r - R 中用户定义函数中的公式

Question

公式是 R 的统计和图形函数的一个非常有用的特性。和大家一样，我也是这些功能的用户。但是，我从未编写过将公式对象作为参数的函数。我想知道是否有人可以通过链接到 R 编程这一方面的可读介绍或提供一个独立的示例来帮助我。

Answer 1

您可以使用 model.matrix()和 model.frame()评估公式:

lm1 <- lm(log(Volume) ~ log(Girth) + log(Height), data=trees)
print(lm1)

form <- log(Volume) ~ log(Girth) + log(Height)

# use model.matrix
mm <- model.matrix(form, trees)
lm2 <- lm.fit(as.matrix(mm), log(trees[,"Volume"]))
print(coefficients(lm2))

# use model.frame, need to add intercept by hand
mf <- model.frame(form, trees)
lm3 <- lm.fit(as.matrix(data.frame("Intercept"=1, mf[,-1])), mf[,1])
print(coefficients(lm3))

这产生

Call: lm(formula = log(Volume) ~ log(Girth) + log(Height), data = trees)

Coefficients: (Intercept)   log(Girth) log(Height)
      -6.63         1.98         1.12

(Intercept)  log(Girth) log(Height)
     -6.632       1.983       1.117  
Intercept  log.Girth. log.Height.
     -6.632       1.983       1.117