTTP request and response, parses the body of the response (depending on the mime-type) and automatically extracts the data records from the nested data structure as data-frames. This enables us to fetch data from different APIs providing data in different formats with essentially the same command.
Consider, for example, the World Bank Indicators API which provides time series data on financial indicators of different countries.12 We want to use data from that API to investigate how the United States’ public debt was affected by the financial crisis in 2008. All we need in order to download and extract the data is the URL to the respective resource on the API.13
R> u <- paste0(“http://api.worldbank.org/v2/countries/USA/indicators”, # address + “/DP.DOD.DECN.CR.GG.CD?”, # query method + “&date=2005Q1:2013Q4”) # parameters R> usdebt <- getTabularData(u)
R> require(zoo) R> plot(as.ts(zoo(usdebt$value, as.yearqtr (usdebt$date))), ylab=”U.S. public debt(in USD)”)
The underlying data structure is relatively simple in this example. The World Bank Indicators API is specifically devised to provide data for statistical analysis. The next section considers examples where the provided data cannot easily be thought of as one single table.
The high-level function getTabularData() automatically handles nested data and converts them to a list of various data-frames. It does not, however, provide any information on the initial nesting structure and can only handle individual requests. Alternatively, we can use apiData() in order to exploit RWebData’s internal classes and methods to handle requests to APIs. The simplest way to call apiData() is again with a URL-string pointing to an API resource. apiData() returns an object of class apiresponse. Such apiresponse objects contain additional information about the executed request to the API and support generic plot and summary functions illustrating the structure and content of the retrieved web document. The following example demonstrates the summary methods for apiresponse objects with data from the Ergast API (http://ergast.com/mrd/) on Formula 1 race results.
R> f1 <- apiData(“http://ergast.com/api/f1/2013/1/results.json”, + shortnames=TRUE) R> summary(f1) API data summary: ================= The API data has been split into the following 2 data frames: Length Class Mode metadata 19 data.frame list Results 27 data.frame list The respective data frame(s) contain the following variables: 1. metadata: xmlns, series, url, limit, offset, total, season, round, 1, raceName, circuitId, 2, circuitName, lat, long, locality, country, date, time, 2. Results: number, position, positionText, points, driverId, permanentNumber, code, url, givenName, familyName, dateOfBirth, nationality, constructorId, url, name, nationality, grid, laps, status, millis, time, rank, lap, time, units, speed, path,
The summary method called by the generic summary(), provides an overview of the variables included in the data and shows how RWebData has split the data into several data-frames. This is particularly helpful in an early phase of a research project when exploring an API for the first time.
The next example demonstrates the visualization of nested data returned from the Open States API.15 We query data on a legislator in the Council of the District of Columbia with apiData() and call the generic plot() on the returned apiresponse object. Figure 3 shows the result of the plot command below. Note that the Open States API is free to use for registered users. Upon registration an api-key is issued that is then added as a parameter-value to each API request (indicated with “[YOUR-API-KEY]” in the example below).
R> url <- “http://openstates.org/api/v1/legislators/DCL000004/ + ?apikey=[YOUR-API-KEY]” R> p <- apiData(url) R> plot(p, type=”vertical”)
The apiresponse plot method illustrates the nesting structure of the original data by drawing entities (variables or types) as nodes and nesting relationships as edges in a Reingold-Tilford tree-graph  (see also  for the implementation in R on which RWebData relies). The option type=”vertical” displays the node labels vertically in order to make long node names better readable in the graph.
The transformed data (in the form of a list of data-frames) saved in an apiresponse-object can then be accessed with getdata (). In the current example, the data has been split into two data-frames: one with meta data containing general variables describing the legislator, and one containing data on the legislator’s roles in the 2011–2012 session.
R> pdata <- getdata(p) R> summary(pdata) Length Class Mode metadata 22 data.frame list 2011–2012 11 data.frame list
So far, we have only considered individual requests to APIs. In practice, users might want to query an API in many ways in an interactive session to explore the data and assemble the data set that meets their needs. This can easily be done with RWebData. We continue with the Open States API example. First, we fetch general data on all legislators in the DC Council.
