convert matrix to data frame (export lasso result)

I intend to convert output from LASSO regresson to csv file.

First, I try to convert matrix to data.frame:
lasso_model = glmnet(x,y,alpha=1,lambda=c(0.0000000001,0.000000001,0.00000001,0.0000001,0.000001,0.00001,0.0001,0.001,0.01,0.1,1))
a<-coef(lasso_model)
class(a)
as.data.frame(t(a))

Then, there appears this problem:

Error in as.data.frame.default(a) :
cannot coerce class ‘structure("dgCMatrix", package = "Matrix")’ to a data.frame

Could you please help me ?

Just convert to matrix first:

a <- as.matrix(a)
as.data.frame(t(a))

However, when running your code, the error I get is Error in t.default(a) : argument is not a matrix so this example is not reproducible, there might be another problem.

Thank you so much, problem solved !