What is the difference between data and data.frame in R? - r

Question

I know data.frame is a 2-D matrix with columns with different types. I think data is another type of data structure in R, which can take multiple data.frames.
In RStudio, now I have two data: dcd and pdb:
I was trying to understand the properties of them:
> dcd
Total Frames#: 101
Total XYZs#: 19851, (Atoms#: 6617)
[1] 65.59 84.65 90.92 <...> 59.76 55.48 83.68 [2004951]
+ attr: Matrix DIM = 101 x 19851
> class(dcd)
[1] "xyz" "matrix"
> dcd$xyz
Error in dcd$xyz : $ operator is invalid for atomic vectors
> pdb
Call: read.pdb(file = pdbfile)
Total Models#: 1
Total Atoms#: 6598, XYZs#: 19794 Chains#: 2 (values: L H)
Protein Atoms#: 6598 (residues/Calpha atoms#: 442)
Nucleic acid Atoms#: 0 (residues/phosphate atoms#: 0)
Non-protein/nucleic Atoms#: 0 (residues: 0)
Non-protein/nucleic resid values: [ none ]
Protein sequence:
DIQMTQSPSSLSASVGDRVTITCKASQNVRTVVAWYQQKPGKAPKTLIYLASNRHTGVPS
RFSGSGSGTDFTLTISSLQPEDFATYFCLQHWSYPLTFGQGTKVEIKRTVAAPSVFIFPP
SDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLT
LSKADYEKHKVYACEVTHQGLSSPVTKSFNRGECEVQLVESGGGL...<cut>...TSAA
+ attr: atom, xyz, calpha, call
> class(pdb)
[1] "pdb" "sse"
> pdb$xyz
Total Frames#: 1
Total XYZs#: 19794, (Atoms#: 6598)
[1] 24.33 14.711 -3.854 <...> -34.374 -6.315 14.986 [19794]
+ attr: Matrix DIM = 1 x 19794
My questions are:
Is dcd similar to a matrix with 101 rows and 19851 columns?
class(dcd) outputs "xyz" and "matrix", does it mean the dcd belongs to both "xyz" and "matrix" types in the same time?
How can I create a data like pdb which includes multiple data.frame?
e.g. if I have
students <- data.frame(c("Cedric","Fred","George"),c(3,2,2))
names(students) <- c("name", "year")
teachers <- data.frame(c("John","Alice","Mike"),c(6,9,5))
names(teachers) <- c("name", "year")
how can I combine students and teachers into a data called people, so that I can use people$students or people$teachers?

Answer 1

If you're asking how to create a dataframe named people, so you can access the names of the people using people$students or people$teachers, then the code to achieve that is:
people <- data.frame(students = students$name, teachers = teachers$name)
people$students
people would be a dataframe that looks like this:
If you want a list, you can create a list object like the following:
people2 <- as.list(c("students" = students, "teachers" = teachers))
people2$students.name
# returns [1] Cedric Fred George
And people2 would be a list:
See the $ (dollar sign) next to each item in the list? That tells you how to access them. If you wanted teachers.name, then print(people2$teachers.name) will do that for you.
As for your other questions:
Is dcd similar to a matrix with 101 rows and 19851 columns?
You can verify the dimension of a matrix-like object using dim(), ncol() or nrow(). In your case yes it has 101 rows and 19851 columns.
class(dcd) outputs "xyz" and "matrix", does it mean the dcd belongs to both "xyz" and "matrix" types in the same time?
Simplistically, you can think of it inheriting a matrix class as well as xyz. You may want to read about classes and inheritance in R.
How can I create a data like pdb which includes multiple data.frame?
Look at my code above. people2 <- as.list(c("students" = students, "teachers" = teachers)) creates a list of "multiple" dataframes.