如图 在用college的数据，在lasso方法寻找最优lamda的时候，不改数据形式显示'list' object cannot be coerced to type 'double'改之后Error in rep(1, N) : 'times'参数不对


library(alr4)
library(glmnet)
names(College)
#划分数据集
xtrain <- College[1:500,2:17]
ytrain <- College[1:500,1]
xtest <- College[501:777,2:17]
ytest <- College[501:777,1]
# Lasso
xt<-as.numeric(unlist(xtrain))
yt<-as.numeric(unlist(ytrain))
cv_lasso <- cv.glmnet(xt,yt,alpha=1)
cv_lasso = cv.glmnet(xtrain,ytrain，alpha=1)



应该怎么改呢