Summary of knowledge points of other objects in R language

Other objects

matrix

Two-dimensional vector

Matrix operations are more similar to vectors, rather than vectors or vector lists of vectors.

Subscripts can be used to refer to elements, but do not reflect the storage method of the matrix.

The matrix does not have a definite property

Array

Vectors with more than two dimensions

Arrays can be used to represent data of the same type in multiple dimensions

The underlying storage mechanism of arrays is vectors

There are no definite class attributes in the array

factor

Factor variables represent classification information

Factor variables are usually a collection of ordered items

All values ​​that a factor variable can obtain are called factor levels

The output results of factor variables are not quotation marks and are clearly displayed.

> ( <- factor(c("brown", "blue", "blue", "green", "brown", "brown", "brown")))
[1] brown blue blue green brown brown brown
Levels: blue brown green
> class()
[1] "factor"

R displays the order of factor levels when outputting ordered factors

Factor data is stored as integer data inside the computer

Factor level maps each integer data to a factor level

Because integer data occupies less storage space, this method saves more storage space than string vectors

You can convert factor variables into integer arrays

> ( <- unclass())
[1] 2 1 1 3 2 2 2
attr(,"levels")
[1] "blue" "brown" "green"
> class()
[1] "integer"

The serpentine attribute can also reconvert integer vectors to factors

> (class() <- "factor")
[1] "factor"
> 
[1] brown blue blue green brown brown brown
Levels: blue brown green

(Note: I don't understand how the integer value matches the factor level internally)

Data frame

Used to display table data, the contents of each column can be classified into different types

Each row in the data frame must have the same length

Usually, every column in the data frame has a column name, and sometimes the row also has a name.

Columns in data frames are often used to represent variables

library(nutshell)
data()

> 
      city rank
1    Seattle 100
2   Washington  96
3    Chicago  94
4    New York  93
5    Portland  93
6    St Louis  92
7     Denver  90
8     Boston  90
9  Minneapolis  89
10    Austin  87
11 Philadelphia  85
12 San Francisco  84
13    Atlanta  82
14  Los Angeles  80
15  Richardson  80

> typeof()
[1] "list"
> class()
[1] ""

The method of referencing elements in the list can also be used in the data frame.

Formula class formula

Used to describe the relationship between variables

y is a function of x1, x2 and x3

>  <- (y ~ x1 + x2 + x3)
> class()
[1] "formula"
> typeof()
[1] "language"

The meaning of different items contained in the formula

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