R语言中的表达式函数

狭义表达式指表达式(expression)类对象，由expression函数产生；而广义的表达式既包含expression类，也包含R“语言”类(language)。expression和language是R语言中两种特殊数据类：

getClass("expression")		#expression由向量派生得到

getClass("language")	#虚拟类(程序控制关键词和name/call子类)
Virtual Class "language" [package "methods"]
No Slots, prototype of class "name"
Known Subclasses: 
Class "name", directly
Class "call", directly
Class "{", directly
Class "if", directly
Class "<-", directly
Class "for", directly
Class "while", directly
Class "repeat", directly
Class "(", directly
Class ".name", by class "name", distance 2, with explicit coerce
           

1.expression

expression(..)
#产生未评估求值(evaluate)的表达式(expression类型的向量/列表,[,[[,$进行子集化), ..-R对象(通常是call类型,符号或常量),返回值的长度等于参数个数
is.expression(x)
as.expression(x,..)	#x任意R对象
######################
length(ex1<-expression(1 + 0:9)) # 1
eval(ex1) # 1:10

length(ex2<-expression(u, v, 1+ 0:9)) # 3
ex3<-expression(u, v, z=1+0:9)
#子集化
mode(ex3 [3])   # expression
mode(ex3[[3]])  # call
mode(ex3$z)	  # call
typeof(ex3$z)   # language
mode(ex3[[2]])  # name
typeof(ex3[[2]])	#symbol
ex3$z<-NULL	  # 删除

#注：expression函数将参数当做列表处理,故参数要符合列表元素的书写规则
           

2.parse

一般用于从文件中读取文本作为表达式，返回expression类

parse(file = "", n = NULL, text = NULL, prompt = "?", keep.source = getOption("keep.source"), srcfile, encoding = "unknown")

cat("x<-c(1,4)\n  x^3-10; outer(1:7,5:9)\n", file="xyz.Rdmped")
parse(file="xyz.Rdmped", n=2)	#expression(x<-c(1,4)\n  x^3-10)
unlink("xyz.Rdmped")

parse(text="a+b") 
expression(a+b)
           

3.quote

quote(expr)	#expr-任何语法上有效的R表达式,返回值一般情况下是call类,expr是单个变量时为name类,expr为常量时返回值的存储模式与相应常量的存储模式相同
cl<-quote(1 + sqrt(a) + b^c)	#1 + sqrt(a) + b^c 
mode(cl)	#"call"
typeof(cl)	#"language" 
cl <- quote(a)
mode(cl)	#"name"
length(cl)	# 1
typeof(cl)	#"symbol" 
cl<-quote(1)	
mode(cl)	#"numeric" 
typeof(cl)	#"double"
           

4.substitute

若不使用环境变量或环境变量参数，substitute函数得到的结果与quote函数相同

substitute(expr,env)	#expr-任何语法上有效的R表达式,env-环境或列表对象,默认为当前环境
substitute(1 + sqrt(a) + b^c) == quote(1 + sqrt(a) + b^c)	#TRUE
ss <- substitute(y == sqrt(a, b), list(a = 3, b = 2))

a=1
substitute(a+b,list(a=a))	#1+b
           

5.deparse

将未评估的表达式转换成字符串

deparse(expr, width.cutoff = 60L, backtick = mode(expr) %in% c("call", "expression", "(", "function"), control = c("keepInteger", "showAttributes", "keepNA"), nlines = -1L)
#expr-any r expression,"call"/"expression"/"function"/"("
#backtick-逻辑值,当不遵循标准语法时-符号名称是否应该用反引号括起来
#width.cutoff-[20,500]中的整数,确定尝试换行的截止(以字节为单位)
#control="all"最接近deparse()为parse()的反转

deparse(args(lm))
deparse(args(lm), width = 500)

e<-quote(`foo bar`)	#name类
deparse(e)	#"foo bar"
deparse(e, backtick = TRUE)	#"foo bar"

e <- quote(`foo bar`+1)
deparse(e,backtick=FALSE)
deparse(e,backtick=TRUE)
deparse(e,control = "all")		#"quote(`foo bar` + 1)"
deparse(e,backtick=FALSE,control = "all")