R语言快速语法参考指南

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"R Quick Syntax Reference" R是一种编程语言，专为用户提供强大的数据分析和图形分析功能。它既灵活又广泛，从简单的加法运算到复杂的ARIMA模型拟合，R都能处理各种数值计算任务。这本书是R语法的快速参考指南，旨在帮助用户迅速掌握R的基本用法和高级特性。书中的内容分为五个部分： 1. R基础：这部分从下载R和设置文件系统开始，介绍了R提示符的使用，以及赋值和操作符的基础知识。在第1章，读者将学习如何下载R并设置工作环境，第2章则会讲解R命令行的交互方式。第3章深入讨论了R中的赋值操作和各种运算符。 2. 对象类型：第4章和第5章涵盖了R中的对象模式和类。第4章讲解了R对象的五种基本模式（向量、矩阵、数组、列表和因子），而第5章则讨论了对象的类，包括数据框、时间序列等复杂数据结构。 3. 函数：在第6章和第7章，读者将了解R中的预定义函数和自定义函数。第6章介绍如何使用R中的包和内置函数，第7章则讲解如何创建自己的函数，提升代码的复用性。第8章进一步说明了如何有效使用和理解函数。 4. 数据输入、创建、输出及对象操作：这部分从第9章到第11章，涵盖了数据导入与创建、数据导出以及描述性统计和对象操纵。第9章讲解了从不同格式导入数据的方法，如CSV、Excel等。第10章则介绍如何将R的结果导出为各种格式。第11章提供了用于描述性统计和数据操纵的函数，如汇总统计、排序和过滤等。 5. 流程控制：最后两章（第12章和第13章）专注于流程控制，包括条件语句、循环和函数的应用。第12章介绍了if/else语句、switch函数和循环结构（如for、while）。第13章通过实例展示了如何在实际问题中运用这些控制流工具。此外，书中还有索引和目录供读者快速查找所需信息。无论你是R的新手还是有经验的用户，这本书都是一个宝贵的资源，能帮助你更好地理解和利用R的强大功能。

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Chapter 3

Assignments and Operators

R works with objects. Objects can include vectors, matrices, functions, the results from a

function, or a number of other kinds of objects. Objects make working with information

easier. This chapter covers assigning names to objects, listing and removing objects, and

object operations. Part II (Chapters 4 and 5) covers the possible forms of objects.

Some objects come with the packages in R. Other objects are user-created. User-created

objects have names that are assigned by the user. Knowing how to create, list, and remove

user-created objects is basic to R.

Types of Assignment

Names in R must begin with a letter or a period, cannot have breaks, and can contain

letters, numeric digits, periods, and underscores. The names that begin with a period are

hidden and are used by R for startup defaults, the random seed, and other such things.

The indexing symbols [],[[]],$, and @ have special meanings with regard to R names,

as explained the “Subscripting Operators” section of this chapter.

R originally used five types of assignment, four of which are still current. The four

types are

a <- b,

which assigns b to a,

a -> b,

which assigns a to b,

a <<- b,

which assigns b to a and can be used inside a function to bring the assignment up to the

workspace level, and

a ->> b,

which assigns a to b and brings an assignment in a function up to the workspace level.

CHAPTER 3 ■ AssignmEnTs And OPERATORs

The ls( ) and rm( ) Functions

To see the objects present in the workspace, use the function ls(). Entering ls() at the R

prompt for the above example gives

> ls()

[1] "abc" "bcd" "cde"

which are the three objects created above.

Although functions are covered in detail in Part III, one interesting property of

functions to note here is they can have arguments that the user enters. Two of the possible

arguments for ls() are pattern and all.names.

The first argument is entered as pattern = “a string”, where “a string” is any part

of an object name. For example, in the above workspace, searching for those objects

containing bc in the name gives abc and bcd, that is

> ls(pattern="bc")

[1] "abc" "bcd"

The argument pattern can be reduced to pat, as in ls(pat=“bc”). The shortening

of arguments of functions is a property of R. All arguments in R can be reduced to the

shortest unique form, but they are usually given in the full form in manuals.

The second argument is all.names=, which can equal TRUE or FALSE. If set

to TRUE, the all.names argument instructs R to list all of the files in the workspace,

including those that begin with a period. FALSE is the default value and does not need to

be entered. For the example workspace above, setting all.names equal to TRUE gives

> ls(all.n=T)

[1] ".commander.done" ".First" ".Random.seed" ".Traceback"

[5] "abc" "bcd" "cde"

The [1] refers to “.commander.done” since “commander.done” is the first element of

the vector, and the [5] refers to “abc” since “abc” is the fifth element of the vector. In R, if the

elements of a vector have not been given a name, the convention for listing the elements is

to show the index of the first element in each line of the lines of listed elements.

