如何强制model.matrix（）函数将NA视为一个组的级别，就像＆＃34;黄色＆＃34;例如，不要删除该记录？ 我的代码：
a <- c(4,NA, 8, 7, 12, 6, 9, 10, 14, 7,9)
b <- as.factor(c(1, 0, 1,NA, 1, 1, 0, 1,1, 0, 0))
c <- c(0.86, 0.45, 0.99, 0.84, 0.85, 0.67, 0.91, 0.29, 0.88,1,0.5)
d <- as.factor(c(0, 1, 1, 2, 2, 3, 2, 0, NA,NA,NA))
e <- as.factor(c(0, 2, 2, 2, 2, 3, 2, 0, 0 , 2,0))
f <- as.factor(c("blue", "blue", "yellow", "red", "red", "yellow",
"yellow", "red", "yellow" , "blue" , "red"))
g <- c(1, 1, 0, 1, 0, 0, 0, 1, 1 , 0,0)
mat <- model.matrix(g ~ b + d + e + f)[, -1]
mat #reduce the number of rows containing NA
非常感谢任何帮助。