如何从 R 中的 lm 打印出设计矩阵？

Question

Assume x2-x4 are continuous predictors occupying one column each in the design matrix created using lm() in R. I want to include x1 a categorical variable that has 3 levels.

Regression R code:
fit <- lm(y~as.factor(x1)+x2+x3+c4,data=mydata)

How can I print the design matrix from lm() in R and what would it look like? I need to know the default coding used in R so I can write contrast statements properly.

Answer 1

I think the model.matrix() function is what you're after.

As kjetil b halvorsen says you can specify the model in the argument. Or you can just hand model.matrix the defined model.

fit <- lm(y~as.factor(x1)+x2+x3+c4,data=mydata)
model.matrix(fit)

You can even get a design matrix for new data: model.matrix(fit, data=newdata)

Answer 2

The comment by kjetil b halvorsen helped me the most:

call res <- lm() with the argument x=TRUE then the design matrix will be returned in the model object res Then call str(res) to see the structure of res , and you will now how to get the design matrix from it. But easier is to call model.matrix(y ~ x + f, data=...) with the same model formula you use in lm .