函数如下：

cv.lm <- function(obj) {
data <- data.frame(y = obj$model[,1], x = obj$model[, -1])
N <- nrow(data)
{
   ytest <- data[, 1]
   newrsq <- numeric(N)
  ypred <- numeric(N)
   for(i in 1:N){
     newtrain <- data[-i, ]
     xtest <- data[i, -1]
   newfm <- lm(y ~., data = newtrain)
    newrsq <- summary(newfm)$r.squared
   ypred <- predict(newfm, xtest)
      }
  }
q.squared <- 1 - (sum((ytest - ypred)^2) / sum((ytest - mean(ytest))^2))
SDEP <- sqrt(sum((ytest - ypred)^2) / N)
return(list(q.squared = q.squared, SDEP = SDEP, newrsq = newrsq))
}




q.squared我计算出来的一直是负数……