what is the "class" parameter in structure()? - r

Question

I am trying to use the structure() function to create a data frame in R.
I saw something like this
structure(mydataframe, class="data.frame")
Where did class come from? I saw someone using it, but it is not listed in the R document.
Is this something programmers learned in another language and carries it over? And it works. I am very confused.
Edit: I realized dput(), is what actually created a data frame looking like this. I got it figured out, cheers!

Answer 1

You probably saw someone using dput. dput is used to post (usually short) data. But normally you would not create a data frame like that. You would normally create it with the data.frame function. See below
> example_df <- data.frame(x=rnorm(3),y=rnorm(3))
> example_df
x y
1 0.2411880 0.6660809
2 -0.5222567 -0.2512656
3 0.3824853 -1.8420050
> dput(example_df)
structure(list(x = c(0.241188014013708, -0.522256746461544, 0.382485333260912
), y = c(0.666080872170054, -0.251265630627216, -1.84200501106852
)), .Names = c("x", "y"), row.names = c(NA, -3L), class = "data.frame")
Then, if someone wants to "copy" your data.frame, he just has to run the following:
> copied_df <- structure(list(x = c(0.241188014013708, -0.522256746461544, 0.382485333260912
+ ), y = c(0.666080872170054, -0.251265630627216, -1.84200501106852
+ )), .Names = c("x", "y"), row.names = c(NA, -3L), class = "data.frame")
I put "copy" in quotes because note the following:
> identical(example_df,copied_df)
[1] FALSE
> all.equal(example_df,copied_df)
[1] TRUE
identical yields false because when you post your dput output, often the numbers get rounded to a certain decimal point.

Answer 2

'class' is not a specific argument to the structure function - that's why you didn't find it in the help file.
structure takes an object and then any number of name/value pairs and sets them as attributes on the object. In this case, class was such an attribute. You can try this to add fictional 'foo' and 'bar' attributes to a vector:
x <- structure(1:3, foo=42, bar='hello')
attributes(x)
#$foo
#[1] 42
#
#$bar
#[1] "hello"
And as Joshua Ulrich and Xu Wang mentioned, you should not create a data.frame like that.

Answer 3

I'm scratching my head, wondering what "R Document" would not have said something about "class". It's a very basic component of the the language and how functions get applied. You should type this and read:
?class
?methods