The function rm() can be used to remove objects from the workspace. For rm(),

the names of the objects to be deleted are put within the parentheses and separated by

commas. For example,

rm(a,b,c)

will remove objects a, b, and c. To remove all objects,

rm(list=ls())

works.

For more information about ls() or rm(), enter ?ls or ?rm at the R prompt.

CHAPTER 3 ■ AssignmEnTs And OPERATORs

Operators

Operators operate on objects. Operators can be logical, arithmetic, matrix, relational,

or subscripting, or they may have a special meaning. Each of the types of operators is

described here.

For operators, elementwise refers to performing the operation on each element

of an object or paired elements for two objects. If two objects do not have the same

dimensions, the operator will cycle the smaller object against the larger object. The

cycling proceeds through each dimension. For example, for matrices the first dimension

is the rows and the second dimension is the columns, so the cycling is down rows starting

with the first column.

The letters NA are used to indicate that an element is missing data. Most operators

have rules for dealing with missing data and may return an NA if data is missing.

CRAN gives a help page of information about operation precedence. Enter

??“Operator Syntax and Precedence” at the R prompt to see the page.

Logical Operators and Functions

Logical operators return the values TRUE, FALSE, or NA, where NA refers to a missing

value. The logical operators are the not operator, two or operators, two and operators, the

exclusive or function (which is a function that acts as an operator), and the any function

(which is a function that operates on a logical object). For logical operators, if the two

objects do not have the same dimensions, the number of elements in the larger object

must be a multiple of the number of elements in the smaller object for cycling to occur.

The logical operators and two logical functions are listed in Table 3-1.

Table 3-1. The Logical Operators and Functions

Operator Operation Description

! not negation operator—e.g., !a

| or elementwise or operator—e.g., a|b

|| or or operator, just evaluates the first elements in the

objects—e.g., a||b

& and elementwise and operator—e.g., a&b

&& and and operator, just evaluates the first elements in the

objects—e.g., a&&b

xor() exclusive or exclusive or function—e.g., xor(a,b)

any() logical test tests if TRUE is present in a logical object—e.g., any(a)

The logical operators operate on objects that are logical, numeric, or raw. When a

numeric object is coerced to logical, all of the nonzero values are set to TRUE and the

zero values are set to FALSE. For raw vectors, the operators are applied bitwise.

CHAPTER 3 ■ AssignmEnTs And OPERATORs

The negation operator changes TRUE to FALSE and FALSE to TRUE in a logical

object. The operator | compares the two objects elementwise and, for each pair of

elements, returns TRUE if TRUE is present, and FALSE otherwise. The operator ||

compares the first element of the first object to the first element of the second object and

returns TRUE if TRUE is present, or FALSE otherwise.

The operator & compares two objects elementwise and, for each pair of elements,

returns TRUE if both elements are TRUE, and FALSE otherwise. The operator &&

compares the first element of the first object to the first element of the second object and

returns TRUE if the first elements are both TRUE, otherwise FALSE.

The xor() function compares objects elementwise and returns TRUE if the paired

elements are different and FALSE if the paired elements are the same.

For a logical vector or a vector that can be coerced to logical, the function any() will

return TRUE if any of the elements are TRUE, and FALSE otherwise.

For more information about the logical operators, the CRAN help pages for logical

operators can be found by entering ??“logical operators” at the R prompt. The help page

for any() can be accessed by entering ?any at the R prompt.

Arithmetic Operators

Arithmetic operators can have numeric operands or operands that can be coerced to

numeric. For example, for logical objects TRUE coerces to 1 and FALSE coerces to 0.

For some types of objects, specific operators have a different meaning, but those types of

objects will not be covered in this chapter.

Arithmetic expressions are evaluated elementwise. If the number of elements is not

the same between the objects in an expression, the smaller object cycles through the

larger one until the end of the larger one. The numbers of elements in the larger object do

not have to be a multiple of the smaller object for cycling. Expressions are evaluated from

left to right, under the rules of precedence.

The arithmetic operators are the standard * for multiplication, / for division, + for

addition, and - for subtraction. The exponentiation symbol is ^. The operator %% gives

the modulus of the first argument with respect to the second argument. The operator

%/% performs integer division. Expressions can be grouped using parentheses, for

example (a+b)/c. Table 3-2 lists the arithmetic operators.

Table 3-2. Arithmetic Operators

Operator Operation Example

* multiplication a*b

/ division a/b

+ addition a+b

- subtraction a-b

^ exponentiation a^b

%% modulus a%%b

%/% integer division a%/%b

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