R> url <- “http://openstates.org/api/v1/legislators/?state=dc&chamber=upper + &apikey=[YOUR-API-KEY]” R> dc_council <- getTabularData(url)
The goal is to combine these general data with data on the legislators’ roles. In particular, we want to download the detailed role data on all legislators and combine these within one data set. As there is no API method provided to obtain these data with one call to the API, we use apiDownload() to handle all individual requests at once. By inspecting the dc_council data-frame from above, we see that the fifth column of that data-frame contains all the IDs (id) pointing to the respective resources on individual council members. We simply paste these ids together with the legislators-method and use the resulting URLs as the function argument to apiDownload() in order to obtain all the data compiled in one data-frame.
R> head(names(dc_council))  “fax” “last_name” “updated_at” “full_name” “id”  “first_name” R> api_requests <- paste0(“http://openstates.org/api/v1/legislators/”, + dc_councilid, + “/?apikey=”, + “YOUR-API-KEY”) R> dclegroles <- apiDownload(api_equests)
During the download, apiDownload() indicates the number of queries processed on a progress bar printed to the R console. Moreover, apiDownload() periodically saves the processed data locally in a temporary file. This relieves the working memory assigned to the R-session and makes large downloads more robust to network interruptions. A summary of the resulting dc_leg_roles is presented in the Appendix.
In the context of an ongoing research project, it might be more comfortable to have a specific function for specific queries to the same API. The further development of RWebData is aimed at providing a way to write such functions automatically. The current implementation of this functionality is still being tested and made more robust.16
The following example illustrates how this newest extension of the package can be applied. Given the parameters and the base URL for the respective API method/resource, generateQueryFunction() writes a function that includes all the functionality of the package to handle queries to that specific API method/resource. The same works for APIs that accept requests via form URLs, such as the MediaWiki API.17 Here, only the base URL (i.e., the endpoint of the API) and the respective parameters have to be specified.
R> endpoint <- “https://en.wikipedia.org/w/api.php?” R> # set parameters (set to NA if no default value in query-function wanted) R> query_params <- data.frame (parameters=c(“action”, “format”, “titles”, “prop”), + values = c(“query”, “xml”, NA, NA)) R> mediaWiki <- generateQueryFunction(x=query_params, base.url=endpoint) R>
The new function mediaWiki() can now be used to query the MediaWiki API for information about Wikipedia pages.
R> # get meta data on the Wikipedia page about the Internet R> page_info <- mediaWiki(titles = “Internet”, prop = “info”) R> page_info[,1:5] page._idx page.pageid page.ns page.title page.contentmodel 1 14539 14539 0 Internet wikitext R> # get information about the latest revision of the same Wikipedia page R> revisions <- mediaWiki(titles = “Internet”, prop = “revisions”) R> revisions[,1:5] rev.revid rev.parentid rev.minor rev.user rev.timestamp 1 823121130 822163533 Ragityman 2018-01-30T10:58:38Z
The MediaWiki API query method allows batch queries of several Wikipedia page titles (separated with the pipe operator ‘|’). mediaWiki() automatically recognizes the different observations of the same observation-type and returns the data as a data-frame with one row per observation:
R> page_info <- mediaWiki (titles = “USA|France|Germany|Switzerland”, + prop = “revisions”) R> dim(page_info)  4 14 R>
With only three lines of code, we have generated a basic MediaWiki API R-client library that automatically provides the web data as data-frames.
Empirically driven research can benefit substantially from the rapidly growing programmable web as a data source covering countless dimensions of socio-economic activity. However, the purpose and goals that drove the initial development of web APIs focused on providing general data for public use via dynamic websites and other applications, and not on the provision of data formatted for scientific research and statistical analyses. This leads to technical hurdles a researcher has to overcome in order to compile such data in a format that is suitable for statistical analysis. The R package RWebData presented here suggests a simple high-level interface that helps to overcome such technical hurdles (and the respective costs) associated with the statistical analysis of data from the programmable web. The package contributes to a frictionless and well documented raw data compilation and data preparation process that can help to increase the replicability and reproducibility of original research based on data from the programmable web. As pointed out in , empirical research with newly available digital data from novel sources generally poses new challenges to the reuse of data as well as the reproduction of results. Empirical research based on newly available public data from web sources is a challenge demanding scientific rigor. With the further development of RWebData, an R-script documenting how the package’s high-level functions have been applied to compile data as well as any further step in the data preparation and analysis are enough to ensure the replicability of a study based on big public data from the programmable web.
