當有下標時從公式中提取變量

Question

在R中的迴歸公式中有幾個與獲得變量列表相關的帖子 - 基本答案是使用all.vars。例如，

> all.vars(log(resp) ~ treat + factor(dose)) 
[1] "resp" "treat" "dose" 

這很好，因爲它去除了所有的功能和操作符（以及重複，未顯示）。然而，這是有問題的，當配方中含有$運營商或下標，如在

> form = log(cows$weight) ~ factor(bulls[[3]]) * herd$breed 
> all.vars(form) 
[1] "cows" "weight" "bulls" "herd" "breed" 

這裏，數據幀名稱cows，bulls，和herd被識別爲變量，和實際變量的名稱解耦或丟失。相反，我真正想要的是這樣的結果：

> mystery.fcn(form) 
[1] "cows$weight" "bulls[[3]]" "herd$breed" 

什麼是最優雅的方式來做到這一點？我有一個建議，我會作爲答案張貼，但也許有人有一個更優雅的解決方案，並會贏得更多的選票！

Answer 1

一種方法的作品，雖然有點繁瑣，更換運營商$等與變量名合法字符，把字符串返回到一個公式，適用all.vars，並取消裂傷結果：

All.vars = function(expr, retain = c("\\$", "\\[\\[", "\\]\\]"), ...) { 
    # replace operators with unlikely patterns _Av1_, _Av2_, ... 
    repl = paste("_Av", seq_along(retain), "_", sep = "") 
    for (i in seq_along(retain)) 
     expr = gsub(retain[i], repl[i], expr) 
    # piece things back together in the right order, and call all.vars 
    subs = switch(length(expr), 1, c(1,2), c(2,1,3)) 
    vars = all.vars(as.formula(paste(expr[subs], collapse = "")), ...) 
    # reverse the mangling of names 
    retain = gsub("\\\\", "", retain) # un-escape the patterns 
    for (i in seq_along(retain)) 
     vars = gsub(repl[i], retain[i], vars) 
    vars 
} 

使用retain參數指定我們希望保留的模式，而不是作爲運算符對待。默認值是$，[[和]]（所有正式轉義）下面是一些結果：

> form = log(cows$weight) ~ factor(bulls[[3]]) * herd$breed 
> All.vars(form) 
[1] "cows$weight" "bulls[[3]]" "herd$breed" 

變化retain也包括(和)：

> All.vars(form, retain = c("\\$", "\\(", "\\)", "\\[\\[", "\\]\\]")) 
[1] "log(cows$weight)" "factor(bulls[[3]])" "herd$breed" 

的點傳遞到all.vars，這與all.names確實相同，但具有不同的默認值。因此，我們也能獲得的功能和運營商不retain：

> All.vars(form, functions = TRUE) 
[1] "~"   "log"   "cows$weight" "*"   
[5] "factor"  "bulls[[3]]" "herd$breed" 

Answer 2

這是不夠的一般使用情況下，但只是爲了好玩，我想我會帶一條縫吧：

mystery.fcn = function(string) { 
    string = gsub(":", " ", string) 
    string = unlist(strsplit(gsub("\\b.*\\b\\(|\\(|\\)|[*~+-]", "", string), split=" ")) 
    string = string[nchar(string) > 0] 
    return(string) 
} 

form = log(cows$weight) ~ factor(bulls[[3]]) * herd$breed 
mystery.fcn(form) 
[1] "cows$weight" "bulls[[3]]" "herd$breed" 

form1 = ~x[[y]] 
mystery.fcn(form1) 
[1] "x[[y]]" 

form2 = z$three ~ z$one + z$two - z$x_y 
mystery.fcn(form2) 
[1] "z$three" "z$one" "z$two" "z$x_y" 

form3 = z$three ~ z$one:z$two 
mystery.fcn(form3) 
[1] "z$three" "z$one" "z$two"