计算公式中的变量

Question

我想计算进入公式右侧的变量数量.有没有这样做的功能？

例如:

y<-rnorm(100)
x1<-rnorm(100)
x2<-rnorm(100)
x3<-rnorm(100)
f<-formula(y~x1+x2+x3)

然后,我会调用SomeFunction(f)哪个将返回3(因为在等式的右边有3个x变量).SomeFunction存在吗？

Answer 1

您可能需要查看帮助页面中链接的一些相关功能formula.特别是terms:

> terms(f)
y ~ x1 + x2 + x3 + x4
attr(,"variables")
list(y, x1, x2, x3, x4)
attr(,"factors")
   x1 x2 x3 x4
y   0  0  0  0
x1  1  0  0  0
x2  0  1  0  0
x3  0  0  1  0
x4  0  0  0  1
attr(,"term.labels")
[1] "x1" "x2" "x3" "x4"
attr(,"order")
[1] 1 1 1 1
attr(,"intercept")
[1] 1
attr(,"response")
[1] 1
attr(,".Environment")
<environment: R_GlobalEnv>

请注意"term.labels"属性.

Answer 2

这有两种可能性:

length(attr(terms(f), "term.labels"))

length(all.vars(update(f, z ~.))) - 1

Answer 3

如果您想计算估计参数的数量，正如 G. Grothendieck 的答案下面的评论所建议的那样，您可以尝试下面的代码。n.coefficients我为错误项添加了一个，就像 AIC 所做的那样。

n      <- 20                                       # number of observations
B0     <-  2                                       # intercept
B1     <- -1.5                                     # slope 1
B2     <-  0.5                                     # slope 2
B3     <- -2.5                                     # slope 3
sigma2 <-  5                                       # residual variance

x1     <- sample(1:3, n, replace=TRUE)             # categorical covariate
x12    <- ifelse(x1==2, 1, 0)
x13    <- ifelse(x1==3, 1, 0)
x3     <- round(runif(n, -5 , 5), digits = 3)      # continuous covariate
eps    <- rnorm(n, mean = 0, sd = sqrt(sigma2))    # error
y      <- B0 + B1*x12 + B2*x13 + B3*x3 + eps       # dependent variable
x1     <- as.factor(x1)

model1 <- lm(y ~ x1 + x3)                          # linear regression
model1

summary(model1)

n.coefficients <- as.numeric(sapply(model1, length)[1]) + 1
n.coefficients

# [1] 5

这是代码的更直接的替代方案n.coefficients：

# For each variable in a linear regression model, one coefficient exists
# An intercept coefficient exists as well
# Subtract -1 to account for the intercept
n.coefficients2 <- length(model1$coefficients) - 1
n.coefficients2

# [1] 5