Before you can work with data you have to get some. This course will cover the basic ways that data can be obtained. The course will cover obtaining data from the web, from APIs, from databases and from colleagues in various formats. It will also cover the basics of data cleaning and how to make data “tidy”. Tidy data dramatically speed downstream data analysis tasks. The course will also cover the components of a complete data set including raw data, processing instructions, codebooks, and processed data. The course will cover the basics needed for collecting, cleaning, and sharing data.
Reading JSON
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來自 Johns Hopkins University 的課程
Getting and Cleaning Data
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Week 1
In this first week of the course, we look at finding data and reading different file types.
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Jeff Leek, PhDAssociate Professor, Biostatistics
Bloomberg School of Public Health
Roger D. Peng, PhDAssociate Professor, Biostatistics
Bloomberg School of Public Health
Brian Caffo, PhDProfessor, Biostatistics
Bloomberg School of Public Health
This lecture's gonna be about reading JSON.
JSON's another file format.
It's a little bit similar to XML in the sense that it's structured, and
it's also very commonly used on the internet.
So JSON is short for Javascript Object Navigation.
It's lightweight for data storage.
So it's very common in application programming interfaces
which are ways that you can programmatically detect access to data for
companies like Twitter or Facebook through URLs.
Similar structure to XML in the sense that it's a structured language, but
it's a very different syntax and format.
The data are stored as Numbers, Strings, Booleans, Arrays, or Objects.
So if you want to know a little bit more about JSON the best place to start again,
as usual, is actually Wikipedia.
This link will take you to a lot more information about JSON.
So this is an example JSON file.
It actually comes from, this is the API for github.
And this is actually the data about the repos that I am contributing to.
And so, what you can do is go here and look at this file.
And so, there's a overall curly bracket that
represents the sort of entire JSON object.
And then, each of the different repos is inside of its own separate curly bracket.
And so each repo has a bunch of variables associated with it, so for
example the ID variable or the full name of the repo and all that.
And so what you see is the structure of a JSON file and
sort of the ID followed by a colon followed by the value of the variable.
And so you can have nested sort of structures within, so
for example an array can be a component of the JSON objects.
So, for example, the owner variable here actually has an array as the value,
so you could see there's the colon, and then there's another open curly bracket.
And then there's a whole bunch of variables, like my login, and
the avatar URL, and all of that different information.
So there is actually a very nice package for reading this data in.
It's called the jsonlite package.
So what you do is you actually take the URL where the JSON appears, so
here's that URL, and you pass it to the fromJSON function.
And what you get out is a structured data frame.
So if you look at the names of that data frame, it's gonna be all the top level
variables, things like ID, and name, and full_name and so forth.
So one of those variables as you can see is owner.
So that's one of the components of this data frame.
So if we drill down a little bit and look at the names of that particular variable,
you see names(jsonData$owner).
So usually this would access one column of the data frame.
And so it turns out you can actually, because of the flexibility of data frames
in R store the data frame within a data frame.
So within that column of the data set,
the owner column, there's actually a bunch of other information.
Cuz you remember owner corresponding to an array of values.
So you get again, the login, the id, say, the avatar_url and so forth.
So we can drill down even further and so we can access, say, for
example the log n of all the different repos on this particular page.
And since we're only looking at the API for my repos, you can see that the login
is the same for all of them, it's jtleek in every single case.
And so you can see there's one time for
every single repo that exists in that data set.
And so another thing that you can do is actually,
you can take data sets that are data frames in R.
So one example of the very classic data set, the Iris data set.
You can turn it into a JSON dataset by saying, toJSON.
So this is nice if you're going to be exporting data that's gonna be
used by an API that requires JSON formatted data.
So here I've set pretty=TRUE, and that would give you nice indentations so
it's easy to read the file when you look at it.
So if you look at this variable, this variable is not a text variable, and
so what we can do is we can use the cat command to print it out and
you could see that it's nicely structured like that.
So you can actually take that JSON file we just created.
So we took the iris dataset, we turned it into a JSON file with the toJSON command.
We can then take that text and
send it right back to a data frame using the fromJSON command.
So the interesting thing here is,
in the first example that we used it was actually a URL here.
We actually passed it a URL for the JSON file.
And here we're just passing it a variable name which corresponds to a text variable
that can then be converted into a data frame.
So then if we look at the top of that data frame using the head command
you see that you get back the iris data sets.
So, if you just looked at the iris data set by itself,
it'll look exactly the same as using this file here.
So, if you want a little bit more information you can go to json.org.
This'll give you more information on JSON in general.
And then a really nice tutorial on jsonlite that was the basis for
a lot of this lecture is right here at r-bloggers.
So you can take that, take a look at that.
The jsonlite vignette is also quite good, it gives you a lot more information.
So if you have a more complicated JSON structure then you need to get access to,
you can go check out those resources and
they might give you a little bit more information.