RWebData parses and coerces the raw web data to a nested list representing the tree-structure of the data. Call this list.
The data mapping algorithm consists of two main parts. First,is split into n lists representing sub-trees, one for each observation type in the data. The key problem that the algorithm has to solve at this step is the identification of cutting points (i.e., what part of the tree belongs to what observation type). Second, each resulting sub-tree is then split into individual character vectors. One for each observation. The individual observations are then stacked together in one data-frame with each vector (observation) as a row. Algorithm 1 presents a formal description of these procedures. In the resulting data-frames, each row i represents one observation, and each column j represents a variable/characteristic of the n observations. The data-frames are then returned in a list.
In order to make the formal descriptions of the procedures in the data mapping algorithm conveniently readable, the pseudo-code describes simple versions of these procedures (that are not necessarily most efficient). The algorithms’ R-implementations in RWebData are more efficient and contain more control statements to ensure robustness. The actual R-implementation relies partly on existing R functions and favors vectorization over for-loops in some cases.
Algorithm 1 Data mapping algorithm
R> url <- “http://openstates.org/api/v1/legislators/?state=dc&chamber=upper + &apikey=[YOUR-API-KEY]” R> c_council <- getTabularData(url) R> api_requests <- paste0(“http://openstates.org/api/v1/legislators/”, dc_council$id, “&apikey=”, “YOUR-API-KEY”) | | | 0% | |============================== | 50% | |============================================================| 100% R> # explore the data R> summary(dc_leg_roles) Length Class Mode metadata 55 data.frame list 2013-2014 13 data.frame list 2011-2012 13 data.frame list offices 7 data.frame list
The data mapping algorithm has been tested in unit tests with various formats (see https://github.com/umatter/RWebData/blob/master/testing.Rmd as well as https://github.com/umatter/RWebData/blob/master/unit_testing.R for details on the tests and https://github.com/umatter/RWebData/blob/master/testing.html for the test results). RWebData is delivered with a set of documents with hierarchical data structures in different formats on which the package’s own examples are based:
R> example(XMLtoDataFrame) R> example(YAMLtoDataFrame) R> example(JSONtoDataFrame)
In addition to unit testing, the code has already been tested extensively in several real world applications (both on Mac OSX and Ubuntu), both in the course of the author’s own ongoing research projects in the areas of political economics and political science  as well as with regard to various openly accessible web APIs. A list of these APIs is provided and further maintained and extended in the code repository of RWebData (see https://github.com/umatter/RWebData/blob/master/testing.Rmd).
RWebData is compatible with any operating system that is compatible with R 3.2.3.
RWebdata is written in and for R (>= 3.2.3).
There are no additional system requirements apart from (for most parts of RWebData) a working Internet connection.
igraph (>= 1.0.1), RCurl (>= 1.95–4.8), jsonlite (>= 1.3), XML (>= 3.98–1.5), plyr (>= 1.8.4), stringr (>= 1.2.0), XML2R (>= 0.0.6), httr (>= 1.2.1), mime (>= 0.5), yaml (>= 2.1.14), RJSONIO (>= 1.3–0), methods (>= 3.2.3), (devtools to install the package from GitHub)
Ulrich Matter: University of St. Gallen, SEPS-HSG/SIAW, Bodanstr. 8, 9000 St. Gallen, Switzerland, email@example.com. Ingmar Schlecht: Faculty of Business and Economics, University of Basel, Peter Merian-Weg 6, 4002 Basel, Switzerland.
Licence: GPL-2, GPL-3
Publisher: Ulrich Matter
Version published: 0.2.1
Date published: 01/29/2018
Licence: GPL-2, GPL-3
Date published: 12/02/2015
RWebData (both in the current and previous versions) has already been used in the course of several research projects (see, for example, Matter and Stutzer ). With the increasing expansion of the programmable web and thus the amount of digital data covering various aspects of social, political, and economic processes, the reuse potential for RWebData is expected to be high, especially in disciplines where R is widely used for statistical analysis and data preparation such as economics and political science.
