Subsetting - Lists

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Del curso dictado por Johns Hopkins University

R Programming

13870 calificaciones

Johns Hopkins University

R Programming

13870 calificaciones

Curso 2 de 10 en SpecializationData Science

In this course you will learn how to program in R and how to use R for effective data analysis. You will learn how to install and configure software necessary for a statistical programming environment and describe generic programming language concepts as they are implemented in a high-level statistical language. The course covers practical issues in statistical computing which includes programming in R, reading data into R, accessing R packages, writing R functions, debugging, profiling R code, and organizing and commenting R code. Topics in statistical data analysis will provide working examples.

De la lección

Week 1: Background, Getting Started, and Nuts & Bolts

This week covers the basics to get you started up with R. The Background Materials lesson contains information about course mechanics and some videos on installing R. The Week 1 videos cover the history of R and S, go over the basic data types in R, and describe the functions for reading and writing data. I recommend that you watch the videos in the listed order, but watching the videos out of order isn't going to ruin the story.

Conoce a los instructores

Roger D. Peng, PhD
Associate Professor, Biostatistics
Bloomberg School of Public Health
Jeff Leek, PhD
Associate Professor, Biostatistics
Bloomberg School of Public Health
Brian Caffo, PhD
Professor, Biostatistics
Bloomberg School of Public Health

So subsetting a list is a little bit different.

Because you can use the double bracket or the dollar sign operator.

You can also use the single bracket operator.

So here I've got a list, the first element is called, is a named element called foo.

That's an ind, and it's a sequence 1 through 4.

And the second element is named bar, and it's the number 0.6.

So this is a list of two elements in it.

I can extract the first element by using the single square bracket.

And I get, when I, remember the single square bracket always returns the element

that's the same class as the original.

So if x is a list, than x bracket 1 is going to be a list too.

So what I get back is a list that has element call foo,

which is a sequence 1 through 4.

Now if I use, so if I use the double bracket then if I said x double bracket 1,

what I get back is just a sequence, 1 through 4.

So, so the difference here is that in the first example, I got a list that contained

the sequence 1 through 4, and in the second example, I got just the sequence.

That's the difference between the single bracket and the double bracket operator.

In the next example here I'm using a dollar sign.

And so I'm saying, x dollar bar.

And that what that mean is that,

that gives me that element that is associated with the name bar.

So in that case it's the, it's a single number 0.6.

I can also use the double bracket operator and pass in a string.

So x double bracket quote bar is the same as doing as x dollar bar and

it just gives me 0.6.

If I use the single bracket with the name,

I can say x bracket quote bar, that gives me a list with the element bar in it.

So remember, because the single bracket always returns a list if I'm

going to subset a list.

So the nice thing about being able to subset an element using its name,

is that you don't have to remember where it is in the list.

So if I couldn't remember whether bar was the first element or

was the second element, I don't have to remember whether, what,

where it is in order to use the numeric index.

I can just use its name, and then I don't have to,

then it will automatically extract that, extract that element from the list.

If you want to extract multiple elements of a list then you need to

use the single bracket operator.

So for example, if I want the third, the first and the third element here,

in which case, which is the foo and the baz element, I can pass a,

a vector, a 1, 3, the numeric vector 1, 3 to x using the single bracket operator.

And that returns to me a list with the elements foo and the elements baz.

So that's how I extract multiple elements of a list.

There's, you cannot use the double bracket or

the dollar sign operators when you only extract multiple elements of a list.

The nice thing about the double bracket operator, which is different from

the dollar sign, is that you can use the double bracket operator to, to,

to index it a list, where the index itself was computed.

So, notice that when I used the dollar sign before, I had to,

I actually typed out the word bar.

I had to type out the name of the object.

Sometimes the name of, sorry the name of the element.

But sometimes the name of the element is actually the result of some computation.

So for example here I've got a list with three elements, foo, bar, and baz.

And then I create a variable called name which is actually the string foo.

So if I use the double bracket operator on this variable here.

Notice that the there's no element in the list that has the name, name in it.

But there is an element in the list that has the name foo in it.

So now when I, when I pass this variable called name into the double

bracket operator, it returns me the, the element that was associated with foo.

because that's what the value of the name variable is.

So if I can, if I compute the index that I want to use,

then I have to use the double bracket operator.

If I use the dollar sign, then it's going to literally look for

an element of the list that's, that has the word name associated with it,

and that of course doesn't exist in this list.

So to use the dollar sign I need to use a literal symbol.

Now, the double bracket operator can take an integer sequence in as,

rather than a single number, and

the way you can think of this is that it kind of recurses into the list.

So if you look at this current list I've got here,

with the first element a is another list which has elements 10, 12 and 14.

So suppose I wanted to extract the number 14.

Well, that's really the third element of the first element, right?

So it's the third element of the list,

which happens to be the first element of the other list.

And so I can extract the 1, 3 element term by passing the vector 1,

3 to it to the x list using the double bracket operator.

And that's equivalent to kind of doing this double sub-setting of one and three.

I can also extract the first element of the second element by use,

by passing the integer vector 2,1 to get 3.14

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