「R」从公式中提取组分

f <- y ~ x1 + x2

# 观察f结构
str(f)
#> Class 'formula'  language y ~ x1 + x2
#>   ..- attr(*, ".Environment")=<environment: 0x1e46710>

# 获得每一部分
f[[1]]
#> `~`
f[[2]]
#> y
f[[3]]
#> x1 + x2

# 转换为列表观察
as.list(f)
#> [[1]]
#> `~`
#>
#> [[2]]
#> y
#>
#> [[3]]
#> x1 + x2
#>
#> <environment: 0x1e46710>

f2 <- ~ x1 + x2
as.list(f2)
#> [[1]]
#> `~`
#>
#> [[2]]
#> x1 + x2
#>
#> <environment: 0x1e46710>

str(f[[1]])
#>  symbol ~
str(f[[2]])
#>  symbol y
str(f[[3]])
#>  language x1 + x2

# Look at parts of the langage object
str(f[[3]][[1]])
#>  symbol +
str(f[[3]][[2]])
#>  symbol x1
str(f[[3]][[3]])
#>  symbol x2

as.character(f[[1]])
#> [1] "~"
as.character(f[[2]])
#> [1] "y"

# The language object gets coerced into a string that represents the parse tree:
as.character(f[[3]])
#> [1] "+"  "x1" "x2"

# You can use deparse() to get a more natural looking string
deparse(f[[3]])
#> [1] "x1 + x2"
deparse(f)
#> [1] "y ~ x1 + x2"

environment(f)
#> <environment: 0x1e46710>