RWebData is explicitly made for researchers without a background in web technologies/programming and who are predominantly using R as statistics software for their every-day empirical research. Together with the increasing relevance of digital data from the web for empirical research in various disciplines and fields, RWebData has a broad and growing potential user base. In addition, the publication of this software (particularly the extensions mentioned in the outlook) provides opportunities for other researchers to build on the code provided in RWebData in order to write their own custom made API clients for data collection.
1I use the term programmable web synonymously with “Semantic Web” or “Web of Data” and as conceptually motivated by .
3The number of publicly accessible web APIs grew from around 1,000 at the end of 2008 to over 10,000 at the end of 2013 . Turner et al.  estimate that the share of available digital data stemming from embedded systems will rise to 10% by 2020 and will include up to 32 billion devices connected to the Internet. See also  for a discussion of how this ‘Internet of things’ could be expanded on a crowd-sourced basis and employed for big-data analytics.
4See  for an introduction to REST APIs.
5Technically, a data-frame can contain other R-objects than just scalars (i.e., another data-frame). However, in the vast majority of applications in the context of statistical analysis and data visualization, data-frames are used for a table-like flat data representation.
6See, i.e., data from CSV- and similar text files: read.table() in  or similar functions in , Microsoft Excel files: read.xls() in , data from other statistical computing environments such as Stata: read.dta() in , or data from ODBC-data bases sqlQuery() in .
7See, e.g., the classical problem of mapping a set of XML-documents to a relational data base scheme (RDBS; i.e., a set of linked tables) with or without knowing the scheme behind the XML-documents [9, 7]. See also, in the R context, the different implementations of mapping JSON to R-objects in , , or  as well as a detailed discussion of this matter in .
8See, e.g., the CRAN Task View on web technologies and services (http://cran.r-project.org/web/views/WebTechnologies.html), the CRAN Task View on open data (https://github.com/ropensci/opendata), as well as the rOpenSci (https://ropensci.org/packages/) for an overview of popular R-packages that parse data from the web and map it to R-objects. Some contributions in this area also focus on the automated information extraction from traditional websites made for human interaction. Such methods are commonly summarized under the terms “web scraping” or “screen scraping”. See, e.g., the R-package rvest  for functions that facilitate web scraping as well as  for a practical introduction to web scraping with R.
9In the case of JSON, either the parser provided by Ooms et al.  or (in order to increase robustness in special cases) the one contributed by Temple Lang  is applied. XML-documents or RSS-documents are parsed with the parser provided in  and YAML-documents with the parser provided by Stephens .
10Details on all these options are provided by the respective R help files. Users are generally encouraged to read the detailed package documentation of RWebData (https://github.com/umatter/RWebData/blob/master/RWebData.pdf).
11The URL to the resource which a user wants to query is very easy to find in any REST API documentation. How to call the respective resource methods of an API with URLs is the essential part of such documentations.
12Note that this is an example of an API which is – unlike most APIs – explicitly made for researchers retrieving data from the web. Nevertheless, retrieving data from this API with functions from, e.g., the XML package  is not necessarily straightforward for users without a background in web technologies.
13How to build query-URLs in the specific case of the World Bank Indicators API is well documented on https://datahelpdesk.worldbank.org/knowledgebase/articles/898581-api-basic-call-structure.
14The World Bank Indicators API provides time series data by default as XML in a compressed text file. Handling solely one API query of this type might already involve many steps to fetch and extract the data with the existing lower level functions. And, thus, might be tedious for a user who only has limited experience with web technologies.
15See https://sunlightlabs.github.io/openstates-api/ for details.
16Note that this additional feature is built on top of the matured features shown in the examples above. The early stage and future development of this particular extension thus does thus not affect the other functionalities of the package that have already been tested extensively.
17See https://en.wikipedia.org/w/api.php? for a detailed documentation of this API.
I am grateful to Dietmar Maringer, Armando Meier, Reto Odermatt, Michaela Slotwinski, Alois Stutzer, as well as seminar participants at the University of Basel and the University of Oxford for helpful remarks. Special thanks go to Ingmar Schlecht for many productive discussions on software development and the methodological aspects of this paper. I also thank Joerg Kalbfuss for excellent research assistance.
The author has no competing interests to declare.
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